A broth-disc technique for the assay of antibiotic synergism

Abstract

A broth-disc technique has been developed to demonstrate the antibiotic synergism of some broad-spectrum penicillins plus an aminoglycoside against Pseudomonas aeruginosa and enterococci. Utilizing the zone sizes obtained from Kirby–Bauer susceptibility tests, approximate minimum inhibitory concentrations of each antibiotic upon individual isolates were derived using standard regression curves. One-fourth of each value was calculated and the approximate desired concentration then prepared in Mueller–Hinton broth through use of the labelled antibiotic disc content. Tubes were inoculated with a standardized suspension of organism and incubated at 35 °C for 18–24 h. Subsequently, inhibitory end points were recorded.This approach to synergy testing showed good correlation with the standard checkerboard method. The broth-disc synergistic assay is also reproducible, financially feasible, and less time-consuming to execute.