Listeria monocytogenes: illuminating adaptation with proteomics John P Bowman, Rolf E Nilsson, Chawalit Kocharunchitt and Tom Ross Food Safety Centre, Tasmanian Institute of Agriculture, University of Tasmania, College Road, Sandy Bay, Tas 7005, Australia Tel: +61 3 6226 6380, Fax: +61 3 6226 2642, Email: john.bowman@utas.edu.au With increased consumption of minimally processed ready-to-eat foods the potential for exposure to Listeria monocytogenes has increased. Thus, there is a need to maintain a balance between food convenience and safety. L. monocytogenes is not a homogenous species; certain strains are more resilient to stressful conditions while others are potentially more virulent. To understand the basis of these differences we are applying proteomics to determine the molecular mechanism of adaptations of L. monocytogenes in food-relevant environments. The goal is to define how this species grows, behaves and survives thus allowing us to fine tune food safety risk management, espe- cially when developing new minimal food processes or con- sidering introduction of unpasteurised food such as raw milk cheeses. Listeria monocytogenes causes listeriosis, a serious disease primar- ily acquired by food consumption, and that mainly impacts immu- nocompromised people, the elderly, and neonates. Listeriosis also occurs in livestock and was originally discovered in animals. Though the prevalence of listeriosis in Australia is relatively low it remains a major concern for the food industry. A case in point is the January 2013 brie and camembert cheese-associated outbreak that occurred in south-east Australia, associated with 3 deaths and one miscar- riage. L. monocytogenes mainly occurs as an environmental con- taminant and can enter food anywhere along the industrial food supply chain including within domestic settings. Despite being relatively nutritionally fastidious this tendency to be a frequent contaminant is due to the species inherent hardiness since it is low water activity and acid tolerant and able to grow at refrigeration temperatures. Fortunately it is readily eliminated by standard pasteurisation or by cooking so that the threat is limited to certain ready-to-eat foods, typically those with long refrigerated shelf-lives. L. monocytogenes mainly causes disease by invading gastrointesti- nal epithelial cells by encouraging endocytosis 1 . Once inside host cells other proteins aid intracellular survival, mobility, and cell-to- cell spread. L. monocytogenes is adept at evading and surviving within cellular and humoral immune systems. A fascinating aspect of L. monocytogenes is its ability to shift from an environmental saprophytic state where virulence genes are turned off to a parasitic state within animal or human hosts 2 . At 378C, if carbohydrate levels are low, many virulence genes are activated. This switch is mainly controlled by temperature sensitive small RNAs 3 . The ability to respond to stress is also intertwined in this transitive process. The many sequenced genomes of L. monocytogenes are rich in tran- scriptional regulators controlled in overarching regulons by various “master” regulators, which functionally overlap in a complex net- work. This network allows L. monocytogenes to rapidly respond to changing environments, including switching on and off stress defence and virulence genes 4 . Using comprehensive proteomics we are attempting to understand more holistically the mechanistic basis of L. monocytogenes’ adap- tation to different situations. State-of the art proteomics is now a very powerful tool and is becoming more cost-effective. Employing gel- free and label-free liquid chromatography (operated in either one or two dimensional modes) and sensitive, high resolution ion trap mass spectrometry, it is possible to take complex protein mixtures digested by a peptidase such as trypsin and identify and quantify peptides en-masse after bioinformatic comparison to proteome databases. This is possible due to better separation of individual Under the Microscope MICROBIOLOGY AUSTRALIA * MAY 2013 10.1071/MA13026 75