In vitro antimicrobial activities of Meropenem, Colistin, and Sulbactum in double and triple combinations against multidrug-resistant Acinetobacter Baumannii

Abstract

Acinetobacter baumannii has emerged as a major cause of nosocomial opportunistic infections in immunocompromised patients, particularly in the intensive care unit (ICU). It is well known as a multidrug-resistant (MDR) pathogen which is resistant to most commonly available antimicrobial agents. Non-traditional antimicrobial agents such as colistin and sulbactam have been studied for the treatment of infections caused by MDR A. baumannii. However, there is no data available on the activity of meropenem, colistin, and sulbactam in double and triple combinations against clinical isolates of MDR A. baumannii collected in Thailand. Therefore, the objective of this study is to determine the in vitro antimicrobial activities of meropenem, colistin, and sulbactam in double and triple combinations against 30 clinical isolates of MDR A. baumannii. All isolates were considered to be MDR isolates due to the resistance of 3 to 5 broad-spectrum agents from the disk diffusion test. From the agar dilution method, all isolates were resistant to meropenem (MICs range = 64–256 mug/ml) but were susceptible to colistin (MICs range = 0.5-2 mug/ml) while the MICs of sulbactam range from 4-64 mug/ml. From the results of combination effects studied by checkerboard method, the synergistic effects of the double combination of meropenem with sulbactam, meropenem with colistin, and sulbactam with colistin were observed in 70%, 73.33%, and 53.33% of all isolates of MDR A. baumannii, respectively, whereas the triple combination showed synergistic effects against 96.67% of the isolates. In the time kill study, 10 MDR A. baumannii isolates were tested. After given meropenem 50 mug/ml, the bactericidal activity (99.9% killing or >3 log CFU/ml decreased) was not observed at any time. For 30 µg/ml sulbactam, 99.9% killing was observed in 2 isolates at the 8[superscript th]hour. Whereas, in colistin 0.5 mug/ml, 99.9% killing was observed at the 2[superscript nd] to 8[superscript th] hour. However, the regrowth was shown at the 24[superscript th] hour when the agent was given alone. In the combination of meropenem and sulbactam, bactericidal was observed at the 4t[superscript th] to 24t[superscript th] hour but the regrowth was also found in 8 isolates (80%) at 24[superscript th] hour, bacteriolytic area for 24 hours (BA[subscript 24]) was significantly different from meropenem alone but not different from sulbactam alone. When meropenem combined with colistin, 99.9% killing could be observed since the 2[superscript nd] to 24[superscript th] hour, and BA[subscript 24] was significantly different from meropenem alone but not different from colistin alone. Similarly with the combination of sulbactam and colistin that 99.9% killing could be observed since the 2[superscript nd]to 24[superscript th] hour but BA[subscript 24] was not different from sulbactam and colistin alone and the regrowth at 24[superscript th] hour was shown more in the latter combination (4 and 6 isolates, respectively). In the triple combination of meropenem, sulbactam, and colistin, 99.9% killing could also be observed since the 2[superscript md] to 24[superscript th] hour. The regrowth was found at 24[superscript th] hour in only 1 isolate. The BA[subscript 24of the triple combination was significantly different from meropenem and sulbactam alone but not different from colistin alone. There were no statistical difference in BA[subscript 24] between the double combinations and the triple combination and the combination of meropenem and colistin was as effective as the triple combination. However, the regrowth at 24[superscript th] hour occurred less in the triple combination accompanied with the synergistic effects shown more in the triple combination. Therefore, the triple combination of meropenem, colistin, and sulbactam could be the promising alternative for the treatment of infections due to MDR A. baumannii