Microbial interactions in Staphylococcus aureus–Candida albicans dual-species biofilms is a relevant research topic given the significant contribution of these microorganisms to hospital-acquired infections. Therefore, the purpose of our investigation was to study the interaction of opportunistic C. albicans and S. aureus in vivo and in vitro, both with the participation of normal microflora and in mice with antibacterial dysbiosis. The study of mentioned interactions was carried out on 100 white male mice weighing approximately 18 grams in vivo and using smears prepared from the grown mixed cultures of C. albicans and S. aureus and the Japan JEM 1400 transmission electron microscope for the purpose of electron microscopic study of microorganisms in vitro. Healthy mice forming control groups and mice with antibiotic-induced dysbiosis (after introduction of vancomycin, gentamicin, ampicillin) were divided into groups to create a mono- and associative infection: Ι group was given 1×107 CFU of C. albicans, II group – 1×108 CFU of S. aureus, and III group – a mixture of specified concentrations of C. albicans and S. aureus in the same proportion. Microorganisms causing monoinfection were being isolated from the body of animals treated with antibiotics till the end of the experiments in large quantities unlike in case of the healthy mice. Co-inoculation of these microbes in the same dose to animals (co-infection), which were injected with antibiotics, turned out to be fatal for them, whereas an adhesive bond was seen between the cells of C. albicans vs. S. aureus in vitro. As can be seen, such bacterial-fungal co-infection reduce substantially the effectiveness of antibiotic therapy and the likelihood of successful treatment and can not be ignored when choosing the appropriate treatment.