Gel microdroplet technique leaving microorganisms alive for sorting by flow cytometry

Fluorescent labels used to disclose cellular function and the like are generally needed for cytometric analysis, but suitable ones are not always available. Thiamin, an example of such a label, was used as a model in this study. Thiamin in cells of bakers' yeast can be chemically converted to thiochrome, which fluoresces strongly, but the reaction kills cells not protected inside a gel microdroplet (GMD). Our new procedure for preparation of a small amount of GMDs uses a glass filter with micropores uniform in size to make an emulsion of molten agar in mineral oil that is then chilled for gelation of the agarose. Cells were suspended and mixed with the warm agarose, which was used to make GMDs. Cells were grown into microcolonies in the GMDs, which were then treated to convert thiamin to thiochrome. The thiochrome in microcolonies in single GMDs could be detected by flow cytometry. Plating showed that enough cells survived for this method to be used for screening. This technique can be used for all applications of GMDs.