A Medium for the Presumptive Detection of Enterobacter sakazakii in Infant Formula: Interlaboratory Study
Abstract:A standard method for the detection of Enterobacteriaceae was modified for the presumptive detection of Enterobacter sakazakii, and the modified method was validated in an interlaboratory trial with 16 laboratories from 8 European countries. The modification included a differential-elective medium for the isolation of E. sakazakii, consisting of nutrient agar (NA) supplemented with 4-methyl-umbelliferyl α-D-glucoside (α-MUG). A 25 g sample was added to 225 mL buffered peptone water. After incubation at 35° or 37°C for 16 or 20 h, 10 mL nonselective enrichment was transferred into 90 mL selective enrichment. The selective enrichment was streaked on violet-red bile glucose agar (VRBGA) and incubated at 37°C for 24 h. It was streaked in parallel on NA plates supplemented with α-MUG at 50 mg/L and incubated at 25°C for 16 h, and afterwards for an additional 24 h at room temperature in the dark. E. sakazakii appeared as vivid yellow colonies under normal light and showed blue/violet fluorescence under UV light on NA + α-MUG plates. Validation samples represented powdered infant formula without E. sakazakii (blanks) and with low (1–10 colony-forming units [CFU]/25 g) and medium (1–10 CFU/g) contamination levels. All samples contained Pseudomonas aeruginosa and Lactobacillus spp. as background flora. The specificity for blank samples was 100%. The sensitivity of the low contamination level was similar for VRBGA and NA + α-MUG, i.e., 66.7% (66.7% accordance, 53.9% concordance). For the medium level the sensitivities were 96.7% (93.3% accordance, 93.5% concordance) for VRBGA and 98.3% (96.9% accordance, 96.9% concordance) for NA + α-MUG.
Document Type: Research Article
Affiliations: Central Science Laboratory, Department for Environment, Food and Rural Affairs, Sand Hutton, York YO41 1LZ, United Kingdom
Publication date: May 1, 2004
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