Activity of Three Aminoglycosides and Two Penicillins Against Four Species of Gram-Negative Bacilli

Three aminoglycoside antibiotics and two penicillins were compared for their in vitro activity against 60 isolates of Serratia, Pseudomonas, Proteus mirabilis , and indole-positive Proteus sp. Testing was done by the agar dilution method using Mueller-Hinton broth solidified with 1.5% agar. The activity of amikacin, aminodeoxybutirosin, and gentamicin against Proteus and Pseudomonas , as related to their peak blood levels, showed no significant differences. Amikacin was the most active against Serratia marcescens . Results using Mueller-Hinton media in broth dilution tests correlated with the agar dilution method except for Pseudomonas aeruginosa . The minimal inhibitory concentration for aminoglycosides in agar was considerably greater than the minimal inhibitory concentration in Mueller-Hinton broth, and the disparity was related to the higher divalent cation concentration of agar. BL-P1654 and carbenicillin were similar except that carbenicillin was much more active against indole-positive Proteus sp. Additionally, the ratio of bactericidal to bacteriostatic concentrations of BL-P1654 was considerably greater than for carbenicillin.