Recoveries of coliforms in water and food samples were increased by adding sodium pyruvate to the Tryptic Soy Agar (TSA) base layer, and Violet Red Bile (VRBA) overlay, of the Modified VRBA procedure described in Standard Methods for the Examination of Dairy Products. Six pyruvate levels (0, 0.005, 0.01, 0.05, 0.1, and 1.0%) were tested. Counts were significantly lower (P≤0.05) on media containing 0% and 1.0% pyruvate than on the other media. Although 0.05% yielded the highest counts overall, there were no significant differences (P≤0.05) among 0.005, 0.01, 0.05, and 0.1% pyruvate. Analysis by using a general linear model procedure revealed that 0.02% pyruvate was the statistically predicted optimal level to use in the Modified VRBA procedure.

This study evaluated the effects of changing the composition of the pre-enrichment medium buffered peptone water (BPW) on the growth of stressed and unstressed Gram-negative foodborne pathogens in a one-broth enrichment strategy. BPW supplemented with an available iron source and sodium pyruvate, along with low levels of 8-hydroxyquinoline and sodium deoxycholate (BPW-S) improved the recovery of desiccated Cronobacter spp. from powdered infant formula. Growth of Salmonella and STEC was comparable in all BPW variants tested for different food matrices. In products with high levels of Gram-negative background flora (e.g. sprouts), the target organisms could not be reliably detected by PCR in any of the BPW variants tested unless the initial level exceeded 103cfu/10g of sprouts.Based on these results we suggest BPW-S for a one-broth enrichment strategy of stressed Gram-negative foodborne pathogens from dry products. However, a one-broth enrichment strategy based on BPW variants tested in this evaluation is not recommended for produce with a high level of Gram-negative background flora due to very high detection limits.