Effect of Salt Concentration on the Apparent In-Vitro Susceptibility of Pseudomonas and Other Gram-Negative Bacilli to Gentamicin

Abstract

A strain of Pseudomonas aeruginosa isolated from blood, an R factor-containing strain of Serratia marcescens, Escherichia coli K12, and a streptomycin-resistant mutant of E. coli K12 were all much more susceptible to gentamicin in nutrient broth than in Mueller-Hinton, glucose-phosphate, or trypticase soy broths. Addition of large amounts of NaCI or moderate amounts of MgCl2 to nutrient broth raised the MICs of gentamicin of all four strains to levels similar to those in the other broths. Successive addition of small increments of a divalent cation to all broths progressively increased MICs of gentamicin to levels at which Pseudomonas is considered resistant, but not Serratia or E. coli. Variations in concentrations of salts may increase or decrease the susceptibility of Pseudomonas and other bacilli to gentamicin in vivo, as they do in vitro.