Antimicrobic diffusion susceptibility tests on a defined medium solidified with a synthetic polymer (neutra-gel)

A synthetic polymer (Neutra-Gel) has been developed to serve as a substitute for agar. Since the polymer is neutral and inert, it is ideally suited for in vitro studies of antimicrobial agents. Diffusion susceptibility tests were performed on three types of media: a synthetic amino acid medium (SAAM) with agar, SAAM with the synthetic hydrogel, and Mueller-Hinton agar. Ten different antimicrobics were tested, each giving somewhat different results. Varying results were also obtained with 171 different clinical isolates. BecauseStaphylococcus aureus grew poorly on SAAM, diffusion tests were not entirely statisfactory. With the enteric bacilli, the defined synthetic medium is advantageous especially when testing drugs which interact with agar, i.e., polymyxins and aminoglycosides.