[Diminution of the antibacterial activity of antibiotics in cultures and in experimental mixed infections].

We have studied interactions between Staphylococcus aureus and Pseudomonas aeruginosa in the absence and the presence of four different antimicrobials in mixed cultures and experimental infections. These two bacterial species, in addition to having different properties, are known to be opportunistic pathogens often present in human microflora. Two main aspects have been investigated and they are related to modifications in two species affecting their equilibrium in the mixed bacterial population and also their pathogenicity markers. Our results indicate that individual growth of S. aureus and P. aeruginosa is not modified in vitro in mixed cultures in the absence of antimicrobials; in vivo, in mouse peritoneal cavity, there is a synergism favorable to S. aureus. In the presence of rifamycin SV and three cell wall inhibitors, pencillin G,D-cycloserine, and vancomycin, we have observed that P. aeruginosa protected S. aureus against the inhibitory effect of these antimicrobials in vitro and in vivo. Such results were obtained in different conditions of culture, stationary, shaken, and in special apparatuses, an "Ecologen" and a "Chemostat." When any one of the antimicrobials was allowed to be in contact for 6 to 8 h with P. aeruginosa cells in a culture, we observed a decrease in their inhibitory effects against S. aureus. These results were supported by microscopical observation. It seems that the inhibitory effects of the antimicrobials have hindered the formation of toxic products of S. aureus, e.g., alpha toxin, and that it was not restored in the presence of P. aeruginosa. Conversely, P. aeruginosa remained apparently unchanged through all these experiments. Our observations may imply that the inhibitory effect of an antimicrobial towards a bacterial species may be significantly decreased in the presence of another species, sometimes present in human microflora.