title: Studies on the additive effect of polymyxin B and the bactericidal activity of human serum against Serratia marcescens.
abstract: 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.
mesh_terms: D000818:Animals; D001647:Bile Acids and Salts / Q000494:pharmacology; D001770:Blood Bactericidal Activity / Q000187:drug effects; D003470:Culture Media; D003864:Depression, Chemical; D004352:Drug Resistance, Microbial; D004926:Escherichia coli / Q000187:drug effects; D006386:Hemagglutination Tests; D006358:Hot Temperature; D006801:Humans; D008826:Microbial Sensitivity Tests; D011113:Polymyxins / Q000494:pharmacology*; D012706:Serratia marcescens / Q000187:drug effects; D012756:Sheep; D013997:Time Factors
chemical_list: D001647:Bile Acids and Salts; D003470:Culture Media; D011113:Polymyxins