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The study of the staphylococcal population heterogeneity with respect to methicillin resistance by 2 methods revealed different numbers of the resistant cells in the population. Thus, when the microbial suspension was plated on an agarized medium with methicillin (50 gamma/ml), only 0.0007--0.0005 per cent of the resistant cells were found. When the colonies were replicated from a medium without methicillin to a medium containing methicillin (50 gamma/ml), 84.3--97.3 per cent of the resistant microbial cells were found in the population of the same strains. The main mechanism in the heterogeneity of the staphylococcal population with respect to methicillin resistance was impairement of the phenotype manifestation of the antibiotic resistance under definite conditions.

The dynamics of staphylococci sensitivity to antibiotics in purulent foci of 242 in patients was studied. Two criteria were used for estimation of the above process, i.e. (I) sensitivity to the antibiotics of the whole staphylococcal population isolated and (2) sensitivity to the antibiotics of separate strains isolated from the population. In 10 or more days the number of the in-patients with staphylococci resistant to streptomycin, monomycin or oxacillin in the purulent foci increased. The number of the in-patients with penicillin, neomycin or chloramphenicol resistant staphylocci was statistically doubtful. The average number of the antibiotic resistant microbial cells in the staphylococcal populations of the foci in the patients during their stay in the hospital increased 2-4 times.

The effect of the incubation temperature, i.e. 37 and 30 degrees and the effect of addition of 5 per cent of sodium chloride to the nutrient medium was studied with respect to the results of sensitivity determination of staphylococci to methicillin by 3 methods, such as serial dilutions in liquid and solid media and replica method applied to separate colonies. With the use of all 3 methods incubation of the samples at a temperature of 30 degrees increased the rate of staphylococcal cultures resistant to methicillin. Addition of sodium chloride to the solid nutrient medium increased the level of detecting methicillin resistant cultures in the samples incubated at a temperature of 37 degrees and decreased it in the samples incubated at 30 degrees.

Two methods were compared for determination of drug resistant staphylococci on the nasal mucosa of patients, i. e. the routine method for determination of staphylococcal sensitivity to antibiotics and the method of direct plating out of the starting material onto agarized media containing antibiotics. Staphylococci resistant to streptomycin, tetracycline, chloramphenicol, erythromycin, monomycin, axacillin and less frequent to penicillin were found more often with the 2nd method. A method of proportions was developed for testing sensitivity of staphylococci in purulent inflammatory foci of the patients. It provided characterization of the staphylococcal population from the foci by the number of the antibiotic resistant microbial cells.

The dynamics of sensitivity to penicillin of staphylococcal populations in purulent inflammatory foci of patients treated and not treated with antibiotics was estimated according to 4 indices. No reliable differences in the dynamics of sensitivity to penicillin were found in 2 groups of the patients, when estimation was performed with respect to the frequency of the penicillin resistant or penicillin sensitive staphylococci and detection of the penicillin resistant staphylococci by direct inoculation of the focal excretion to the medium with penicillin. A reliable increase in the percentage of the penicillin resistant staphylococci in the microbial population was observed only in the patients treated with penicillin.

The literature and experimental data on stability of resistance ot benzylpenicillin, streptomycin, tetracycline, chloramphenicol, erythromycin, kanamycin and methicillin in staphylococci stored for prolonged periods of time on artificial nutrient media were systematized. When the staphylococcal cultures were not in contact iwth the antibiotics for 5 to 18 months, sensitive cells to separate drugs were detected in 51.0--67.4 per cent of the strains. The number of such cells in the populations of some strains ranged from 1 to 100.0 per cent. The effect of the resistance multiplicity on the stability of resistance to benzylpenicillin, tetracycline, chloramphenicol and erythromycin was not observed.

A method for analysis of antibiotic sensitivity restoration in staphylococcal populations in humans is described. The mechanism of streptomycin sensitivity restoration in staphylococcal populations on the nasal mucosa of 59 patients 7 months after discharging from surgical stationary was studied. Quantitative estimates for loss of superinfecting streptomycin-resistant staphylococci, initial streptomycin-resistant staphylococci and resistance determinants by the initial streptomycin-resistant staphylococci are presented.

Transmissivity of drug resistance in wild strains of E. coli was shown to be determined not only by the presence of the transmissive factor and r-determinants in the microbial cell, but also by the functional activity of the R-factor. The capacity of the R-factor for transmission depended to a certain extent on the number of the r-determinants in the R-factor and the presence of various r-determinants in the R-factor.

Antistaphylococcal bacteriostatic and bactericidal activity was detected in the preparations of human leucocytic interferon of the native type. Such an activity could not be neutralized by the interferon antibodies. However, it was usually lost during the process of interferon purification. It is concluded that this antistaphylococcal activity is not related to interferon and is due to other factors which can be isolated with induced leucocytes.

The experiments showed that among 110 Salmonella strains resistant to 1 or more antibiotics 54.5 per cent of the cultures transfered their resistance to E. coli when grown in vitro. The frequency of the transmissive drug resistance in Salmonella was directly proportional to the number of r-determinants in R-factor. The cultures resistant to ampicillin and kanamycin proved to be the most competent donors.