A Rapid, simple method for demonstrating synergy of amikacin and penicillin against various microorganisms

A simple disk diffusion technique with use of triple-layer agar and enzymatic inactivation of penicillin demonstrated synergy between penicillins and aminoglycosides. The method was applied to measurement of the synergistic activity of amikacin and penicillin G against a strain of Streptococcus faecalis sensitive to both drugs, amikacin and ampicillin against a strain of S. faecalis sensitive to amplicillin but resistant to amikacin, and amikacin and carbenicillin against a strain of Proteus mirabilis sensitive to both drugs. The degree of synergy was related to the specific organism tested. For example, syngery of penicillin G and amikacin for S. faecalis could be demonstrated only when the organism was subjected first to the action of penicillin G alone. In addition to providing useful clinical information, the triple-layer technique appears to be a promising tool for the study and understanding of synergy between antimicrobial agents.