In vitro activity of piperacillin, a new semisynthetic penicillin with an unusually broad spectrum of activity.

The in vitro activity of piperacillin (T-1220), a new semisynthetic derivative of aminobenzylpenicillin, was investigated. The majority of streptococci and pneumococci were inhibited by 0.12 micrograms/ml; the staphylococci and enterococci were inhibited by 2 micrograms/ml. Piperacillin was slightly more active against Neisseria and Haemophilus influenzae than was ampicillin. Piperacillin was active against all members of the Enterobacteriaceae including the Klebsiella, 58% of which were inhibited by 8 micrograms/ml. The activity of piperacillin was at least equivalent, but generally superior, to that of ampicillin or carbenicillin on species susceptible to these drugs. Most striking was its activity on Pseudomonas aeruginosa: 50% were inhibited by 2 micrograms/ml, and 83% were inhibited by 4 micrograms/ml. The minimum bactericidal concentrations were very close to the minimum inhibitory concentrations, and in most species only a slight inoculum effect was observed on the minimum bacterial values except for certain P. aeruginosa strains. A complete parallel resistance exists between piperacillin and ampicillin or carbenicillin. However, the clinical importance of this is largely mitigated by the intrinsically higher activity of piperacillin.