Susceptibility of Recent Isolates of Pseudomonas aeruginosa to Gentamicin, Polymyxin, and Five Penicillins, with Observations on the Pyocin and Immunotypes of the Strains

The susceptibilities of recently isolated strains of Pseudomonas aeruginosa to gentamicin, polymyxin B, carbenicillin, ampicillin, penicillin G, and two newer penicillins were tested with the inocula-replicating technique by using undiluted and 10(-3) dilutions of the cultures. With either inoculum, polymyxin B was the most active agent, and a comparison with previous data from this laboratory showed that the susceptibility of P. aeruginosa to this antibiotic had not changed over the past 20 years. Gentamicin was nearly as active as polymyxin, all but 2 of the 141 strains tested with the diluted inoculum being inhibited by 6.25 mug/ml or less. AB-2288, an agent resembling carbenicillin, was four times more active than carbenicillin or BLP-1654; the last two were equally active against the 10(-3) inoculum. A more marked inoculum effect was noted with the penicillin analogues tested, the increase in minimum inhibiting concentration with the undiluted culture being eight-fold for carbenicillin and at least 16-fold for AB-2288 and BLP-1654. Pyocin typing and serotyping failed to demonstrate any clearly predominating types.