Comparison of four agar plating media with and without added novobiocin for isolation of salmonellae from beef and deboned poultry meat

Abstract

Four plating media, Hektoen enteric (HE), xylose-lysine deoxycholate (XLD), tryptic soy-xylose-lysine (TSXL), and tryptic soy-brillant green (TSBG) agars with and without 10 mg of added novobiocin per ml, were evaluated for recovery of Salmonella from roast beef and deboned turkey. Colonies producing a reaction typical of H(2)S-positive salmonellae (alkaline with black centers) were picked. On the media without novobiocin, from 109 determinations on 75 samples, number of salmonellae found and false-positives were, respectively: HE-13, 58; XLD-17, 18; TSXL-23, 0; TSBG-22, 7. When novobiocin was present the corresponding results were: HE-17, 24; XLD-21, 2; TSXL-23, 3; TSBG-20, 7. A total of 25 determinations were positive on one or more agars. False-positives on HE and XLD without novobiocin were predominantly Proteus, which were almost totally eliminated by addition of 10 mg of novobiocin per liter. If alkaline H(2)S-negative colonies had been considered, many more false-positives would have been found on HE and XLD but not on TSBG or TSXL. Addition of novobiocin markedly improved isolations of salmonellae from XLD and HE and reduced the number of false-positives. Addition of novobiocin did not improve performance of TSXL and slightly impaired differentiation of salmonellae from Citrobacter on TSBG. XLD with novobiocin and TSXL are highly specific for H(2)S-positive salmonellae, and the appearance of Salmonella-like colonies on these media can be considered a presumptive test for H(2)S-positive salmonellae.