Antimicrobial susceptibility of mucoid Pseudomonas aeruginosa and their spontaneously occurring non-mucoid derivatives

Fifteen strains of mucoid Pseudomonas aeruginosa and their respective spontaneously occurring isogenic non-mucoid derivatives were examined with respect to antimicrobial susceptibility to various antibiotics, including gentamicin, amikacin, kanamycin, carbenicillin, and rifampin. Except for kanamycin, mucoid strains were more susceptible to each antibiotic than the non-mucoid variant as measured by geometric mean minimum inhibitory concentrations. Nevertheless, several mucoid strains were more resistant than non-mucoid variants, suggesting a possible role in antibiotic resistance for the exopolysaccharide capsule. R-factor carriage tended to abolish any differences in MICs between mucoid and non-mucoid strains, thereby diminishing the effect of the capsule. We conclude that in most instances the emergence of mucoid isolates of Ps. aeruginosa probably does not result from differences in antimicrobial susceptibility between mucoid strains and spontaneously occurring non-mucoid variants.