Aims:  To review and characterize the enrichment protocols used for detecting all Shiga‐Toxin producing Escherichia coli (STEC) from different matrices. Methods and Results:  Firstly, the frequency distribution of the factors characterizing the enrichment protocols is described; secondly, a multiple correspondence analysis is performed to display profiles of association of these factors, and thirdly, published results concerning the relative performances of the protocols are summarized. Trypticase Soy Broth (TSB) is reported as the most frequently used enrichment broth. More often, one antibiotic is added in enrichment broths and these broths are incubated for a duration of 16–24 h at 35–37°C. It also appears that the incubation temperature does not seem to be related to the type of serogroup looked for and that antibiotics are used regardless of the matrix analysed. Finally, results relating to the enrichment protocol efficacy are rare and differ from one study to another. Conclusions:  Statistical studies must be conducted so as to assess the efficacy of the main enrichment protocols investigated in this study. Significance and Impact of the Study:  This study reviews the most commonly used enrichment protocols and highlights the lack of results as to their relative efficacy.

Aims:  To investigate the assumption that usage of novobiocin (20 mg l−1) in Shiga toxin‐producing Escherichia coli (STEC) enrichment broths could achieve false‐negative results. Methods and Results:  First, the minimum inhibitory concentration (MIC) of 74 E. coli O157:H7 and 55 non‐O157:H7 STEC strains to novobiocin was determined. Second, to visualize the potential impact of novobiocin on the STEC growth during the enrichment step, the growth experiments were carried out in trypticase soy broth (TSB) with and without 20 mg l−1 of novobiocin. The MIC values varied from 32 to >64 mg l−1 for the 74 E. coli O157:H7 strains, and from 16 to >64 mg l−1 for the 55 non‐O157:H7 STEC strains. The E. coli O157:H7 strains were significantly (P < 0·001) more resistant to novobiocin than the non‐O157:H7 STEC strains. The present study shows that the addition of novobiocin into enrichment broths inhibits the growth of some non‐O157:H7 STEC strains, and slows down the growth of some STEC strains. Conclusions:  Enrichment broths supplemented by novobiocin could lead to false‐negative results for detecting STEC from food. Significance and Impact of the Study:  We strongly suggest that novobiocin should not be systematically added into enrichment broths for detecting STEC from food.

The simultaneous growth of Escherichia coli O157:H7 (O157) and the ground beef background microflora (BM) was described in order to characterize the effects of enrichment factors on the growth of these organisms. The different enrichment factors studied were basal medium (Trypticase soy broth and E. coli broth), the presence of novobiocin in the broth, and the incubation temperature (37°C or 40°C). BM and O157 kinetics were simultaneously fitted by using a competitive growth model. The simple competition between the two microfloras implied that O157 growth stopped as soon as the maximal bacterial density in the BM was reached. The present study shows that the enrichment protocol factors had little impact on the simultaneous growth of BM and O157. The selective factors (i.e., bile salts and novobiocin) and the higher incubation temperature (40°C) did not inhibit BM growth, and incubation at 40°C only slightly improved O157 growth. The results also emphasize that when the level of O157 contamination in ground beef is low, the 6-h enrichment step recommended in the immunomagnetic separation protocol (ISO EN 16654) is not sufficient to detect O157 by screening methods. In this case, prior enrichment for approximately 10 h appears to be the optimal duration for enrichment. However, more experiments must be carried out with ground beef packaged in different ways in order to confirm the results obtained in the present study for non-vacuum-and non-modified-atmospherepacked ground beef.

Cattle are an important reservoir for STEC and eating food contaminated with fecal material is a frequent source of human STEC infection. It is thus essential to reliably determine the prevalence of STEC contamination in cattle. Currently, different enrichment protocols are used before the detection of Shiga-Toxin producing Escherichia coli (STEC) in fecal samples. However, there have not been any studies performed that have compared the effectiveness of these various enrichment protocols for the growth of non-O157 STEC in fecal samples. The objective of this present study was to characterize the effects of different enrichment factors on the simultaneous growth of the feces background microflora (BM) and two non-O157 STEC strains. The different factors studied were the basal medium (TSB and EC), the effect of novobiocin in the broth (N+ or N-) and the incubation temperature (37 or 40 degrees C). The BM and STEC growth data were simultaneously fitted by using a competitive growth model. The STEC final levels obtained after 24h were higher for the protocols with novobiocin and/or EC compared to the others. However, novobiocin inhibited the growth of one STEC strain. We observed that the addition of novobiocin into broths is not advisable for optimal growth conditions. Moreover, given high BM and low STEC levels often observed in feces, predictions made with the growth model highlighted that false negative results could more likely appear with protocols using TSB without novobiocin than with protocols using EC. In conclusion, the use of EC broth in enrichment protocols seems to be more appropriate for detecting non-O157 STEC from bovine fecal samples. This can help avoid false negative results that cause an underestimation of the STEC prevalence in cattle.

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