[Bactericidal activity and capacity for selection of mutants resistant to imipenmen and meropenem in strains of Proteus, Morganella and Providencia].

BACKGROUND

To compare the bactericidal activity and frequency of mutation for meropenem and imipenem against Proteus mirabilis, Morganella morganii and Providencia rettgeri.

MATERIAL AND METHODS

Minimum inhibitory concentrations (MIC) were determined by agar dilution method. Bactericidal activities were evaluated by killing curves method employing 4 and 16x MIC. One-step resistant mutant selection was performed by spreading more than 5 x 10(8) UFC/ml on cystine-lactose-electrolyte deficient agar containing 4 times the MIC of meropenem or imipenem.

RESULTS

MIC were 8 to 16 times lower for meropenem. After 24 h, bactericidal activity was observed for meropenem at 4 and 16 x MIC against 76.7 and 100% of the strains in comparison to 26.7 and 83.3% with imipenem. After 24 h incubation with imipenem, re-growth occurred in 80 and 90% of P. mirabilis and M. morganii strains, respectively. Imipenem resistant mutants were selected from 3 strains of P. mirabilis. One of them was stable and MIC of meropenem and imipenem were 8 to 16-fold higher.

CONCLUSIONS

From the laboratory point of view we consider that meropenem is more active against Proteeae because it was more potent in terms of inhibitory and bactericidal activity. In addition the risk to select for resistant mutants was significant with imipenem and P. mirabilis.