Amoxicillin: in Vitro Susceptibility of "Blood Culture Strains" of Gram-Negative Bacilli and Comparisons with Penicillin G, Ampicillin, and Carbenicillin
One hundred sixty-eight strains of nine species of gram-negative bacilli isolated from patients hospitalized at Boston City Hospital in 1972 were tested for susceptibility to amoxicillin, penicillin G, ampicillin, and carbenicillin. The tests were done on heart infusion agar containing twofold dilutions of the antibiotics; a 10−3 dilution of each culture grown overnight in brain heart infusion broth was applied with the replica inoculator. By this method the activity of amoxicillin most resembled that of ampicillin; the former appeared to be slightly more active against strains of Proteus mirabilis and Serratia marcescens and somewhat less active against those of Klebsiella pneumoniae and Enterobacter. Strains that were highly resistant (MIC > 400 µg/ml) to one were similarly resistant to the other. The MICs for all the susceptible strains were greater when an undiluted culture was used as inoculum, the difference varying with the organism and the antibiotic; the smallest inoculum effect was exhibited by amoxicillin against Escherichia coli, and the largest was that of all four penicillins against the strains of P. mirabilis. Strains of Pseudomonas aeruginosa were susceptible only to carbenicillin.