The effect of the combination of erythromycin with new beta-lactam antibiotics against gram-negative aerobic respiratory pathogens.

The effect of erythromycin on the in vitro activity of cefotaxime, ceftizoxime, cefoperazone, moxalactam, ceftazidime, cefmenoxime, aztreonam, imipenem, piperacillin, and gentamicin against 89 bacteria isolated from sputum and tracheal aspirates of patients admitted to intensive care units was evaluated. There were 30 Pseudomonas aeruginosa, 27 Klebsiella pneumoniae, 9 Enterobacter cloacae, 5 Escherichia coli, 4 Enterobacter aerogenes, 3 each of Klebsiella oxytoca, Pseudomonas maltophilia, Serratia marcescens, 2 each of Morganella morganii, Acinetobacter anitratus; 1 Proteus mirabilis. All isolates were resistant to erythromycin. Organisms were screened for synergy by fixed erythromycin concentration at 1, 8 and 16 micrograms/ml. There were 16 isolates that showed a greater than four-fold difference in MICs. These strains were analyzed by the checkerboard broth method. No antagonism was seen for any drug combination with erythromycin. One Enterobacter cloacae showed synergy of ceftizoxime and erythromycin, and 1 E. cloacae showed synergy with cefotaxime. Addition was found for 2 P. aeruginosa, 1 each E. aerogenes, E. coli, P. mirabilis, K. pneumoniae and S. marcescens. At concentrations of erythromycin achievable in blood or pulmonary tissue, the activity of newer beta-lactams and gentamicin is not altered by erythromycin.