Evaluation of Ceftazidime as an Antibiotic Supplement in Mannitol-Yolk-Polymyxin B Agar Used for Enumeration of Bacillus cereus in Ready-to-Eat Vegetables

ABSTRACT

The Bacillus cereus group of bacteria, which causes foodborne diseases, can be detected by culture on selective media. However, the presence of competing flora is the most common factor preventing the accurate enumeration of B. cereus on selective agars. In this study, we improved the selectivity of mannitol-yolk-polymyxin B agar (MYPA) and its modified version containing trimethoprim (mMYPA) developed in our previous study by supplementation with ceftazidime (16 μg/mL). Ceftazidime-supplemented MYPA (C-MYPA16) and mMYPA (C-mMYPA16) were evaluated for bacteria recovery and selectivity with three types of ready-to-eat vegetables. Four B. cereus and one Bacillus thuringiensis strains were mixed and artificially inoculated into vegetable salad, radish sprouts, and sprout mix and then recovered on MYPA, mMYPA, C-MYPA16, and C-mMYPA16. In all tested vegetables, mMYPA, C-MYPA16, and C-mMYPA16 culture resulted in similar recovery of B. cereus and B. thuringiensis (P > 0.05), whereas radish sprout and sprout mix colonies grown on MYPA were undistinguishable. C-mMYPA16 was the most selective medium because it eliminated most of the competing flora, especially that in sprouts, without negatively affecting the recovery of B. cereus and B. thuringiensis. Our results indicate that supplementation of mMYPA with ceftazidime may improve the selectivity of this medium for B. cereus and B. thuringiensis in food testing.

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Detection of Campylobacter jejuni from Fresh Produce: Comparison of Culture- and PCR-based Techniques, and Metagenomic Approach for Analyses of the Microbiome before and after Enrichment

ABSTRACT

In this study, we compared the efficiency of culture-based methods with or without membrane filtration, real-time PCR, and digital droplet PCR (ddPCR) for the detection of Campylobacter in fresh produce. Alfalfa sprouts, clover sprouts, coleslaw, and lettuce salad spiked with Campylobacter jejuni were enriched in Bolton broth for 48 h, and enrichment cultures were either directly inoculated onto modified charcoal-cefoperazone-deoxycholate agar or applied on membrane filters placed on the surface of plating media. In parallel, 2-mL Bolton broth cultures were taken to extract DNA for real-time PCR and ddPCR assays and bacterial community analysis. A developed primer set for ddPCR and real-time PCR was evaluated for its inclusivity and exclusivity using pure culture of C. jejuni and non-C. jejuni strains, respectively. In pure culture, the primer set reacted only with C. jejuni strains and showed negative reaction to non-C. jejuni strains. There was no significant difference (P > 0.05) in the detection efficiency of positive Campylobacter isolates from coleslaw and lettuce salad using four detection methods. However, for sprout samples, the detection efficiency of the culture method was significantly (P < 0.05) lower than those of the two PCR assays and the filtration method. The analysis also revealed the presence of Pseudomonas and Acinetobacter as the most prevalent competing microbiota in enriched culture and only Acinetobacter on agar plates in the selective culture step.

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