Human leukocytes containing less than 2C DNA per cell (damaged or dead cells) were detected and quantified by flow cytometry and DNA-specific staining with ethidium bromide and mithramycin in whole blood infected with Staphylococcus aureus or Yersinia pestis. Addition of live S. aureus to the blood (100 microbe cells per one leukocyte) resulted in rapid degradation of leukocyte DNA within 3 to 6 hours of incubation at 37 °C. However, only about 50 percent cells were damaged and the leukocytes with the intact genetic apparatus could be found in the blood for a period up to 24 hours. The leukocyte injury was preceded by an increase of DNA per cell content (as compared to the normal one) that was likely to be connected with the active phagocytosis of S.aureus by granulocytes (2C DNA of diploid phagocytes plus the all bacterial DNA absorbed). In response to the same dose of actively growing (at 37 °C) virulent Y.pestis cells, no increase in DNA content per cell could be observed in the human blood leukocytes. The process of the leukocyte DNA degradation started after a 6-hours' incubation, and between 18 to 24 hours of incubation about 90 percent leukocytes (phagocytes and lymphocytes) lost their specific DNA fluorescence.These results demonstrated a high potential of flow cytometry in comparative analysis in vitro of the leukocyte DNA degradation process in human blood in response to bacteria with various pathogenic properties. They agree with the modern idea of an apoptotic mechanism of immunosuppression in plague.

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