Medium for the presumptive identification of Aeromonas hydrophila and Enterobacteriaceae

Abstract

A medium was devised for the rapid presumptive identification of Aeromonas hydrophila. It also offered good differentiation of Klebsiella, Proteus, and other enteric species. Mannitol fermentation, inositol fermentation, ornithine decarboxylation and deamination, indole production, motility, and H2S production from sodium thiosulfate and cysteine could be recorded in a single tube of the medium. The medium contains (in grams per liter): proteose peptone (Difco Laboratories, Detroit, Mich.), 5; yeast extract (Difco), 3; tryptone (Difco), 10; L-ornithine hydrochloride, 5; mannitol, 1; inositol, 10; sodium thiosulfate, 0.4; ferric ammonium citrate, 0.5; bromocresol purple, 0.02; agar, 3. To prepare the medium, one must thoroughly mix the ingredients in 1 liter of distilled water and adjust the pH to 6.7. The medium is heated to a boil, dispensed in 5-ml quantities in tubes (13 by 100 mm), and autoclaved at 121°C for 12 min. Colonies of Aeromonas and enteric bacteria can be picked directly from isolation plates or membrane filters and inoculated into the medium by stabbing to the base of the tube with a straight needle. The inoculated tubes are incubated at 35°C for 18 to 24 h, after which reactions are recorded. For the detection of indole production, 3 to 4 drops of Kovacs reagent are added to each tube.