1. Infection of Macrophages with H. capsulatum, C. albicans
<ol>
	<li>16 hr prior to infection, seed macrophages at desired density in 24-well plates. In this protocol, a density of 3.0 x 105 cells/well was used.</li>
	<li>Add fungal cells in log-phase growth at a desired multiplicity of infection (MOI). This protocol can accommodate a range of MOI (0.2 - 5.0). For the infection of RAW264.7 macrophages with H. capsulatum, we used an MOI of 0.2.</li>
	<li>Following 1.5 hr to allow for phagocytosis, wash macrophages three times with PBS to remove extracellular fungi.</li>
	<li>Incubate for desired number of hours prior to sample analysis. At low MOI (0.2 in our experiment), infected macrophage cells remain viable for several days, and samples can be analyzed approximately every 12-24 hr.</li>
</ol>
2. Macrophage Lysis
<ol>
	<li>To liberate fungi from macrophage cells, remove media, wash 3 times with PBS, and add 0.5 ml sterile water. Under these conditions, macrophages will lyse and fungal cells will remain intact.</li>
	<li>Incubate for 5 min at room temperature.</li>
	<li>Transfer lysate to sterile tube, keep on ice.</li>
	<li>Transfer 20 &mu;l lysate to a separate tube, then add 20 &mu;l AO/PI solution. Proceed directly to step 5: "Sample Preparation for Image Cytometric Analysis"</li>
</ol>
3. CFU Plating
<ol>
	<li>Perform ten-fold dilution of lysates (from step 2.3) in media.</li>
	<li>Plate 100 &mu;l of each dilution, in duplicate, on HMM-agarose plates. Incubate plates in a humidified chamber at 37 &deg;C with 5% CO2 for 7-8 days.</li>
	<li>Manually count colonies on plates displaying a minimum of 100 and a maximum of 1,000 distinct colonies.</li>
</ol>
4. Visualization of Fungi within Live Macrophages
<ol>
	<li>To collect live macrophages, wash cells 3 times with PBS. Add 0.5 ml PBS and incubate for 30 min at 4 &deg;C.</li>
	<li>To remove macrophages from tissue culture wells, gently pipette up and down several times. Transfer liquid to sterile tube, keep on ice.</li>
	<li>Transfer 20 &mu;l sample to a separate tube, then add 20 &mu;l AO/PI solution. Proceed directly to step 5: "Sample Preparation for Image Cytometric Analysis".</li>
</ol>
5. Sample Preparation for Image Cytometric Analysis
<ol>
	<li>Pipette the target sample thoroughly and then transfer 20 &mu;l of sample into the disposable cell counting chamber.</li>
	<li>Allow the cells to settle in the chamber for 30 sec.</li>
	<li>Insert counting chamber into the image cytometer.</li>
</ol>
6. Cellometer Instrument Setup
<ol>
	<li>Insert the Fluorescence Optics Modules: VB-535-402 and VB-660-502 into the system and make sure they are locked in place.
	<ol>
		<li>VB-535-402 (excitation at 475 nm, emission at 535 nm) is used for acridine orange and GFP detection.</li>
		<li>VB-660-502 (excitation at 540 nm, emission at 660 nm) is used for propidium iodide detection.</li>
	</ol>
	</li>
	<li>Turn on the image cytometer and open the accompanying software.</li>
</ol>
7. Cellometer Software Setup
<ol>
	<li>Select the preset "Assay Type" and "Cell Type" in the Assay Dropdown Menu.
	<ol>
		<li>For viability, the assay is optimized for acridine orange and propidium iodide detection.</li>
		<li>For detection of Candida albicans infection, the assay is optimized for GFP detection.</li>
	</ol>
	</li>
	<li>Select "Options" on the top and click on "Take Background Image", and allow the operation to complete.</li>
	<li>Click on "Preview Bright-Field Image".</li>
</ol>
8. Image Acquisition Procedure
<ol>
	<li>Insert the prepared sample chamber into the image cytometer.</li>
	<li>Use the focus knob and adjust the focus.</li>
	<li>Once in focus, click on "Count", and allow the image acquisition operation to complete.</li>
	<li>Remove the disposable counting chamber and dispose of appropriately.</li>
</ol>
9. Image Data Analysis
<ol>
	<li>Concentration and viability measurement
	<ol>
		<li>Once the counting is completed, the concentration and viability of the target cells are displayed in the result page.</li>
		<li>Click on "Export" to export the data into FCS Express 4 for cell population analysis of GFP in the Candida albicans infection experiment.</li>
	</ol>
	</li>
	<li>FCS Express analysis of Candida albicans-infected cells
	<ol>
		<li>Import the ".NXDAT" file into FCS Express 4 and plot the results in a fluorescence histogram.</li>
