In vitro antimicrobial activities of double and triple combinations of Meropenem, Ciprofloxacin and Colistin against meropenem-resistant Pesudomonas Aeruginosa

Abstract

Determination of in vitro antimicrobial activities of double and triple combinations of meropenem, ciprofloxacin and colistin against 30 clinical isolates of P. aeruginosa found that twenty-six isolates (86.67%) were multidrug-resistant isolates (MDR) and all isolates were meropenem-resistant (MIC90= 16 µg/mL). Only 4 isolates (13.33%) were susceptible to ciprofloxacin (MIC90=128 µg/mL). Whereas all isolates were susceptible to colistin (MIC90= 2 µg/mL). The results from checkerboard method have shown that the combination of meropenem with ciprofloxacin had synergistic effect against only 9 isolates (30%) at concentrations ranged from 4-16 µg/mL and 0.25-64 µg/mL, respectively. For combination of meropenem and colistin, it had synergistic effect against 11 isolates the same as ciprofloxacin combined with colistin. Synergistic effect of meropenem and colistin was observed at concentrations ranged from 1-8 µg/mL and 0.5-1 µg/mL, respectively, whereas such effect was observed at ciprofloxacin and colistin concentrations ranged from 0.06-32 µg/mL and 0.5-1 µg/mL, respectively. In addition, triple combination of these agents showed synergistic effect against 18 isolates (60%) at meropenem, ciprofloxacin and colistin concentrations ranged from 1-16 µg/mL, 0.06-64 µg/mL and 0.06-1 µg/mL, respectively. In time-kill study, 5 isolates of meropenem-resistant P. aeruginosa were tested. When exposed to meropenem 16 and 25 µg/mL alone, 99.9% killing was observed in 2 isolates at 8th hour with meropenem 25 µg/mL and then the regrowth occurred at 24th hour. For ciprofloxacin 4 µg/mL alone, only 1 isolate was killed at level of 99% killing at 2nd hour and the regrowth was observed at 24th hour. Whereas, 0.5xMIC colistin alone, 99.9% killing was observed in all isolates at 2nd hour. However, such effect was observed in only 4 isolates at 8th hour then the regrowth of all isolates occurred at the 24th hour. The combinations of meropenem (16 or 25 µg/mL) and ciprofloxacin seem not to increase antimicrobial activity, only 1 isolate was killed at level of 99.9% killing. On the contrary, the combinations of meropenem (16 or 25 µg/mL) with 0.5xMIC colistin, 99.9% killing was observed in all isolates at 2nd-24th hour. The same results were observed in triple combinations. In addition, ciprofloxacin in combination with colistin showed the same results as colistin alone. BA24 value of 0.5xMIC colistin alone showed significantly different from control (p<0.05). BA24 values of meropenem (16 or 25 µg/mL) in combination with 4 µg/mL ciprofloxacin were not significantly different from meropenem alone (p>0.05) the same as ciprofloxacin in combination with colistin compared to colistin alone. Whereas meropenem (16 or 25 µg/mL) combined with 0.5xMIC colistin were significantly different from meropenem at each concentration alone and colistin alone as well (p<0.05). Moreover, there were not significantly different from triple combinations (p>0.05). The morphology change of such P. aeruginosa exposed to meropenem combined with colistin occurred with alteration of bacterial membrane resulted in cell break and cell debris when compared to control. The results have shown that antibacterial activity of meropenem (16 or 25 µg/mL) combined with 0.5xMIC colistin were most effective. Therefore, the combination of meropemem with low dose of colistin could be the promising alternative treatment for infections caused by meropenem-resistant P. aeruginosa. The lower dose of colistin could also lead to the decreased nephrotoxicity in patients.