Changes in the pathogenicity of opportunistic enterobacteriaceae are currently acquiring more and more medico-biological significance. The rates and scale of transformations of symbiont bacteria do not fit into the norm of the phenotype reaction to changes in environmental conditions. All this determines the intensification of studies of the phenomenon of pathogenicity at the molecular genetic level. This paper presents the results of a molecular genetic analysis of the interactions of E. faecalis in association with Blastocystis spp. in vitro. It was found that the nucleotide sequences of genes that determine the synthesis of pathogenicity factors are found in the studied enterococci in different ratios, and the frequency of detection of fragments of the desired genes changes after their cocultivation with blastocysts isolated from the feces of gastroenterological patients. The simplest Blastocystis spp. caused an increase in the frequency of occurrence of the gene gelE (gelatinase) in enterococci after their joint cultivation, which is evidence of their influence on the ability to realize the pathogenic potential of associative symbionts