		<li>Apply cell population gate to the histogram to determine the population percentages of Candida albicans-infected cells.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Worsham, P. L., Goldman, W. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantitative+plating+of+Histoplasma+capsulatum+without+addition+of+conditioned+medium+or+siderophores.">Quantitative plating of Histoplasma capsulatum without addition of conditioned medium or siderophores.</a> J. Med. Vet. Mycol. 26, 137-143 (1988).</li><li>Goihman-Yahr, M., Pine, L., Albornoz, M. C., Yarzabal, L., de Gomez, M. H., San Martin, B., Ocanto, A., Molina, T., Convit, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Studies+on+plating+efficiency+and+estimation+of+viability+of+suspensions+of+Paracoccidioides+brasiliensis+yeast+cells.">Studies on plating efficiency and estimation of viability of suspensions of Paracoccidioides brasiliensis yeast cells.</a> Mycopathologia. 71, 73-83 (1980).</li><li>Bhatti, M. A., Hjertstedt, J., Hahn, B. L., Sohnle, P. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Inefficient+delivery+of+yeast+cells+as+an+explanation+for+reduced+plating+efficiency+of+Candida+albicans.">Inefficient delivery of yeast cells as an explanation for reduced plating efficiency of Candida albicans.</a> Med. Mycol. 40, 465-469 (2002).</li><li>Chang, W. L., vander Heyde, H. C., Klein, B. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Flow+cytometric+quantitation+of+yeast:+a+novel+technique+for+use+in+animal+model+work+and+in+vitro+immunologic+assays.">Flow cytometric quantitation of yeast: a novel technique for use in animal model work and in vitro immunologic assays.</a> Journal of Immunological Methods. 211, 51-63 (1998).</li><li>Green, L., Petersen, B., Steimel, L., Haeber, P., Current, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid+determination+of+antifungal+activity+by+flow+cytometry.">Rapid determination of antifungal activity by flow cytometry.</a> Journal of Clinical Microbiology. 32, 1088-1091 (1994).</li><li>Kirk, S. M., Callister, S. M., Lim, L. C., Schell, R. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid+susceptibility+testing+of+Candida+albicans+by+flow+cytometry.">Rapid susceptibility testing of Candida albicans by flow cytometry.</a> Journal of Clinical Microbiology. 35, 358-363 (1997).</li><li>Chan, L. L. -. Y., Lai, N., Wang, E., Smith, T., Yang, X., Lin, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+rapid+detection+method+for+apoptosis+and+necrosis+measurement+using+the+Cellometer+imaging+cytometry.">A rapid detection method for apoptosis and necrosis measurement using the Cellometer imaging cytometry.</a> Apoptosis. 16, 1295-1303 (2011).</li><li>Chan, L. L. -. Y., Shen, D., Wilkinson, A. R., Patton, W., Lai, N., Chan, E., Kuksin, D., Lin, B., Qiu, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+novel+image-based+cytometry+method+for+autophagy+detection+in+living+cells.">A novel image-based cytometry method for autophagy detection in living cells.</a> Autophagy. 8, 1371-1382 (2012).</li><li>Chan, L. L., Wilkinson, A. R., Paradis, B. D., Lai, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid+Image-based+Cytometry+for+Comparison+of+Fluorescent+Viability+Staining+Methods.">Rapid Image-based Cytometry for Comparison of Fluorescent Viability Staining Methods.</a> Journal of Fluorescence. 22, 1301-1311 (2012).</li><li>Chan, L. L., Zhong, X., Pirani, A., Lin, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+novel+method+for+kinetic+measurements+of+rare+cell+proliferation+using+Cellometer+image-based+cytometry.">A novel method for kinetic measurements of rare cell proliferation using Cellometer image-based cytometry.</a> J. Immunol. Methods. 377, 8-14 (2012).</li><li>Chan, L. L., Zhong, X., Qiu, J., Li, P. Y., Lin, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cellometer+Vision+as+an+alternative+to+flow+cytometry+for+cell+cycle+analysis,+mitochondrial+potential,+and+immunophenotyping.">Cellometer Vision as an alternative to flow cytometry for cell cycle analysis, mitochondrial potential, and immunophenotyping.</a> Cytom. Part A. 79, 507-517 (2011).</li><li>Hull, C. M., Johnson, A. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Identification+of+a+mating+type-like+locus+in+the+asexual+pathogenic+yeast+Candida+albicans.">Identification of a mating type-like locus in the asexual pathogenic yeast Candida albicans.</a> Science. 285, 1271-1275 (1999).</li><li>Berkes, C. A., Chan, L. L., Wilkinson, A., Paradis, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid+Quantification+of+Pathogenic+Fungi+by+Cellometer+Image-Based+Cytometry.">Rapid Quantification of Pathogenic Fungi by Cellometer Image-Based Cytometry.</a> J. Micro. Meth. 91, 468-476 (2012).</li><li>Nguyen, V. Q., Sil, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Temperature-induced+switch+to+the+pathogenic+yeast+form+of+Histoplasma+capsulatum+requires+Ryp1,+a+conserved+transcriptional+regulator.">Temperature-induced switch to the pathogenic yeast form of Histoplasma capsulatum requires Ryp1, a conserved transcriptional regulator.</a> Proc. Natl. Acad. Sci. U.S.A. 105 (12), 4880-4885 (2008).</li><li>Wenisch, C., Linnau, K. F., Parschalk, B., Zedtwitz-Liebenstein, K., Georgopoulos, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid+susceptibility+testing+of+fungi+by+flow+cytometry+using+vital+staining.">Rapid susceptibility testing of fungi by flow cytometry using vital staining.</a> Journal of Clinical Microbiology. 35, 5-10 (1997).</li></ol>

