Did you just eat that? Novel approaches to control Listeria monocytogenes on ready-to-eat-foods

Abstract

The Gram-positive bacterium Listeria monocytogenes is an ubiquitous, intracellular pathogen which has been causative organism in several outbreaks of foodborne disease. Listeriosis has a mortality rate of about 24%, making it one of the leading causes of deaths associated with foodborne illness. Severe forms of listeriosis mainly affect pregnant women, their fetuses, newborns, and immunocompromised persons, with symptoms of abortion, neonatal death, septicemia, and meningitis. L. monocytogenes can contaminate a variety of foods, with recent high incidences in dairy products. L. monocytogenes can survive and grow in a variety of environments and refrigeration, making it difficult to control and highlighting the importance of optimizing control strategies against this pathogen. The overall goal of the work presented here was to (i) investigate the role of the environment on L. monocytogenes sensitivity to two methods (i.e. nisin and bacteriophage) currently used on foods, to inhibit bacterial contamination and (ii) understand how changing environmental conditions can lead to transmission of L. monocytogenes to humans and subsequently cause disease. Nisin and phage reduced L. monocytogenes counts on cheese in temperature- and pH-dependent manners. Nisin-mediated reduction of L. monocytogenes was more pronounced at lower storage temperatures, whereas phage was more effective at higher temperatures. However, both control strategies were able to reduce L. monocytogenes numbers when cheese was formulated at higher pH. Furthermore, serotype was found to also affect the sensitivity of L. monocytogenes to both nisin and phage treatment. Serotype 1/2 strains showed significantly higher susceptibility to both treatments than serotype 4b strains. L. monocytogenes must rapidly adapt to changes in the environment for survival and to cause disease. Previously, micro-array studies have been used to characterize virulence regulation in L. monocytogenes. Here, we use a bioinfromatics approach to expand our knowlege of virulence, the PrfA regulon, in L. monocytogenes. We identified a PrfA-dependent gene with a novel PrfA-box and putative sigA-dependent promoter region. Overall, this work provides experimental evidence that environment, as well as serotype affect L. monocytogenes sensitivity to control strategies, and that environmental conditions should be carefully considered when applying interventions against this important foodborne pathogen. Understanding the conditions that surround the interaction between pathogen and control strategy could prevent foodborne infections. Furthermore, better understanding of sigB and PrfA-dependent regulation of virulence related genes under different environmental conditions can allow for further improvement of Listeria control strategies.