According to electron microscopy the repeated contacts of Aerococcus viridans 167 (industrial strain for the probiotic Aerobact production) with Salmonella typhimurium and Staphylococcus aureus caused the deep changes of cellular structure, down to bacteriolysis. Remaining viable pathogenic cells gave posterity with changed biochemical properties and virulence. S. typhimurium completely lost typical biochemical properties, its mobility and ability to be agglutinated by O-specific Salmonella serum reduced after the fifth joint passage. Sharp decrease of the leucotoxic and loss of the lethal and necrotic activity of the S. aureus toxin were observed. The latter was characterized by the smaller number of protein fractions in comparison with control series on electrophoregrams and densitograms. Pathogenic bacteria, playing a role in formation of the pathological intestinal microbiocenosis (S. typhimurium) and being hospital infections (S. aureus) acquired increased sensitivity to antibiotics. The carried out researches open some mechanisms of bactericidal and bacteriostatic action of A. viridans (group 12, Bergey, 1994) and their metabolic products, causing antagonistic effect during direct action on infectious microorganisms.