The authors Leo Li-Ying Chan and Benjamin Paradis are employees of Nexcelom Bioscience LLC.

Image cytometry allows the user to capture high quality images and, using specialized software, perform rapid quantification of cells. One potential challenge to the adoption of image cytometry in the field of microbial pathogenesis is that the microbes to be counted are present in a mixed population of cells, including mammalian host cells. Here, we demonstrate that image cytometry can be used for the quantification of viable pathogenic yeasts during in vitro macrophage infection. Our methodology not only faithfully recapitulates trends in fungal growth and viability observed during in vitro macrophage infection assays, but also displays improved sensitivity compared to the traditional CFU assay 13.
Image cytometry offers several practical advantages when compared with either CFU enumeration or flow cytometry for quantification of yeast cells. When plating CFUs, laboratory workers must plate several dilutions in order to ensure a countable number of cells on each plate, which can be very labor-intensive. Image cytometry eliminates the need for multiple dilutions. Previously, we have shown that colony formation on solid media is often variable, and at very low concentrations fungi may fail to form colonies at all, whereas image cytometry enables enumeration of cells regardless of concentration 13. The ability to detect and count fungi at very low concentrations can be very useful during early stages of a low-dose infection, or during clearance, when numbers and viability may be at the margin of detection. Additionally, data generated by image cytometry are available instantly, whereas colonies may take several days to become visible. While many fungal species can be detected by flow cytometry, the possibility of cross-contamination is a concern in shared facilities. The counting chamber used in this study offers a significant advantage in terms of biosafety, as it is disposable and self-contained. Furthermore, image cytometry offers the practical advantages of lower price and a smaller footprint when compared with conventional flow cytometry, which can be important for smaller research laboratories 11.
In addition to its practical advantages, image cytometry of yeast cells provides information that CFU enumeration cannot. First, CFU enumeration is only able to detect fungi that are able to establish colony growth on the medium chosen, which may not accurately represent the number of viable fungi present in an infection experiment in real time. Also, the ability to establish colony growth on solid media can differ when comparing wild-type and mutant strains of fungi, which can be a source of experimental bias 14, and these hidden biases may be revealed by comparison of image cytometry and CFU quantification of wild-type and mutant strains. Once the behavior of a particular strain has been characterized by both CFU enumeration and image cytometry, we believe that image cytometry can be used as an alternative means of fungal quantification.
One limitation of image cytometry in general is that it does not allow collection of as many data points as flow cytometry. Since the image cytometer derives quantitative data from a limited number of captured images, it is difficult for the system to collect data on hundreds of thousands to millions of cells per sample. However, this limitation may be circumvented by grouping multiple samples into one set of data for analysis in order to reach similar data points as a flow cytometer. We would also like to stress that the AO/PI staining method used here represents one way of measuring of cellular viability, and does not necessarily indicate cellular vitality and/or the ability to establish growth on solid medium in any given sample. Therefore, while our method provides a facile method of live/dead quantification of yeasts, it will also be interesting to investigate the use of image cytometry in combination with cellular metabolism indicators such as FUN-1 15.
Image cytometry software identifies and counts cells of interest based on size and shape, and therefore, it is absolutely critical that these parameters are set carefully during each experiment. When performing image cytometry with a new yeast strain, we suggest that one first compare data generated from a pure yeast culture to data generated from counting a mixture of yeasts plus host cells to ensure that the parameters are set such that the software can distinguish between the two cell types.
In the future, this work will be the basis for further development of methods to detect and quantify microbes via image cytometry. For example, development of image cytometric methods to detect fungi within organ homogenates will be of great use in the field. Another future challenge will be to develop and refine image cytometric methods and software such that it can be used for quantification of bacterial cells. With the advancement in optical systems, digital cameras, and a wide variety of fluorescent stains in the field, the ability to image and analyze bacteria population can be implemented in image cytometers, which can provide a more rapid method for concentration and viability or vitality measurement in comparison to the CFU or optical density methods. In summary, the work presented here is a proof-of-concept showing that image cytometry is a sensitive method for quantification of pathogenic yeasts. The sophistication of image-based cytometric analysis software offers the opportunity to analyze interactions between host cells and pathogenic fungi in a highly quantitative manner. This technology may be adapted to address a range of research questions in the field of fungal pathogenesis.

Studies of pathogenic fungi in association with their hosts and/or host cells often require quantification of viable fungal cells over a time course or under different infection conditions. Enumeration of colony forming units (CFU) is the standard method by which the number of viable fungal cells has been measured, however, this technique has several drawbacks and limitations. First, many fungal species are slow-growing. Growth of visible colonies on solid media can take 1-2 weeks, significantly slowing the pace of research. Second, manipulation of samples during CFU plating is a laborious process, since several dilutions must be plated to ensure a countable number of colonies. Third, the number of CFU is typically lower than the number of viable organisms plated because the plating efficiency is well below 100%. For example, plating efficiencies for the dimorphic fungal pathogen Histoplasma capsulatum can be as high as 90%, but are routinely as low as 30% and are even lower (10%) for the related fungal dimorph Paracoccidioides brasiliensis 1, 2. Plating efficiency for Candida albicans is also subject to variability 3. Finally, CFU analysis accounts for only live and actively dividing cells capable of establishing growth on solid media, whereas in many situations, it would be useful to determine the presence and concentration of dead and/or metabolically inactive cells.
Previously, flow cytometric methods for quantification of several pathogenic fungal species has been described 4-6. However, due to biosafety containment issues involved with using biosafety level 2 (BSL2) or BSL3-level pathogens on shared flow cytometers, adoption of this technique has been limited. Like flow cytometry, image cytometry is a sensitive and rapid method of cell quantification. However, image cytometry can be performed at a fraction of the cost with comparable results 7-11. Here, we describe methods for performing image cytometry of pathogenic fungi in association with host cells. We demonstrate our methods using two human fungal pathogens: Histoplasma capsulatum and Candida albicans. H. capsulatum is a dimorphic fungal pathogen that causes respiratory disease; in humans, it grows as budding yeast and replicates within alveolar macrophages. Candida albicans is a human commensal species that occasionally causes candidiasis. We show that image cytometry allows rapid quantification of these yeasts, together with visualization capability.

