Effect of pH on the activity of erythromycin against 500 isolates of gram-negative bacilli.

Erythromycin was found to be a more effective inhibitor of gram-negative bacilli in alkaline medium than in neutral or acid medium. A definite effect was noted with all of 500 recent clinical isolates of Escherichia coli, Klebsiella-Enterobacter, Pseudomonas aeruginosa, and Proteus mirabilis studied, but it was most striking with E. coli. At pH 8.5 all strains, except for 14% of those of P. mirabilis, were inhibited by concentrations of erythromycin readily achieved in urine with common therapeutic doses.