Effect of assay medium on the antibacterial activity of certain penicillins and cephalosporins.

It is well known that the composition of the assay medium greatly affects the antimicrobial activity of aminoglycoside antibiotics. A similar response has now been observed with certain penicillins and cephalosporins. In the case of these compounds, this effect is apparently governed by the chemical nature of the penicillin 6- and cephalosporin 7-side chains. In comparison with their activity in Nutrient Broth, the activity of some of the beta-lactam antibiotics that have a weakly basic or basic group in their side chain was reduced as much as 40-fold in one or more of the following media: Mueller-Hinton, Trypticase soy, antibiotic assay, and heart infusion broths. In contrast, the assay medium had no effect on the activity of those compounds possessing an acidic or a nonionizable function in their side chain. The extent to which medium influences the antibacterial activity was also dependent upon the assay method and the organism, the effect being more pronounced in broth dilution than in agar dilution tests and occurring more frequently with gram-negative than with gram-positive organisms.