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A total of 4801 Salmonella strains of 67 serovars isolated from various sources in 1968-1982 were studied for their sensitivity to 12 drugs. The Salmonella strains of groups A, B, C, D and E amounted to 0.8, 84.6, 6.4, 5.8 and 2.0 per cent, respectively. The Salmonella strains of the other groups amounted to 0.4 per cent. During the 5-year periods, the number of the antibiotic resistant Salmonella strains increased from 57.7 to 93.4 per cent. The conjugative drug resistance was detected in 16 out of the 67 serovars. The Salmonella strains usually resistant to 4-10 drugs were the donors of this conjugative drug resistance. The strains of S. typhimurium responsible for the hospital infections were characterized by the highest levels of the antibiotic polyresistance and the greatest variety of the conjugative R plasmids. The structure of the multiple and conjugative drug resistance of the organisms causing salmonellosis was more frequently defined by the conjugative plasmids of the antibiotic resistance of the S. typhimurium serovar. The Salmonella R plasmids determined various levels of the antibiotic resistance. The highest resistance levels were observed to benzylpenicillin, chloramphenicol and erythromycin and the lowest resistance levels to tetracycline.

The pathogenicity level of antibiotic sensitive and streptomycin resistant strains of S. typhimurium, S. paratyphi B and S. kottbus changed under the effect of identical R plasmids more frequently in contrary directions. The conjugative plasmids of antibiotic resistance widened the ranges of the virulence changes in the Salmonella serovars for albino mice. It was found that 7 out of 8 plasmids studied significantly decreased and increased the virulence of the antibiotic sensitive Salmonella strains. As a rule, R plasmids of various origin decreased the virulence of all the tested streptomycin chromosome resistant causative agents of salmonellosis.