Chapter 21: Culture Media for Food Spoilage Bacteria of the Order Pseudomonadales: Pseudomonas, Acinetobacter and Psychrobacter spp. Check Access

The common Gram-negative food spoilage bacteria consist primarily of Pseudomonas, Acinetobacter and Psychrobacter spp. The genus Pseudomonas has undergone extensive taxonomic changes in recent years, with species being removed from the order Pseudomonadales and placed in other orders. Selective isolation of Pseudomonadales is facilitated by higher resistance to antimicrobials than other Gram-negative bacteria. Cephaloridine, fucidin and cetrimide can be used to select for Pseudomonas spp., while cefsulodin (a specific inhibitor of Ps. aeruginosa) can be incorporated into media for selection of Acinetobacter spp. Isolation of Psychrobacter spp. is facilitated by their ability to grow at very low temperatures. A fundamental problem in designing media for these organisms is that they have a respiratory metabolism, which does not generate enough acid to lower the pH of media sufficiently to enable differentiation strategies based on sugar metabolism. Differentiation of fluorescent and pigment producing strains is possible on media designed to enhance these traits. Samples with low numbers of competing flora can be examined using non-selective pre-enrichment. However, a selective medium is required for most food and environment samples and a balance needs to be achieved between selectivity, to inhibit competing organisms, and sensitivity, to optimise the recovery of stressed cells. As with all selective media, confirmatory tests need to be made on presumptive positive colonies.