EFFECTS OF ANTIBIOTIC COMBINATIONS AGAINST PSEUDOMONAS AERUGINOSA, KLEBSIELLA PNEUMONIAE AND SERRATIA MARCESCENS

Abstract

Effects of antibiotic combinations against Pseudomonas aeruginosa, Klebsiella pneumoniae and Serratia marcescens which are frequently found in hospitalized patients and resistant to multiple antibiotics were investigated.
By means of the plate checkerboard method, remarkable synergy was observed with the combination of gentamicin (GM) and piperacillin against Pseudomonas aeruginosa, and with amikacin (AMK) and sulfamethoxazole-trimethoprim (ST) against Serratia marcescens. Also synergy was observed with a combination of AMK and cefazolin (CEZ) against Klebsiella pneumoniae. These were confirmed from the distribution of the lower fractional inhibitory concentration indexes of each combination.
Within the concentrations achievable in usual dosage of antibiotics, the combination of GM and sulbenicillin or fosfomycin showed remarkable bactericidal effects when used against Pseudomonas aeruginosa, as investigated from killing curves. The same effects were obtained with a combination of AMK and ST against Serratia marcescens, and with GM and CEZ against Klebsiella pneumoniae.
Neither GM nor carbenicillin (CBPC) alone reduced the number of conglomerated Pseudomonas aeruginosa observed within a suture. However, a combination of both antibiotics was effective in reducing the organism from the suture within 4 to 6 days after the addition of the antibiotics. Moreover, the treatment of CBPC followed GM was much more effective in eliminating the organism from the suture than thc treatment of GM followed by CBPC.