Studies on the additive effect of polymyxin B and the bactericidal activity of human serum against Serratia marcescens

Two of twelve examined S. marcescens strains were promptly killed by 80% (v/v) fresh human serum (within 20 min), analogous to a serum-sensitive control strain of Escherichia coli; ten strains, however, were killed by fresh serum only after extended incubation (2-4 h). The combination of therapeutically achievable concentrations of polymyxin B (range 5 to 1.25 mug/ml) and fresh, but not heat-inactivated human serum was found to exert an accelerated, additive effect against 9 of 10 'delayed serum-sensitive' isolates of S. marcescens, an organism that is characterized by intrinsic resistance against polymyxins. The combination of 80% (v/v) fresh, defibrinated human blood and polymyxin B likewise resulted in an additive effect. Polymyxin B treatment of S. marcescens strains caused a prompt, marked, though reversible bile salt susceptibility of the cells; in contrast, the effect induced by fresh serum was slight and not apparent until several hours after exposure.