IN-VITRO STEPWISE SELECTION OF RESISTANCE TO QUINOLONES, BETA-LACTAMSAND AMIKACIN IN NOSOCOMIAL GRAM-NEGATIVE BACILLI

The ability of six antibiotics to produce resistance by stepwise selec tion on agar medium was assessed in 24 gram-negative rods. Escherichia coli was the strain least prone to selection of resistance, whereas P seudomonas aeruginosa frequently developed resistance to all antibioti cs. When used alone, ciprofloxacin, pefloxacin, amikacin, ceftazidime and cefpirome were associated with a comparable risk of acquired resis tance (in 14 to 17 out of 24 strains); imipenem selected resistant str ains in 10/24 isolates (5/18 in non-pseudomonas strains). The number o f strains exhibiting cross resistance with structurally unrelated anti biotics was 11 after pefloxacin treatment, eight after exposure to cip rofloxacin, six after ceftazidime, and one after imipenem or cefpirome . The combination of ciprofloxacin with amikacin was less efficient in reducing acquisition of resistance than the combination of ciprofloxa cin with a beta-lactam: ciprofloxacin plus cefpirome was especially po tent in this respect.