[Characteristics of the increased resistance of mice to Pseudomonas aeruginosa caused by Staphylococcus aureus].

Mice were protected against a lethal dose of Pseudomonas aeruginosa by a previous sublethal infection with Staphylococcus aureus. Under similar conditions, Staphylococcus epidermidis afforded only slight protection. The following characteristics of this increased resistance that we have observed are (1) survival of mice, and decrease of the number of viable units of P. aeruginosa in the peritoneal cavity of the same mice infected with S. aureus coagulase-positive strains, either from our laboratory collection or from hospital cases, when the time interval between the injection with staphylococci and that of P. aeruginosa was between 4 and 96 h; (2) absence of a net inhibitory effect in vitro on P. aeruginosa with serum from mice infected with a sublethal dose of coagulase-positive S. aureus; (3) changes in the appearance of peritoneal exudate cells after infection with a sublethal dose of S. aureus; P. aeruginosa injected afterwards in the peritoneal cavity of mice was eliminated; when P. aeruginosa was injected alone, "activated macrophages" were not observed and bacterial cells were present in large numbers in the exudate. The immunostimulation induced by a previous sublethal injection of S. aureus coagulase-positive strains seemed to be inhibited by the immunosuppressive drug cyclophosphamide, since mice were no longer protected against a lethal dose of P. aeruginosa. Cell immunity may intervene in such infections with opportunistic species and check the invasiveness of a gram-negative bacterium superinfecting a host already exposed to coagulase-positive S. aureus.