Susceptibility of Pseudomonas aeruginosa to serum bactericidal activity. A comparison of three methods with clinical correlations.

Twenty-nine blood culture isolates of Pseudomonas aeruginosa were tested by three established methods to determine the effect of in vitro conditions on the survival of this organism in human serum. Clinical correlations were made to determine the relationship of serum resistance as defined by each method to clinical outcome. Major differences of bacterial survival in the presence of pooled normal human serum and in classification of isolates (sensitive, intermediate, resistant) were observed among the three methods. Isolates grown in broth for preparation of inocula demonstrated significantly greater sensitivity to serum bactericidal activity than those grown on agar. The use of organisms in early logarithmic growth phase or increased concentrations of serum augmented the serum sensitivity of these isolates. No correlation was observed between serum bactericidal activity and antibiotic susceptibility, pyocine type, patient mortality, or underlying disease. All strains of serotype 6 or 11 (immunotype 1 or 2) were serum-sensitive by one of the three methods. This study indicates that by testing isolates of P. aeruginosa under a variety of in vitro conditions, it is possible to identify a few isolates that are highly sensitive or resistant to serum under all conditions. The survival of the majority of strains of P. aeruginosa in human serum is highly dependent on in vitro conditions. Conclusions regarding the role of serum bactericidal activity in host defense must be drawn cautiously when based solely on in vitro tests.