Starch-Ampicillin Agar for the Quantitative Detection of Aeromonas hydrophila

Interest in Aeromonas hydrophila as a food-borne and human pathogen is increasing. Isolation media from the clinical laboratory were evaluated for food use and either did not give quantitative recovery of A. hydrophila or did not permit ready differentiation of A. hydrophila from the background microflora. A new medium was developed which permitted quantitative recovery of A. hydrophila from foods. The medium consisted of phenol red agar base (Difco Laboratories), soluble starch (10 g/liter), and ampicillin (10 mg/liter). All foods surveyed contained A. hydrophila. Foods sampled included red meats, chicken, raw milk, and seafood (fish, shrimp, scallops, crab, and oysters). The count of A. hydrophila at the time of purchase ranged from 1 x 10/g (lower limit of detection) to 5 x 10/g. In most instances, the count of A. hydrophila increased during 1 week of storage at 5 degrees C. The starch-ampicillin agar developed permitted rapid quantitative recovery of A. hydrophila from foods in the presence of very large numbers of competing microflora.