RESUMO
BACKGROUND: The aim of the present was to study the inhibitory and bactericide activity and risk of selection of mutants resistant or tolerant to vancomycin in strains of Staphylococcus aureus resistant to methicillin from out patient and hospitalary patients. MATERIALS AND METHODS: The minimum inhibitory concentration was obtained by the macrodilution method in liquid medium and by the Etest system. The minimum bactericide concentration was obtained by the macrodilution method in liquid medium. The selection of resistant or tolerant mutants was performed inoculating more than 5 x 10(6) UFC/ml in brain-heart agar plates and in tripteinsoya brought with different concentrations of vancomycin. RESULTS: Vancomycin showed greater inhibitory and bactericide activity in strains sensitive to methicillin of out patient and hospitalary origin with respect to strains resistant to methicillin with a minimum inhibitory concentration > 1 mg/l in 23.3, 20.0 and 70.0%, respectively and a minimum bactericide concentration of 90%, 8.16 and 64 mg/l. The incidence of tolerance was greater in the strains resistant to methicillin in comparison with the out patient and hospitalary strains sensitive to methicillin, since a minimum bactericide/inhibitory concentration greater than 4 was obtained in 63.3 in comparison with 20.0%. Only mutants with diminished sensitivity (minimum inhibitory concentration of 3 and 6 mg/l) or tolerant in strains resistant to methicillin could be selected. The increases in the minimum inhibitory concentrations were not stable, while the tolerant mutants preserved this characteristic after 20 subcultures. CONCLUSIONS: The in vitro selection of mutants with decreased sensitivity or tolerant was only obtained in strains resistant to methicillin. The changes in the minimum inhibitory concentrations were not stable in contrast to the increase in the minimum bactericide concentrations which remained unchanged. The differences in the selection of mutants with diminished sensitivity or tolerant among strains resistant and sensitive to methicillin found in this study were correlated with the differences in minimum inhibitory and bactericide concentrations and the relation between both in the clinical isolates.
Assuntos
Staphylococcus aureus/efeitos dos fármacos , Staphylococcus aureus/genética , Vancomicina/farmacologia , Resistência Microbiana a Medicamentos , Testes de Sensibilidade Microbiana , Mutação , Staphylococcus aureus/isolamento & purificaçãoRESUMO
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.