Characterization of cefsulodin-resistant variants of Pseudomonas aeruginosa

Fifty clinical strains of Pseudomonas aeruginosa were grown in the presence of cefsulodin and 96% acquired resistance to the antibiotic after 8 serial passages. Cross-resistance was observed between cefsulodin, carbenicillin and ticarcillin and growth of the strains on carbenicillin also induced resistance to the other two antibiotics. The resistant progeny fell into two categories, the first included strains which were indistinguishable phenotypically from the original strain and the second group were atypical in some biochemical tests confirmatory for Ps. aeruginosa and were not typable by serology and phage-typing. Neither category reverted to cefsulodin susceptibility after repeated subcultures on antibiotic-free media over a period of 6 months.