Comparison of the performance of lactose and mannitol selenite enriched broths, subcultured to DCA and XLD agars, in the isolation of Salmonella spp. from faeces.

ABSTRACT

Selenite-based enrichment broths using either lactose or mannitol as a carbohydrate source are generally used as selective enrichment media for the isolation of Salmonella spp. from human faeces in the UK, but few studies have compared the relative efficacy of the available formulations. A variety of solid media is used for the routine subculture from these selective broths, but similarly we have been unable to find published evidence as to which medium performs best. Four thousand and nineteen faecal samples were examined in four laboratories and the isolation rates of Salmonella spp. from lactose (LS) or mannitol selenite (MS) broths, plated onto either xylose lysine desoxycholate agar (XLD) or desoxycholate citrate agar (DCA) were compared. MS performed significantly better than LS (p = 0.02), recovering 95 salmonellae compared with 87. No significant difference in isolation rates was found between XLD and DCA, although colonial appearances of suspected salmonellae on XLD were much more specific, resulting in significantly fewer colonies having to be selected for supplementary testing (p < 0.001) and so reducing confirmatory work. An opportunity exists to simplify holdings of media by choosing to use the MS/XLD combination.