Summary. Insertion mutagenesis with the help of the plasmid pFS23 was used to generate Yersinia pestis fra mutants. The results of pFra-strain production under non-selective conditions suggested that such Y . pestis variants may be generated in natural plague foci at high frequency and may participate in supporting the epizootic process. Present data suggest that the reduction of virulence in Fra-strains reported by the majority of investigators was connected with the use of Y. pestis variants carrying additional unidentified mutations. It was shown that the loss of the ability to produce capsular antigen (FI) alone or in combination with absence of murine toxin production did not lead to an increase in LD50 absolute values. Simultaneous loss of these two virulence determinants did not influence the duration of survival of the infected animals. However, absence of only FI antigen production in the infecting strain resulted in prolonged survival of the infected animals. Conversion of plague infection from acute to chronic form is probably dependent on the animal host species and the Y. pestis parent strain subjected to mutagenesis.

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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.

Within the frames of an analytical review, modern approaches are described whose development is considered to be very important in order to rationally operate secure working procedures at the facilities of medical and biological profile. Peculiarities of formulating the methods to assess biological hazards are analyzed together with the ways of their objectivization. The place and the role of biological safety as a scientific trend are discussed in relation with the problem of availability of practical application of the "risk theory" elements with the aim of making governing decisions based on complex, well established estimations. The architecture of the system of modeling and estimation of potential danger associated with handling pathogenic biological agents is enunciated.