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		<tbody>
		 <tr>
			<td></td>
			<td></td>
			<td></td>
			<td>REAGENTS</td>
		 </tr>
		 <tr>
			<td>DMEM</td>
			<td>Life Technologies</td>
			<td>11965-084</td>
			<td></td>
		 </tr>
		 <tr>
			<td>Fetal Bovine Serum</td>
			<td>Life Technologies</td>
			<td>16000044</td>
			<td></td>
		 </tr>
		 <tr>
			<td>AO/PI Solution</td>
			<td>Nexcelom Bioscience</td>
			<td>CSK-0102</td>
			<td></td>
		 </tr>
		 <tr>
			<td>Disposable Counting Chamber</td>
			<td>Nexcelom Bioscience</td>
			<td>CHT4-SD100 </td>
			<td></td>
		 </tr>
		 <tr>
			<td></td>
			<td></td>
			<td></td>
			<td>EQUIPMENT</td>
		 </tr>
		 <tr>
			<td>Cellometer Vision</td>
			<td>Nexcelom Bioscience</td>
			<td></td>
			<td></td>
		 </tr>
		 <tr>
			<td>Cellometer Vision Software</td>
			<td>Nexcelom Bioscience</td>
			<td></td>
			<td></td>
		 </tr>
		</tbody>
 </table>

use of image cytometry for quantification of pathogenic fungi in association with host cells

Studies of the cellular pathogenesis mechanisms of pathogenic yeasts such as Candida albicans, Histoplasma capsulatum, and Cryptococcus neoformans commonly employ infection of mammalian hosts or host cells (i.e. macrophages) followed by yeast quantification using colony forming unit analysis or flow cytometry. While colony forming unit enumeration has been the most commonly used method in the field, this technique has disadvantages and limitations, including slow growth of some fungal species on solid media and low and/or variable plating efficiencies, which is of particular concern when comparing growth of wild-type and mutant strains. Flow cytometry can provide rapid quantitative information regarding yeast viability, however, adoption of flow cytometric detection for pathogenic yeasts has been limited for a number of practical reasons including its high cost and biosafety considerations. Here, we demonstrate an image-based cytometric methodology using the Cellometer Vision (Nexcelom Bioscience, LLC) for the quantification of viable pathogenic yeasts in co-culture with macrophages. Our studies focus on detection of two human fungal pathogens: Histoplasma capsulatum and Candida albicans. H. capsulatum colonizes alveolar macrophages by replicating within the macrophage phagosome, and here, we quantitatively assess the growth of H. capsulatum yeasts in RAW 264.7 macrophages using acridine orange/propidium iodide staining in combination with image cytometry. Our method faithfully recapitulates growth trends as measured by traditional colony forming unit enumeration, but with significantly increased sensitivity. Additionally, we directly assess infection of live macrophages with a GFP-expressing strain of C. albicans. Our methodology offers a rapid, accurate, and economical means for detection and quantification of important human fungal pathogens in association with host cells.

We used the Cellometer Vision image cytometer to monitor growth of H. capsulatum in macrophage cells. RAW 264.7 cells were infected with H. capsulatum yeast cells and at various time points, samples were subjected to AO/PI staining followed by image-based cytometric analysis. In parallel, samples were analyzed by traditional CFU enumeration. At each time point, samples were incubated in water to lyse macrophages, and the liberated yeast cells were identified by the Cellometer Vision software (Figures 1a and 1b). We observe highly comparable trends in the concentration of viable yeasts as detected by image-based cytometric analysis and CFU enumeration at each time point (Figure 1c). As expected, the absolute number of live yeasts detected by AO/PI staining was consistently higher than the number of yeasts capable of colony formation on solid medium as assessed by CFU analysis, highlighting significant numbers of viable but non-cultivatable cells.
Visualization of fungal cells within live macrophages can be accomplished using GFP-expressing yeast strains. Bone marrow-derived macrophages (BMDM) were infected with C. albicans yeast cells expressing green fluorescent protein (GFP) under control of the ADH1 promoter 12 (Figure 2a). Infection of BMDM with GFP-C. albicans yeasts at multiplicities of infection (MOI) ranging from 0.1-10 corresponded to incremental increases in GFP intensity within infected macrophages (Figure 2a) as well as total percentage of infected macrophages (Figure 2b).
<img alt="Figure 1" fo:content-width="3.5in" fo:src="/files/ftp_upload/50599/50599fig1highres.jpg" src="/files/ftp_upload/50599/50599fig1.jpg" /> 
 Figure 1. Comparison of image-based cytometric analysis to CFU enumeration of H. capsulatum during in vitro infection of RAW 264.7 macrophages. (a) RAW 264.7 macrophages were infected with H. capsulatum yeasts at an MOI of 0.2. At 24 hr intervals following infection, macrophages were lysed and viable yeasts assessed by AO/PI staining in parallel with CFU enumeration. (b) Representative brightfield (BR) and FL1/FL2 (green/red) combined images of AO/PI stained H. capsulatum released from lysed RAW 264.7 macrophages. A segmenting algorithm counted only AO and PI positive yeasts in the fluorescent images while the large PI-stained RAW 264.7 macrophages (red) are excluded. Images were captured under 10X magnification. (c) Direct comparison of yeast proliferation as assessed by AO/PI staining and CFU enumeration. Linear regression analysis of the growth trends as measured by CFU and Cellometer analysis yield R2=0.9927. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/50599/50599fig1large.jpg" target="_blank">Click here to view larger figure</a>.
<img alt="Figure 2" fo:content-width="5.5in" fo:src="/files/ftp_upload/50599/50599fig2highres.jpg" src="/files/ftp_upload/50599/50599fig2.jpg" /> 
 Figure 2. Quantification of BMDM infection with a GFP-expressing strain of C. albicans. (a) Captured brightfield (BR) (top) and fluorescent (middle) images of GFP-C. albicans infected BMDMs at increasing MOI. Histograms of fluorescence intensities show an increase in GFP fluorescence intensity as the MOI increases, which represents an increase in the number C. albicans associated with BMDM cells (bottom). The software identified infected BMDMs based on application of a linear marker at the baseline fluorescence intensity displayed by the uninfected control population (MOI = 0). (b) Percent infection as a function of MOI, which shows an increase in infected BMDMs as MOI increases. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/50599/50599fig2large.jpg" target="_blank">Click here to view larger figure</a>.

