Rippey-Cabelli Agar Base

Principle And Interpretation

Aeromonas species are natural inhabitants of aquatic environments worldwide. Their populations are seasonal in all natural waters. Aeromonads cause serious diseases of aquatic animals and represent an economic threat to the aquaculture industry (1). The motile aeromonads have emerged as a serious microbial threat to human populations, especially the immunocompromised (2). Aeromonads can be enumerated in water samples by employing the membrane filter technique. Rippey-Cabelli (RC) Agar, formulated by Rippey and Cabelli (3) is used for this purpose. The medium is differential as it depends on the ability of organisms to ferment trehalose and selective due to the incorporation of selective agents.

Tryptose and yeast extract support the growth of Aeromonas species. Bromothymol blue is the pH indicator, which changes from blue to yellow colour under acidic conditions, created due to fermentation of trehalose. Sodium chloride maintains the osmotic equilibrium whereas potassium chloride, magnesium sulphate and ferric chloride provide essential ions. Ampicillin, sodium deoxycholate and ethanol are the selective agents inhibiting gram-positive bacteria, coliforms, Shigella species, Proteus mirabilis and Actinomyces. Ethanol inhibits overgrowth of Klebsiella species on the filter (4). Most of the Enterobacteriaceae ferment trehalose, therefore it is difficult to distinguish Aeromonas from Enterobacteriaceae. The medium gives higher specificity and sensitivity when pure cultures are used (4, 5).

However, ampicillin is also unsuitable as a selective agent with Plesiomonas (6).

Membrane filters through which water samples have been filtered are aseptically placed on Rippey-Cabelli Agar Base plates (M859) and incubated at 35-37°C for 24 hours. Aeromonas ferments trehalose and forms yellow colonies.