Food-borne Listeria infections: Zinc homeostasis at the host-pathogen interface

The bacterium Listeria monocytogenes causes serious food-borne disease in man, with high mortality rates despite antibiotic intervention. The disease can range from stillbirth in infected women, septicaemia (blood poisoning) and meningitis. People with weakened immune systems are particularly susceptible to this disease. A feature of Listeria is its ability to survive a range of harsh conditions and grow at low temperatures routinely used to store food. In order for Listeria to grow, it must acquire the metal zinc. This is true not only during life in the environment, on food and food preparation surfaces, but also during infection of a host. This proposal aims to understand how Listeria acquires zinc especially in conditions inside the host where available zinc may be in short supply. We have already identified two zinc-uptake systems and shown that these are vital for Listeria to be able to survive when there is very little zinc available and also for it to be able to cause infections. Having discovered these systems, the next step is to gain a more detailed understanding of how each one contributes to obtaining zinc and their roles during Listeria infections. We will also seek to find other systems that are needed by Listeria to handle zinc. An important impact of understanding the mechanisms by which Listeria acquires zinc and controls its zinc levels will be the opportunity to exploit this knowledge to design new ways of combating Listeria infections and design new control strategies to reduce contamination of foods by Listeria.

Obtaining zinc for loading onto zinc-dependent proteins is a vital requirement for bacteria. However, zinc can also be toxic and bacteria must therefore precisely balance the supply and demand for this metal to ensure the correct metal-occupancy of their metalloproteins and avoid toxicity. For bacterial pathogens, such as food-borne Listeria monocytogenes, this challenge becomes exacerbated by host immune defences that can act to restrict access to this metal. We have recently identified two related zinc uptake systems in L. monocytogenes which are required for growth during zinc-limitation and are important for virulence during growth in vitro and in vivo. However, whilst there is some functional redundancy between the two systems, this appears to vary depending upon the environment in which the bacterium is growing. This programme of work will test the hypothesis that the presence of the two specific zinc uptake systems in L. monocytogenes allows access to different sources of zinc within different environments. We will perform a detailed characterisation of these systems and identify their preferred substrates for import. We will also identify other proteins within this organism that we hypothesise also perform a vital role in handling zinc and may contribute to zinc-protein assembly. We will investigate the role of each system in promoting the survival of L. monocytogenes within different host cell types and during infection in vivo. With the incidence of listeriosis on the increase and the requirement for novel antimicrobial strategies, a thorough investigation of the zinc-acquisition systems in L. monocytogenes and their roles in virulence is both timely and necessary.

The programme of work will focus on Listeria monocytogenes which is an important food-borne pathogen and a target for antimicrobial products in the agri-food industry. L. monocytogenes is responsible for a number of serious clinical syndromes in both humans and animals and accounts for 30% of all human fatalities as a consequence of food-borne infection worldwide. Food-borne pathogens and basic/applied microbiology are outlined within the remit of BBSRC committee B (Plants, Microbes, Food & Sustainability). By providing fundamental knowledge of the mechanisms used to survive within different environments and cause infections, this study will help inform the development of new antimicrobials and control strategies to improve microbial food safety and hence is relevant to the BBSRC strategic research priority 'food security' within the strategic objective 'Microbial food safety'. Elucidation of these systems at the molecular level also overlaps with the scientific areas of biochemistry, biological chemistry, structural biology & molecular biology outlined in the remit of committee D (Molecules, Cells & Industrial Biotechnology).

The main impact from this research will come from the acquisition and transfer of knowledge.

This will include:

1. Informing a broad range of researchers in the bioscience and medical sectors, particularly microbiologists, biochemists and cell biologists engaged in the study of the cell biology of metals and/or bacterial pathogenesis.
2. Providing high quality training for early career scientists.
3. Engaging with the media and the public.
4. Generating and sharing strains and detailed protocols as research resources.
5. Engagement with industrial companies via The University of Manchester Intellectual Property Limited (UMIP), with regard to the development of antimicrobials.
6. Engagement with agencies concerned with the control of food-borne infections.