Watch this Scientific Journal Video about Use of Image Cytometry for Quantification of Pathogenic Fungi in Association with Host Cells at JoVE.com

Use of Image Cytometry for Quantification of Pathogenic Fungi in Association with Host Cells

Here, we demonstrate how image cytometry can be used for quantification of pathogenic fungi in association with host cells in culture. This technique can be used as an alternative to CFU enumeration.

Studies of the cellular pathogenesis mechanisms of pathogenic yeasts such as Candida albicans, Histoplasma capsulatum, and Cryptococcus neoformans ...

use-image-cytometry-for-quantification-pathogenic-fungi-association

Nanyang Technological University

Research

JoVE Journal

Immunology and Infection

12.9K Views.  Merrimack College. Here, we demonstrate how image cytometry can be used for quantification of pathogenic fungi in association with host cells in culture. This technique can be used as an alternative to CFU enumeration.

Video: Use of Image Cytometry for Quantification of Pathogenic Fungi in Association with Host Cells

Use of an Optical Trap for Study of Host-Pathogen Interactions for Dynamic Live Cell Imaging

Dynamic live cell imaging allows direct visualization of real-time interactions between cells of the immune system1, 2; however, the lack of spatial and temporal control between the phagocytic cell and microbe has rendered focused observations into the initial interactions of host response to pathogens difficult. Historically, intercellular contact events such as phagocytosis3 have been imaged by mixing two cell types, and then continuously scanning the field-of-view to find serendipitous intercellular contacts at the appropriate stage of interaction. The stochastic nature of these events renders this process tedious, and it is difficult to observe early or fleeting events in cell-cell contact by this approach. This method requires finding cell pairs that are on the verge of contact, and observing them until they consummate their contact, or do not. To address these limitations, we use optical trapping as a non-invasive, non-destructive, but fast and effective method to position cells in culture.
Optical traps, or optical tweezers, are increasingly utilized in biological research to capture and physically manipulate cells and other micron-sized particles in three dimensions4. Radiation pressure was first observed and applied to optical tweezer systems in 19705, 6, and was first used to control biological specimens in 19877. Since then, optical tweezers have matured into a technology to probe a variety of biological phenomena8-13.
We describe a method14 that advances live cell imaging by integrating an optical trap with spinning disk confocal microscopy with temperature and humidity control to provide exquisite spatial and temporal control of pathogenic organisms in a physiological environment to facilitate interactions with host cells, as determined by the operator. Live, pathogenic organisms like Candida albicans and Aspergillus fumigatus, which can cause potentially lethal, invasive infections in immunocompromised individuals15, 16 (e.g. AIDS, chemotherapy, and organ transplantation patients), were optically trapped using non-destructive laser intensities and moved adjacent to macrophages, which can phagocytose the pathogen. High resolution, transmitted light and fluorescence-based movies established the ability to observe early events of phagocytosis in living cells. To demonstrate the broad applicability in immunology, primary T-cells were also trapped and manipulated to form synapses with anti-CD3 coated microspheres in vivo, and time-lapse imaging of synapse formation was also obtained. By providing a method to exert fine spatial control of live pathogens with respect to immune cells, cellular interactions can be captured by fluorescence microscopy with minimal perturbation to cells and can yield powerful insight into early responses of innate and adaptive immunity.

A method is described to individually select, manipulate, and image live pathogens using an optical trap coupled to a spinning disk microscope. The optical trap provides spatial and temporal control of organisms and places them adjacent to host cells. Fluorescence microscopy captures dynamic intercellular interactions with minimal perturbation to cells.

Dynamic live cell imaging allows direct visualization of real-time interactions between cells of the immune system1, 2; however, the lack of spatial and ...

Use of the EpiAirway Model for Characterizing Long-term Host-pathogen Interactions

Nontypeable Haemophilus influenzae (NTHi) are human-adapted Gram-negative bacteria that can cause recurrent and chronic infections of the respiratory mucosa 1; 2. To study the mechanisms by which these organisms survive on and inside respiratory tissues, a model in which successful long-term co-culture of bacteria and human cells can be performed is required. We use primary human respiratory epithelial tissues raised to the air-liquid interface, the EpiAirway model (MatTek, Ashland, MA). These are non-immortalized, well-differentiated, 3-dimensional tissues that contain tight junctions, ciliated and nonciliated cells, goblet cells that produce mucin, and retain the ability to produce cytokines in response to infection. 
This biologically relevant in vitro model of the human upper airway can be used in a number of ways; the overall goal of this method is to perform long-term co-culture of EpiAirway tissues with NTHi and quantitate cell-associated and internalized bacteria over time. As well, mucin production and the cytokine profile of the infected co-cultures can be determined. This approach improves upon existing methods in that many current protocols use submerged monolayer or Transwell cultures of human cells, which are not capable of supporting bacterial infections over extended periods3. For example, if an organism can replicate in the overlying media, this can result in unacceptable levels of cytotoxicity and loss of host cells, arresting the experiment. The EpiAirway model allows characterization of long-term host-pathogen interactions. Further, since the source for the EpiAirway is normal human tracheo-bronchial cells rather than an immortalized line, each is an excellent representation of actual human upper respiratory tract tissue, both in structure and in function4.
For this method, the EpiAirway tissues are weaned off of anti-microbial and anti-fungal compounds for 2 days prior to delivery, and all procedures are performed under antibiotic-free conditions. This necessitates special considerations, since both bacteria and primary human tissues are used in the same biosafety cabinet, and are co-cultured for extended periods.

