Efficacy of natural antimicrobial systems used in foods; bacterial stress response and mode of action

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Although Canada is recognized to have a rather safe food supply, the Public Health Agency of Canada estimates that four million people (one in eight Canadians) get sick every year due to domestically acquired foodborne illnesses and meat remains the most often incriminated food. The 2008 listeriosis crisis related to tainted processed meat reminds us how microorganisms can be devastating and last year, XL Foods, a Canadian own company, was sold to the US branch of a Brazilian consortium following the Escherichia coli O157:H7 contaminated meat incident. Those events are prime examples of sanitary problems Canadians are facing, and that our laboratory is addressing in this proposal. Furthermore, consumer's demand for minimally processed foods with fewer additives, but that remain easy to prepare, is challenging food scientists to find innovative ways to maintain the safety and microbial quality of our food supply. The efficacy of antimicrobial systems is traditionally evaluated by cell enumeration on solid growth medium during challenge studies. However, viable organisms too stressed or injured do not grow directly to form distinct colonies on solid media. These viable but not culturable organisms may recover during the storage period in foods and resume growth. Hence, determination of the efficacy of an antimicrobial treatment can be overestimated if solely based on cell counts. Therefore, to deepen the control of microorganisms in our food supply effectively, our interventions need to be based on sound scientific data. We must understand the efficacy of antimicrobial systems and the microbial physiological response to them at the molecular level in order to develop effective and practical means of microbial control. Powerful molecular tools for proteomics, transcriptomics, RT-PCR, etc. have been used to better understand the physiological response of Escherichia coli to cooking temperatures. Several potential biomarkers of heat treatment efficacy have been identified using microarray analysis. The stability and functionality of the mRNA transcribed from those genes of interest when cells are exposed to severe heat treatment must be established. Also, quantification of heat shock gene expression by real-time PCR revealed that dnaK and groEL mRNA levels decreased significantly above 60°C to reach levels similar to those of control cells at 37°C suggesting that above 60°C, the cell is no longer able to adapt and that the treatment starts to be effective. Our search for new alternatives to control microbial spoilage and pathogens has leaded us to study the efficacy of natural antimicrobials from various sources (e.g., essential oils of plant origins, nisin, peptides from crab hydrolysate) in meat products. In Quebec alone, 15 000 tonnes of onion per year are rejected and found unfit for human consumption for various reasons. Effective use of polyphenols extracted from by-products of plant productions in food or feed is in line with a global vision for sustainable agriculture and economic efficiency. But in order to use those novel antimicrobials to their full potential in a complex food matrix such as meat, a deep understanding of the bacterial stress response and mode of action at the molecular level is mandatory and will be investigated in this proposal.