Abstract
The ability of three quinolones, two β-lactams, and one aminoglycoside to select resistant mutants was examined in tests with 30 isolates of commonly encountered nosocomial pathogens. Ciprofloxacin and norfloxacin, two new quinolone derivatives, were no more likely to select resistant mutants than amikacin, whereas nalidixic acid, an older quinolone derivative, was the most likely of the six drugs examined to select resistant mutants. Mutational frequencies of 10-7 to 10-8 were observed in most instances. In general, the mutants were 8 to 16 times less susceptible to the drug used for selection. Although most quinolone-selected mutants were cross-resistant only to other drugs within this class, certain mutants of Klebsiella pneumoniae selected by nalidixic acid, ciprofloxacin, or norfloxacin were also less susceptible to β-lactam antibiotics. This unusual pattern of multiple drug resistance was associated with changes in outer membrane proteins of the organism. Multiple drug resistance was also observed in β-lactam-selected mutants of Enterobacter cloacae and Pseudomonas aeruginosa (β-lactams), amikacin-selected mutants of Providencia stuartii and P. aeruginosa (aminoglycosides), and β-lactam- or amikacin-selected mutants of Serratia marcescens (β-lactams plus aminoglycosides). These results underscore the need to examine carefully the frequency with which resistance to any new antibiotic develops as well as the patterns of multiple drug resistance which may occur simultaneously.