This method allows characterization of extended bacterial co-culture with EpiAirways, primary human respiratory epithelial tissue grown at the air-liquid interface, a biologically relevant in vitro model. The approach can be used with any microbe that is amenable to long-term co-culture.

Nontypeable Haemophilus influenzae  (NTHi) are human-adapted Gram-negative bacteria that can cause recurrent and chronic infections of the respiratory ...

Fluorescence Microscopy Methods for Determining the Viability of Bacteria in Association with Mammalian Cells

Central to the field of bacterial pathogenesis is the ability to define if and how microbes survive after exposure to eukaryotic cells. Current protocols to address these questions include colony count assays, gentamicin protection assays, and electron microscopy. Colony count and gentamicin protection assays only assess the viability of the entire bacterial population and are unable to determine individual bacterial viability. Electron microscopy can be used to determine the viability of individual bacteria and provide information regarding their localization in host cells. However, bacteria often display a range of electron densities, making assessment of viability difficult. This article outlines protocols for the use of fluorescent dyes that reveal the viability of individual bacteria inside and associated with host cells. These assays were developed originally to assess survival of Neisseria gonorrhoeae in primary human neutrophils, but should be applicable to any bacterium-host cell interaction. These protocols combine membrane-permeable fluorescent dyes (SYTO9 and 4&#39;,6-diamidino-2-phenylindole [DAPI]), which stain all bacteria, with membrane-impermeable fluorescent dyes (propidium iodide and SYTOX Green), which are only accessible to nonviable bacteria. Prior to eukaryotic cell permeabilization, an antibody or fluorescent reagent is added to identify extracellular bacteria. Thus these assays discriminate the viability of bacteria adherent to and inside eukaryotic cells. A protocol is also provided for using the viability dyes in combination with fluorescent antibodies to eukaryotic cell markers, in order to determine the subcellular localization of individual bacteria. The bacterial viability dyes discussed in this article are a sensitive complement and/or alternative to traditional microbiology techniques to evaluate the viability of individual bacteria and provide information regarding where bacteria survive in host cells.

Central to the field of bacterial pathogenesis is the ability to define if and how microbes survive after exposure to eukaryotic cells. This article outlines protocols for the use of fluorescent dyes that reveal the viability of individual bacteria inside and associated with host cells.

Central to the field of bacterial pathogenesis is the ability to define if and how microbes survive after exposure to eukaryotic cells. Current protocols ...

Porphyromonas gingivalis as a Model Organism for Assessing Interaction of Anaerobic Bacteria with Host Cells

Anaerobic bacteria far outnumber aerobes in many human niches such as the gut, mouth, and vagina. Furthermore, anaerobic infections are common and frequently of indigenous origin. The ability of some anaerobic pathogens to invade human cells gives them adaptive measures to escape innate immunity as well as to modulate host cell behavior. However, ensuring that the anaerobic bacteria are live during experimental investigation of the events may pose challenges. Porphyromonas gingivalis, a Gram-negative anaerobe, is capable of invading a variety of eukaryotic non-phagocytic cells. This article outlines how to successfully culture and assess the ability of P. gingivalis to invade human umbilical vein endothelial cells (HUVECs). Two protocols were developed: one to measure bacteria that can successfully invade and survive within the host, and the other to visualize bacteria interacting with host cells. These techniques necessitate the use of an anaerobic chamber to supply P. gingivalis with an anaerobic environment for optimal growth.
The first protocol is based on the antibiotic protection assay, which is largely used to study the invasion of host cells by bacteria. However, the antibiotic protection assay is limited; only intracellular bacteria that are culturable following antibiotic treatment and host cell lysis are measured. To assess all bacteria interacting with host cells, both live and dead, we developed a protocol that uses fluorescent microscopy to examine host-pathogen interaction. Bacteria are fluorescently labeled with 2&#39;,7&#39;-Bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethyl ester (BCECF-AM) and used to infect eukaryotic cells under anaerobic conditions. Following fixing with paraformaldehyde and permeabilization with 0.2% Triton X-100, host cells are labeled with TRITC phalloidin and DAPI to label the cell cytoskeleton and nucleus, respectively. Multiple images taken at different focal points (Z-stack) are obtained for temporal-spatial visualization of bacteria. Methods used in this study can be applied to any cultivable anaerobe and any eukaryotic cell type.

Porphyromonas gingivalis as a Model Organism for Assessing Interaction of Anaerobic Bacteria with Host Cells

This article presents two protocols: one to measure anaerobic bacteria that can successfully invade and survive within the host, and the other to visualize anaerobic bacteria interacting with host cells. This study can be applied to any cultivable anaerobe and any eukaryotic cell type.

Anaerobic bacteria far outnumber aerobes in many human niches such as the gut, mouth, and vagina. Furthermore, anaerobic infections are common and ...

