Citrobacter braakii: A Major Cause of False-Positive Results on MacConkey and Levine's Eosin Methylene Blue Selective Agars Used for the Isolation of Escherichia Coli from Fresh Vegetable Samples

摘要

Fresh vegetables are a frequent cause of foodborne disease outbreaks because they often do not undergo heat treatment prior to consumption. Pathogenic Escherichia coli strains are the major causative agents of produce-related outbreaks. In this study, we investigated the efficiency of conventional E.coli selective medium, MacConkey (MAC) agar, for the screening of fresh vegetable samples and evaluated the discrimination ability of Levine's eosin methylene blue (L-EMB) agar for E.coli-like colonies obtained from MAC agar. A total of 120 samples of lettuces (n=60) and radish sprouts (n=60) were analyzed. Among 17 (14.2%) MAC agar plates containing putative E.coli colonies, only one plate was confirmed E.coli-positive (positive predictive value=5.9%). All 16 false-positive isolates were identified as Citrobacter braakii and formed E.coli-like colonies on L-EMB agar. These results indicate that in order to reduce false-positive results in screening fresh vegetable samples, both MAC and L-EMB selective agars should be modified with the aim of providing reliable differentiation between E.coli and C.braakii.