ABSTRACT
Combinations of tobramycin (TOB) with cephalothin (CET) or carbenicillin (CBPC) were evaluated by in vitro test against 161 clinical isolates of Klebsiella pneumoniae, Escherichia coli, indole-positive Proteus, Enterobacter and Serratia marcescens which were not inhibited by 6.25 microgram/ml of CET. The combinations were considered to show synergy when there was a 4-fold or greater reduction in MIC values (FIC index less than or equal to 0.5) of both antibiotics when combined. Synergy of TOB with CET could be demonstrated against 83% of Klebsiella pneumoniae, 78% of E. coli, 44% of Proteus rettgeri and 39% of Proteus inconstans. Synergy of TOB with CBPC could be demonstrated against 78% of Proteus vulgalis, 45% of Serratia marcescens and 28% of Proteus inconstans. Bactericidal effect showing synergy of TOB with CET could be demonstrated against each 2 strains of Klebsiella pneumoniae and E. coli. Frequency of synergy of TOB with CET under the condition of large inoculum size was significantly higher than that of synergy of TOB with CET under the condition of small inoculum size. Synergy of bactericidal effect of TOB combined with CET against Klebsiella pneumoniae and E. coli was more active when they were combined at the same time, than those of when TOB was combined after 2 hours exposure by CET or when CET was combined after 2 hours exposure by TOB. No antagonistic action was observed in these studies.