The Use of Flow Cytometry to Assess the State of Chromatin in T Cells

During a proper immune response, quiescent T cells become activated upon antigen presentation to their antigen-specific T cell receptor. This leads to clonal proliferation of only those T cells that bear a receptor that recognizes the antigen. Chromatin decondensation is a hallmark of T cell activation and is required for T cells to acquire the ability to proliferate after antigen engagement. This change in chromatin condensation can be detected using antibodies raised against histone proteins. These antibodies cannot bind to their epitopes in na&#239;ve T cells as well as they can in activated T cells. We describe how to simultaneously stain T cell-specific surface markers, track viability with a fixable dead cell stain, and measure chromatin status via intracellular staining of Histone H3 proteins. Stained cells are analyzed by flow cytometry and chromatin condensation status is measured as the mean fluorescence intensity (MFI) of the Histone H3 stain. Chromatin decondensation during T cell activation is demonstrated as an increase in the MFI

Flow cytometry can be utilized to assess the state of chromatin within T cells. This protocol allows scientists to interpret evidence of chromatin decondensation during T cell activation demonstrated by an increase in the mean florescence intensity (MFI) of fluorescent Histone H3 antibodies

During a proper immune response, quiescent T cells become activated upon antigen presentation to their antigen-specific T cell receptor. This leads to ...

Multicolor Flow Cytometry-based Quantification of Mitochondria and Lysosomes in T Cells

T cells utilize different metabolic programs to match their functional needs during differentiation and proliferation. Mitochondria are crucial cellular components responsible for supplying cell energy; however, excess mitochondria also produce reactive oxygen species (ROS) that could cause cell death. Therefore, the number of mitochondria must constantly be adjusted to fit the needs of the cells. This dynamic regulation is achieved in part through the function of lysosomes that remove surplus/damaged organelles and macromolecules. Hence, cellular mitochondrial and lysosomal contents are key indicators to evaluate the metabolic adjustment of cells. With the development of probes for organelles, well-characterized lysosome or mitochondria-specific dyes have become available in various formats to label cellular lysosomes and mitochondria. Multicolor flow cytometry is a common tool to profile cell phenotypes, and has the capability to be integrated with other assays. Here, we present a detailed protocol of how to combine organelle-specific dyes with surface markers staining to measure the amount of lysosomes and mitochondria in different T cell populations on a flow cytometer.

This article illustrates a powerful method to quantify mitochondria or lysosomes in living cells. The combination of lysosome- or mitochondria-specific dyes with fluorescently conjugated antibodies against surface markers allows the quantification of these organelles in mixed cell populations, like primary cells harvested from tissue samples, by using multicolor flow cytometry.

T cells utilize different metabolic programs to match their functional needs during differentiation and proliferation. Mitochondria are crucial cellular ...

An Advanced Murine Model for Nonalcoholic Steatohepatitis in Association with Type 2 Diabetes

Obesity is associated with chronic low-grade inflammation and insulin resistance, contributing to an increasing prevalence of chronic metabolic diseases, such as type 2 diabetes and nonalcoholic steatohepatitis (NASH). Recent research has established that pro-inflammatory immune cells infiltrate obese hypertrophic adipose tissue and liver. Given the emerging importance of immune cells in the context of metabolic homeostasis, there is a critical need to quantify and characterize their modification during the development of type 2 diabetes and NASH. However, animal models that induce pathophysiological features typical of human NASH are sparse.
In this article, we provide a detailed protocol to identify immune cell subsets isolated from liver and adipose tissue in a reliable mouse model of NASH, established by housing high-fat diet (HFD) mice under non-specific pathogen-free (SPF) conditions without a barrier for at least seven weeks. We demonstrate the handling of mice in non-SPF conditions, digestion of the tissues and identification of macrophages, natural killer (NK) cells, dendritic cells, B and T cell subsets by flow cytometry. Representative flow cytometry plots from SPF HFD mice and non-SPF mice are provided. To obtain reliable and interpretable data, the use of antibodies, accurate and precise methods for tissue digestion and proper gating in flow cytometry experiments are critical elements.
The intervention to restore physiological antigen exposure in mice by housing them in non-SPF conditions and unspecific exposure to microbial antigens could provide a relevant tool for investigating the link between immunological alterations, diet-induced obesity and related long term complications.

A simple and reliable diet-induced rodent animal model for nonalcoholic steatohepatitis (NASH) is described, achieved through non-SPF housing of the animals and administration of a specific high-fat diet. We describe identification of hepatic and adipose immune cell subsets to recapitulate human immunological conditions by exposing mice to environmental germs.

Obesity is associated with chronic low-grade inflammation and insulin resistance, contributing to an increasing prevalence of chronic metabolic diseases, ...

Preparation of Whole Bone Marrow for Mass Cytometry Analysis of Neutrophil-lineage Cells

In this article, we present a protocol that is optimized to preserve neutrophil-lineage cells in fresh BM for whole BM CyTOF analysis. We utilized a myeloid-biased 39-antibody CyTOF panel to evaluate the hematopoietic system with a focus on the neutrophil-lineage cells by using this protocol. The CyTOF result was analyzed with an open-resource dimensional reduction algorithm, viSNE, and the data was presented to demonstrate the outcome of this protocol. We have discovered new neutrophil-lineage cell populations based on this protocol. This protocol of fresh whole BM preparation may be used for 1), CyTOF analysis to discover unidentified cell populations from whole BM, 2), investigating whole BM defects for patients with blood disorders such as leukemia, 3), assisting optimization of fluorescence-activated flow cytometry protocols that utilize fresh whole BM.

