Dissecting Bacterial Abscess Formation for Vaccine Design

Antimicrobial resistance (AMR) is an increasing global threat to both human and animal health as infections become increasingly difficult to treat with commonly used antimicrobials. Vaccination is a powerful approach for limiting the impacts of AMR by disease prevention. However, major bacterial pathogens such as Staphylococcus aureus remain refractory to vaccine development due, at least in part, to a lack of translation of vaccine testing in animal models to human protective efficacy. Clinically, S. aureus is a major infectious burden of humans and farmed animals with particular lineages specialized for causing mastitis in ruminants. This bacterial infection of the udder generates a £120 million economic cost to UK sheep farmers annually as well as being a food security concern with 6% of ewes culled annually. In order to achieve the United Nation’s Sustainable Development Goal (SDG) of Ending Hunger, food production efficiency needs to be increased with reductions in underperforming animals. The ability to vaccinate ruminants to eliminate productivity costs associated with mastitis would be hugely beneficial in our fight to create a sustainable future and to limit the impacts of bacterial AMR. To create an effective S. aureus vaccine targeting mastitis we first need to understand what bacterial factors to target and how to promote an effective immune response. A major pathology during chronic sheep mastitis is the development of abscesses that promote persistence. These abscesses are palpable in the udder as lumps and will often inform the farmer about whether to cull a ewe. In humans, such abscesses contain a dense population of viable bacteria at the core called Staphylococcal abscess communities (SACs) that are protected from the immune system and antibiotic treatment by an outer fibrin layer, promoting bacterial persistence and infection recurrence. There is currently a lack of understanding of the bacterial factors required by sheep strains of S. aureus to produce these SACs during chronic mastitis limiting the design of a vaccine targeting sheep mastitis that would prevent abscess formation. This project aims to address this challenge by (i) identifying potential vaccine candidates for S. aureus-mediated sheep mastitis, and (ii) identifying the key immune cell interactions for effective control of ovine S. aureus strains in vitro. These aims will be achieved through the following objectives: Use transposon mutagenesis to identify the bacterial factors required for SAC formation in vitro by ovine aureus strains Evaluate bacterial factors required for SAC formation as vaccine candidates Establish in vitro systems for evaluating the interaction of aureus with ovine immune cells isolated from milk and blood Use transcriptomics to identify markers of aureus control by immune cells isolated from sheep Taken together these aims and objectives will identify potential vaccine candidates involved in abscess formation that could be employed to control sheep mastitis. Additionally, it will identify markers and pathways associated with effective S. aureus control in vitro, which will aid in vaccine design by informing routes of vaccine delivery and enhancing vaccine success. By addressing major gaps in our understanding of the pathogenesis of sheep mastitis, this project has the long-term vision of enhancing ewe welfare, increasing financial return for farmers and contributing to the SDG of Ending Hunger through increased food security globally.