Comparison of Three Chromogenic Media and Evaluation of Two Molecular-Based Identification Systems for the Detection of Enterobacter sakazakii from Environmental Samples from Infant Formulae Factories
Résumé
Enterobacter sakazakii is an occasional contaminant of powdered infant formula that can cause rare but severe foodborne infections in infants. To determine optimal methods for the detection and identification of E. sakazakii, 38 naturally contaminated samples from infant formulae factories were analyzed by two PCR-based methods and by a method (TS 22964/RM 210) developed by the International Organization for Standardization and the International Dairy Federation (ISO-IDF) using three different commercial chromogenic agars. The ISO-IDF method includes two enrichment steps, plating of the second enrichment broth on E. sakazakii isolation agar (a chromogenic selective agar), picking of five typical colonies for transfer onto tryptone soy agar, and subsequent confirmation of yellow-pigmented colonies by biochemical characterization. Twenty-two of the 38 samples were positive by the culture method. E. sakazakii isolation agar (ESIA; AES Laboratoires), COMPASS agar (Biokar Diagnostics), and Druggan-Forsythe-Iversen agar (Oxoid) compared favorably with violet red bile glucose agar (VRBG, a selective medium for Enterobacteriaceae), with positive predictive values of 86.96, 88, and 74.07%, respectively, in contrast to 47.83% for VRBG. One additional positive sample was detected using the nonpatented real-time PCR method evaluated, and those results were in 97.3% concordance with the ISO-IDF results. Some discrepancies between the results of the DuPont Qualicon BAX system and those of the ISO-IDF method could be explained by heterogeneity of contamination and sampling. Thus, both PCR-based systems were suitable for detecting and specifically identifying E. sakazakii within 1 to 2 days, and COMPASS agar and ESIA could be used interchangeably as a first-step medium to isolate presumptive E. sakazakii colonies.