Moxalactam therapy of infections caused by cephalothin-resistant bacteria: influence of serum inhibitory activity on clinical response and acquisition of antibiotic resistance during therapy.

Thirty patients infected predominantly by Serratia marcescens and Pseudomonas aeruginosa were treated in an open trial with moxalactam, a broad-spectrum beta-lactam antibiotic. Twenty-three (76%) had a satisfactory microbiological or clinical response. Among 25 patients for whom serum inhibitory concentrations were measured, those with favorable microbiological responses had significantly higher values than those with poor responses (reciprocal geometric mean concentrations, 49 versus 4.9; P less than 0.01). A serum inhibitory concentration of greater than 1:8 correlated significantly with a favorable outcome (17 of 18 versus 2 of 7 responses; P less than 0.01). Although the overall clinical efficacy of moxalactam was good, resistant organisms of species identical to those of the original infecting isolates were recovered during therapy in seven cases, including five caused by Pseudomonas organisms and two caused by Serratia organisms.