In vitro antibacterial activity of imipenem in combination with colistin against multidrug-resistant Pseudomonas aeruginosa

Abstract

The purpose of this study was to determine the in vitro activity of imipenem in combination with colistin against 30 strains of P.aeruginosa. Nineteen strains (63.33%) were found to be multidrug-resistant strains (MDR) and 8 strains (26.67%) of these organisms were imipenem-resistant with MIC = 16 µg/ml while all strains were susceptible to colistin (MIC range were 0.5-4 µg/ml). Metallo-β-lactamase production could not be detected in all strains. The combination of imipenem and colistin showed synergistic activity against 20 strains (66.67%), partial synergy in 3 strains (10%), and additive in 7 strains (23.33%) by checkerboard method.In the time-kill study, using 8 strains of imipenem-resistant P.aeruginosa, it was shown that 99.9% killing were observed at 2nd hour, when exposed to 1xMIC of imipenem, and increased to 5 strains at 8th hour. The regrowth were observed in 6 strains(75%) at the 24th hour. When the concentration of imipenem was increased to 2xMIC, the number of strains which were 99.9% killing were increased and the regrowth were observed in only 2 strains at the 24th hour. The 0.25xMIC of colistin alone showed only bacteriostatic activity (99% killing) but when the concentration of colistin was increased to 0.5xMIC, the 99.9% killing was observed in 1 strain during 2nd-8th hour. However, all strains regrew at the 24th hour. Whereas, the combination of imipenem 1xMIC with colistin 0.5xMIC could show 99.9% killing in 6 strains (75%) without regrowth while in the combination of 1xMIC of imipenem and 0.25xMIC colistin, bacterial regrowth occurred in 2 strains (25%) at the 24th hour. There were no significantly difference (p>0.05) among the BA24 of the both combination (1xMIC of imipenem+0.5xMIC of colistin and 1xMIC of imipenem+0.25MIC of colistin), while BA24 of the single drug (colistin 0.25xMIC and 0.5xMIC) were significantly difference (p<0.05) and also significantly difference from BA24 of imipenem alone or BA24 of the both combination. Colistin 1xMIC, 2xMIC,4xMIC and 8xMIC showed the 99.9% killing in 2, 4, 6 and 7 strains, respectively. At 24 hours, the regrowth was observed when using colistin < 4xMIC. There no significantly difference (p>0.05) was observed among the BA24 from the different concentrations of colistin. Moreover, the morphology change of P. aeruginosa strain no.10 after was exposed to the combination of imipenem and colistin. The roughly spherical surface and bacterial lysis could be clearly seen under the electron microscope The results suggested that antibacterial activity of imipenem 1xMIC and colistin 0.5xMIC showed the best activity, whereas colistin monotherapy might need the very high concentration (8xMIC) of drug. Bactericidal activity (99.9% killing) of this combination was observed during the 24th hours of study without any regrowth. Therefore, the combination of imipenem and with lower dose of colistin can be the alternative treatment of infections caused by MDR P. aeruginosa. The lower dose of colistin could lead to the decrease in nephrotoxicity.