Determination of antimicrobial MIC by paper diffusion method

Because they are cumbersome, tests to determine the quantitative susceptibility of organisms to antimicrobial drugs are not performed routinely in many diagnostic laboratories. This paper describes a simple method of incorporating the antimicrobial drug in agar. It is an adaptation of the Rolinson and Russell technique which allows the determination of minimum inhibitory concentrations (MIC) of antimicrobial drugs for a large number of organisms. Results are comparable with those obtained when the standard agar dilution method is used. Strains of aerobic Gram-negative bacilli were tested by both methods using ampicillin (86 strains), cephaloridine (72 strains), trimethoprim (72 strains), and gentamicin (72 strains). Of the 302 tests thus performed, a difference in MIC of more than one double dilution was noted in only 11 tests. With one strain of Pseudomonas aeruginosa, however, it was not possible to detect ampicillin resistance by the method described in this paper.