Here, we present a protocol to process fresh bone marrow (BM) isolated from mouse or human for high-dimensional mass cytometry (Cytometry by Time-Of-Flight, CyTOF) analysis of neutrophil-lineage cells.

In this article, we present a protocol that is optimized to preserve neutrophil-lineage cells in fresh BM for whole BM CyTOF analysis. We utilized a ...

Double Labeling Immunofluorescence using Antibodies from the Same Species to Study Host-Pathogen Interactions

Nowadays, it is possible to find a wide range of molecular tools available to study parasite-host cell interactions. However, some limitations exist to obtain commercial monoclonal or polyclonal antibodies that recognize specific cell structures and proteins in parasites. Besides, there are few commercial antibodies available to label trypanosomatids. Usually, polyclonal antibodies against parasites are prepared in-house and could be more challenging to use in combination with other antibodies produced in the same species. Here, the protocol demonstrates how to use polyclonal and monoclonal antibodies raised in the same species to perform double labeling immunofluorescence to study host cell and pathogen interactions. To achieve the double labeling immunofluorescence, it is crucial to incubate first the mouse polyclonal antibody and then follow the incubation with the secondary mouse IgG antibody conjugated to any fluorochrome. After that, an additional blocking step is necessary to prevent any trace of the primary antibody from being recognized by the next secondary antibody. Then, a mouse monoclonal antibody and its specific IgG subclass secondary antibody conjugated to a different fluorochrome are added to the sample at the appropriate times. Additionally, it is possible to perform triple labeling immunofluorescence using a third antibody raised in a different species. Also, structures such as nuclei and actin can be stained subsequently with their specific compounds or labels. Thus, these approaches presented here can be adjusted for any cell whose sources of primary antibodies are limited.

Here, the protocol describes how to perform double labeling immunofluorescence using primary antibodies raised in the same species to study host-pathogen interactions. Also, it can include the third antibody from a different host in this protocol. This approach can be made in any cell type and pathogens.

Nowadays, it is possible to find a wide range of molecular tools available to study parasite-host cell interactions. However, some limitations exist to ...

Analysis of SAMHD1 Restriction by Flow Cytometry in Human Myeloid U937 Cells

Sterile &#945;-motif/histidine-aspartate domain-containing protein 1 (SAMHD1) inhibits replication of HIV-1 in quiescent myeloid cells. U937 cells are widely used as a convenient cell system for analyzing SAMHD1 activity due to a low level of SAMHD1 RNA expression, leading to undetectable endogenous protein expression. Based on similar assays developed in the Stoye laboratory to characterize other retroviral restriction factors, the Bishop lab developed a two-color restriction assay to analyze SAMHD1 in U937 cells. Murine Leukaemia Virus-like particles expressing SAMHD1, alongside YFP expressed from an IRES, are used to transduce U937 cells. Cells are then treated with phorbol myristate acetate to induce differentiation to a quiescent phenotype. Following differentiation, cells are infected with HIV-1 virus-like particles expressing a fluorescent reporter. After 48 h, cells are harvested and analyzed by flow cytometry. The proportion of HIV-infected cells in the SAMHD1-expressing population is compared to that in internal control cells lacking SAMHD1. This comparison reveals a restriction ratio. SAMHD1 expression leads to a five-fold reduction in HIV infection, corresponding to a restriction ratio of 0.2. Our recent substitution of RFP for the original GFP as the reporter gene for HIV infection has facilitated flow cytometry analysis.
This assay has been successfully used to characterize the effect of amino acid substitutions on SAMHD1 restriction by transducing with viruses encoding altered SAMHD1 proteins, derived from site-directed mutagenesis of the expression vector. For example, the catalytic site substitutions HD206-7AA show a restriction phenotype of 1, indicating a loss of restriction activity. Equally, the susceptibility of different tester viruses can be determined. The assay can be further adapted to incorporate the effect of differentiation status, metabolic status, and SAMHD1 modifiers to better understand the relationship between SAMHD1, cell metabolic state, and viral restriction.

Described here is an established method to determine the extent of HIV-1 restriction by the cellular inhibitory protein SAMHD1. Human myeloid lineage U937 cells are transduced with a SAMHD1 expression vector co-expressing YFP, differentiated and then challenged with HIV-RFP. The level of restriction is determined by flow cytometry analysis.

Sterile &#945;-motif/histidine-aspartate domain-containing protein 1 (SAMHD1) inhibits replication of HIV-1 in quiescent myeloid cells. U937 cells are ...

Use of Image Cytometry for Quantification of Pathogenic Fungi in Association with Host Cells

Summary

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Chapters in this video

Use of an Optical Trap for Study of Host-Pathogen Interactions for Dynamic Live Cell Imaging

Use of the EpiAirway Model for Characterizing Long-term Host-pathogen Interactions

Fluorescence Microscopy Methods for Determining the Viability of Bacteria in Association with Mammalian Cells

Porphyromonas gingivalis as a Model Organism for Assessing Interaction of Anaerobic Bacteria with Host Cells

The Use of Flow Cytometry to Assess the State of Chromatin in T Cells

Multicolor Flow Cytometry-based Quantification of Mitochondria and Lysosomes in T Cells

An Advanced Murine Model for Nonalcoholic Steatohepatitis in Association with Type 2 Diabetes

Preparation of Whole Bone Marrow for Mass Cytometry Analysis of Neutrophil-lineage Cells

Double Labeling Immunofluorescence using Antibodies from the Same Species to Study Host-Pathogen Interactions

Analysis of SAMHD1 Restriction by Flow Cytometry in Human Myeloid U937 Cells