STUDIES ON MUTATIONAL CROSS-RESISTANCE BETWEEN CIPROFLOXACIN, NOVOBIOCIN AND COUMERMYCIN IN ESCHERICHIA-COLI AND STAPHYLOCOCCUS-WARNERI

Nalidixic acid resistant mutants of Escherichia coli KL16 were tested against ciprofloxacin, coumermycin and novobiocin. The mutants gyrA, n alB and nal-24 were more resistant than KL16 to ciprofloxacin, whereas the nal-31 strain was hypersensitive. Only the nalB mutant was more r esistant to novobiocin than KL1 6, but gyrA, nal-31 and nal-24 mutants were more sensitive to coumermycin than KL16. Newly-isolated novobioc in-resistant mutants of KL16 were not cross-resistant to coumermycin o r ciprofloxacin. Some coumermycin-resistant mutants were cross-resista nt to novobiocin but not ciprofloxacin, whereas mutants resistant to n ovobiocin and ciprofloxacin were isolated at higher coumermycin concen trations. Two types of Staphylococcus warneri mutant were isolated on media containing novobiocin or coumermycin. Each was resistant to both coumarins, but one was highly resistant to novobiocin and the other t o coumermycin. High level resistance to both coumarins was unstable. E . coli mutants differed in susceptibility to bactericidal concentratio ns of ciprofloxacin, and S. warneri mutants behaved similarly. These r esults suggest the modes of action of the coumarins are not identical.