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Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 4/17/2015 Publication Date: 8/1/2015 Publication URL: http://handle.nal.usda.gov/10113/61198 Citation: Wang, R., Kalchayanand, N., Bono, J.L. 2015. Sequence of colonization determines the composition of mixed biofilms by Escherichia coli O157:H7 and O111:H8 strains. Journal of Food Protection. 78(8):1554-1559. DOI: 10.4315/0362-028X.JFP-15-009. Interpretive Summary: Shiga-toxin producing Escherichia coli (STEC) are important foodborne pathogens. E. coli O157:H7, the most frequently isolated STEC serotype, is one of the leading causes of foodborne diseases. Meanwhile, six other STEC serotypes also have been well associated with serious outbreaks, thus, they are regulated as adulterants in beef products as well. Various STEC serotypes are capable of forming biofilms on food-contact surfaces that, when detached, may lead to food contamination. In nature, bacteria may coexist in multi-species communities to form mixed biofilms, and the interactions among bacterial species could profoundly affect biofilm development and composition. Here we investigated how the coexistence of E. coli O157:H7 and O111:H8, the two most common STECs, would affect bacterial growth competition and mixed biofilm development. Our results indicated that E. coli O157:H7 strains grew better in mixed biofilms than the co-inoculated O111:H8 companion strains. However, the sequence of colonization played a critical role in determining the composition of the mixed biofilms. The first serotype to colonize the surface was able to outgrow the other serotype and became the dominant strain in mixed biofilms. Since the potential risks to food safety of biofilms is largely influenced by the dominant strain in the mixtures, our results on the nature of mixed biofilms composed of multiple microorganisms will help determine the most effective disinfection operations against the risk of contamination from biofilms. Technical Abstract: Bacterial biofilms are one of the potential sources of cross-contamination in food processing environments. Shiga-toxin producing Escherichia coli (STEC) O157:H7 and O111:H8 are important foodborne pathogens capable of forming biofilms, and the coexistence of these two STEC serotypes has been detected in various food samples and in multiple commercial meat plants throughout the U.S. Here we investigated how coexistence of these two STEC serotypes and their sequence of colonization could affect bacterial growth competition and mixed biofilm development. Our data showed that E. coli O157:H7 strains were able to maintain a higher cell percentage in mixed biofilms with the co-inoculated O111:H8 companion strains, even though the results of planktonic growth competition were strain dependent. On solid surfaces with preexisting biofilms, the sequence of colonization played a critical role in determining the composition of the mixed biofilms as early stage pre-colonization significantly affected the competition results between the E. coli O157:H7 and O111:H8 strains. The pre-colonizer of either serotype was able to outgrow the other serotype in both planktonic and biofilm phases. The competitive interactions among the various STEC serotypes would determine the composition and structure of the mixed biofilms as well as their potential risks to food safety and public health, which is largely influenced by the dominant strains in the mixtures. Thus, the analysis of mixed biofilms under various conditions would be of importance to determine the nature of mixed biofilms composed of multiple microorganisms and to help implement the most effective disinfection operations accordingly. |
