Enumeration, viability and heterogeneity in Staphylococcus aureus cultures by flow cytometry

Several fluorochromes (rhodamine 123, bis-oxonol, propidium iodide, SYTO-13 and calcein) were tested by flow cytometry for their ability to determine cell density, viability and heterogeneity in Staphylococcus aureus cultures exposed to heating (60–70–80°C for 2 min), formaldehyde 2% for 20 min and gramicidin-S at 2–5–10 μg/ml for 20 min. Results were validated by viable plate count and counts performed with a particle analyser. Flow cytometry gave quicker results and more accurate information about intermediate states and heterogeneity of S. aureus cultures than viable plate counts. Rhodamine 123 and oxonol were found to be efficient dyes for the assessment of bacterial viability. SYTO-13 was an excellent marker for total counts and calcein can be used to assess metabolic activity.