Chromogenic Agar Medium Identifies Aeromonas in Fecal Samples

Chromogenic agar medium has been found useful for the isolation, differentiation, and potential identification of Aeromonas spp. isolated from several sources.

Chromogenic media have facilitated the identification of pathogenic organisms at the genus level and in some cases at the species level, thus requiring a shorter testing time and simplifying the detection of target bacteria and the interpretation of primary culture results.

Medical microbiologists at National Polytechnic Institute (México) evaluated the microbiological performance of a chromogenic agar medium, using the Miles and Misra method and the Mossel ecometric technique. A total of 62 strains were tested, including 23 isolated from gastrointestinal infection, 13 type strains from the Spanish type culture collection of the Aeromonas genus, 12 strains isolated from water, nine isolated from frozen fish, and six other strains from the American type culture collection.

The medium CromoCen AGN agar (BioCen; Havana, Cuba) was designed as a chromogenic agar medium for the rapid and simultaneous identification of Aeromonas and Pseudomonas and for the identification and/or enumeration of Salmonella, Escherichia coli, and coliform bacteria. Two control media, MacConkey (Bioxon; Mexico City, Mexico), and Ryan Aeromonas (Oxoid Ltd.; Basingstoke, UK) were used for the comparative evaluation using the ecometric test.

CromoCen AGN agar medium was also assessed for the potential identification of Aeromonas spp. in fecal samples that were collected from asymptomatic schoolchildren. Out of 106 processed stool cultures, five were putatively positive (4.7%) and identified as Aeromonas colonies. These five strains were pale or light green in color, and only four had evident proteolytic activity. The CromoCen AGN exhibited 94.73% sensitivity and 100% specificity for the various samples

The authors concluded that if the incidence of Aeromonas diarrhea in the patient population of a given laboratory is high, the routine use of CromoCen AGN agar medium is warranted. This agar has a relatively high plating efficiency for most strains, can simplify the initial genus identification prior to genetic identification. CromoCen AGN agar is useful for the inhibition of Gram-positive bacteria, is easy to use, is simple to interpret and suitable for the identification of the Aeromonas genus. However, due to the incidence of false negative results, it should not be used for species identification. The study was published online on April 27, 2012, in the Journal of Microbiological Methods.

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