No abstract

Staphylococcus aureus and Pseudomonas aeruginosa are often found in succession or in association in infections. To study experimentally their interactions, we have used systems of growth or survival of mixed cultures of both species in vitro in a semisynthetic medium and in vivo in the peritoneal cavity of mice. Conditions for maximum growth in vitro of both species in mixed cultures are about similar to those in pure culture when the pH is maintained between 6.0 and 7.3. The inhibition of S. aureus growth by some antimetabolites or antibiotics, e.g., 5-methyltryptophan and D-cycloserine, is antagonized in mixed cultures. Staphylococcus coagulase, DNase, and alpha toxin were present either in mixed cultures or after mixing pure culture supernatants of both species, but P. aeruginosa slime was not observed in these conditions. In vivo, the survival of S. aureus seemed greater in mixed infections with P. aeruginosa than in those with S. aureus alone. In our systems, S. aureus may have benefited from the presence of P. aeruginosa whereas the reverse was not observed. These observations on interbacterial ecology could help to explain the importance and the behavior of some species at the initiation of pyogenic infections, either their interactions or their selection.

The distribution of the resident oral bacterial species was evaluated in six genetically different mouse strains (BALB/c, C3H/He, C57BL/6, C57BL/10, DBAj2 and CD-1). The results indicate that these mouse strains harbour different oral microbiota upon arrival at our animal facilities. However, this difference persists only in strains C57BL/10 and DBAj2 1 wk after arrival. Except for strains CD-1 and C57BL/10, the oral biota is different upon arrival compared to that observed after 1 wk at our animal facilities. For the same mouse strain, the oral biota can differ significantly from one shipment to another upon arrival but tends to stabilise after 1 wk. This stabilising effect can be related to the increase in the proportion of Lactobacillus murinus among the total cultivable oral microbiota. These results indicate that if the host genetic background influences the distribution of the oral bacterial populations, its effect is masked by environmental factors.

We have studied aspects of interbacterial ecology with nutritionally dependent Staphylococcus aureus strains; they were grown in association with Pseudomonas aeruginosa in systems of mixed cultures and infections in vitro in a semisynthetic medium and in vivo in mouse peritoneal cavity and rabbit skin. In mixed cultures and in P. aeruginosa culture filtrates, thymine and tryptophan deficiencies in staphylococci were partly overcome. This is probably because P. aeruginosa supplied the essential metabolites required to ensure growth; however, other metabolic activities could also be involved. Other experiments showed that the sensitivity of thymineless staphylococci to nucleoside inhibitions was alleviated. In mixed infections with P. aeruginosa, the S. aureus thymineless strain has shown a greater ability to survive in the peritoneal cavity of mice than when injected alone, even when one species was injected after the other with different doses of bacteria. The examination of the liquid from the peritoneal cavity of infected mice by fluorescence microscopy after fluorochroming with acridine orange or auramine O has revealed that Pseudomonas endotoxin seems to damage leucocytes and consequently reduces the phagocytosis of Staphylococcus cells.Necrosis in rabbit skin was mainly due to S. aureus when both species were injected together intradermally; the thymineless strain was less harmful than the parent strain.It seems that survival and even growth of nutritionally dependent strains of a bacterial species can be favored by the metabolic activities of another species in mixed cultures and infections, in this instance S. aureus by P. aeruginosa. This phenomenon among others could be a determinant of bacterial pathogenicity for nutritionally dependent pathogenic bacteria; thus associated organisms could determine the effective pathogenicity of nutritionally dependent bacteria by contributing essential nutrilites at the site where infection is initiated.

Gonococcal strains of different nutritional types did not adhere to the same extent to rabbit mesentery in vitro. Prototrophic strains generally showed much more affinity than strains requiring arginine (or ornithine), hypoxanthine, and uracil (AHU− or OHU−) for growth. Rabbit mesentery may be useful in the comparative study of the adherence properties of prototrophic and AHU− or OHU− auxotrophic strains.