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Effects of incorporating ampicillin, bile salts and carbohydrates in media on the recognition and selection of Aeromonas spp. from faeces

Department of Bacteriology, Royal Postgraduate Medical School, Hammersmith Hospital, Du Cane Road, London W12 ONN

Received August 1, 1989 Revision received November 2, 1989.
The effects of incorporating ampicillin, some bile salts and sugars into media on the primary recognition and selection of aeromonads from faeces were examined. Most (88%) of the 101 Aeromonas strains examined had an ampicillin MIC of <<40 mg/L, and would be isolated on blood agar containing ampicillin 30 mg/L. The strains with an ampicillin MIC of >> 40mg/L were all of human origin and predominantly A. caviae. Although ampicillin at 10, 20 or 30 mg/L in blood agar inhibited faecal bacteria, the ability to detect Aeromonas strains with a high ampicillin MIC was less when the lower concentrations of ampicillin were used, without any improvement in the isolation of those strains with a low ampicillin MIC. Thirty-seven strains were tested for sensitivity to several different bile salts and found to be at least as resistant to them as Escherichia coli NCTC 10418. Bile salt sensitivity was not related to the species or source of a strain. There were minor differences in sensitivity to bile salts between some strains which related to whether strains had been isolated originally in the presence of bile salt or not. The effects of the presence of E. coli, Klebsiella spp. and Enterococcus faecalis on the growth of Aeromonas strains in mixed culture on media with and without carbohydrate were examined. The colony size of some Aeromonas strains was reduced in mixed culture but colony counts were not affected with any Aeromonas strains. The effect of carbohydrate in the medium on Aeromonas colonies was apparent in the presence of other bacteria that fermented carbohydrate, when the non-fermenting Aeromonas colonies were often indistinguishable from those of the fermenters. When xylose was added to the medium the recognition of colonies of the non-fermenting aeromonads among those of other fermenting bacteria was difficult because their colonies were often of the same colour. This may explain the difficulty of isolating aeromonads from mixed cultures on some carbohydrate-containing media. It is recommended that if aeromonads are to be recognised in selective culture from faeces by their failure to ferment carbohydrate, a medium be used which inhibits the growth of non-aeromonad bacteria as much as possible.