Failure of Polymyxin B Nonapeptide to Augment Bactericidal Activities of Novobiocin, Rifampin, and of Defibrinated Human Blood against <i>Serratia marcescens</i>

Abstract

Polymyxin B (PB) and polymyxin B nonapeptide (PBNP), when combined with rifampin or novobiocin, but not vancomycin, yielded additive inhibitory effects against test strains of Serratia marcescens of three varieties: those that produced cocarde growths around PB disks (coc⁺); those that grew adjacent to PB disks (coc^-, 6); and those that yielded clear inhibition zones around PB disks (coc^-, clear). However, time kill curve experiments disclosed that only the combination of rifampin + PB exerted a potent bactericidal effect against coc⁺ strains of S. marcescens; rifampin + PBNP and novobiocin + PB or PBNP merely effected transient decreases of colony counts. Assays involving 50% (v/v) of fresh defibrinated human blood + PB or PBNP revealed that only PB clearly augmented the antibacterial activity of blood against coc⁺, and less so against coc^- test strains of S. marcescens.

Agents that increase the permeability of the outer membrane.

“…Notably, they are also resistant to numerous other cationic agents (see below). Other polymyxinand PMBN-resistant gram-negative bacteria include neisseriae and Serratia marcescens (190,218,221).…”