Thesis (M.Sc.)--Chulalongkorn University, 2007

The purpose of present study is to determine the in vitro antibacterial activity of imipenem in combination with colistin against 30 strains of multi-drug resistant A. baumannii. Thirty A. baumannii strains that were found to be multi-drug resistant strains and all strains were resistant to imipenem (MIC range 8-128 µg/ml) but susceptible to colistin (MIC range 0.5-2 µg/ml). The metallo-β-lactamase activity could not be detected in all strains. Checkerboard method served to determine the activity of imipenem in combination with colistin. Combination of the imipenem with colistin showed synergistic effect against 27 strains (90%) and partial synergistic effect against 3 strains (10%) of A. baumannii strains tested. In the time kill study using 15 A. baumannii strains resulted in synergistic effect. Imipenem 32µg/ml, the concentration was equal to the mean serum level of imipenem at the therapeutic dose showed bactericidal activity (99.9% killing) in 1 strain (6.67%) at 8 hours of growth and 1 strain (6.67%) at 24 hours of growth, 1/16MIC of colistin alone showed no antibacterial activity against all strains tested, whereas 1/4MIC of colistin alone showed bacteriostatic activity (90.0% killing) in 1 to 2 strains at 4, 6 and 8 hours of growth. However, the regrowth was observed in 7 (46.67%), 15 (100%) and 15 (100%) strains at 24 hours by imipenem alone, 1/16MIC and 1/4MIC of colistin alone, respectively. The combination of imipenem (32µg/ml) plus 1/16MIC of colistin showed bactericidal activity against 2 strains (13.33%) at 8 hours of growth and 4 strains (26.67%) at 24 hours of growth. The combination of imipenem (32µg/ml) plus 1/4MIC of colistin showed bactericidal activity against 1 strain (6.67%) at 2 hours of growth and against 10 strains (66.67%) at 24 hours of growth. In addition, the combination of imipenem (32µg/ml) plus 1/16MIC and 1/4MIC of colistin showed regrowth against 4 strains (26.67%) and 2 strains (13.33%) at 24 hours of growth, respectively. The amount of bacteria killed by the combination of imipenem plus 1/16 MIC of colistin [BA24 = 158.12 log CFU/ml.h] were not significantly higher than those killed by imipenem alone [BA24 = 127.66 log CFU/ml.h] while the amount of bacteria killed by the combination of imipenem plus 1/4MIC of colistin [BA24 = 193.72 log CFU/ml.h] were significantly higher than those kill by imipenem alone and the combination of imipenem plus 1/16MIC of colistin. In addition, the bactericidal effect was observed by morphological akteration and cell wall destruction after exposure to combination of imipenem (32µg/ml) plus colistin (1/4 MIC) more than exposure to imipenem alone (32µg/ml) and colistin alone (1/4 MIC). The results obtained suggested that antibacterial activity of the combination of imipenem and colistin were higher than antibacterial activity of single drug alone. It is concluded that the combination of imipenem and colistin could be promising alternative in the treatment of infection due to multi-drug resistant A. baumannii.