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Applied and Environmental Microbiology, January 2006, p. 261-268, Vol. 72, No. 1
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.1.261-268.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Modeling and Predicting the Simultaneous Growth of Escherichia coli O157:H7 and Ground Beef Background Microflora for Various Enrichment Protocols

A. Vimont,1* C. Vernozy-Rozand,1 M. P. Montet,1 C. Lazizzera,2 C. Bavai,1 and M.-L. Delignette-Muller1

Unité de Microbiologie Alimentaire et Prévisionnelle, Ecole Nationale Vétérinaire de Lyon, 1 avenue Bourgelat, 69280 Marcy l'Etoile, France,1 Unité de Bactériologie Bovine et Sécurité des Viandes, Afssa de Lyon, 31 avenue Tony Garnier, 69364 Lyon, France2

Received 13 May 2005/ Accepted 25 September 2005

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-atmosphere-packed ground beef.

* Corresponding author. Mailing address: Unité de Microbiologie Alimentaire et Prévisionnelle, Ecole Nationale Vétérinaire de Lyon, 1 avenue Bourgelat, 69280 Marcy l'Etoile, France. Phone: 33-4-78-87-25-50. Fax: 33-4-78-87-25-54. E-mail: a.vimont{at}vet-lyon.fr

Applied and Environmental Microbiology, January 2006, p. 261-268, Vol. 72, No. 1
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.1.261-268.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.





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