We are only just beginning to understand the dense and diverse microbial communities that live in and on us. A key benefit of the microbiome is its ability to provide protection against pathogens, known as colonisation resistance. Resident strains of bacteria compete with incoming pathogens and help prevent them from colonising and causing infection. In my DPhil, I am investigating the ecological interactions between symbiont strains and pathogens in order to further our understanding of colonisation resistance. My research focuses on the gut microbiome and I use the species Klebsiella pneumoniae as a model pathogen. K. pneumoniae can colonise the gastrointestinal tract of healthy individuals, where it does not usually cause disease, but acts as a reservoir for opportunistic infection of other parts of the body such as the lungs and urinary tract. It is an important nosocomial pathogen that, due to the widespread emergence of antibiotic-resistant strains, poses an increasing threat to human health. Probiotics that engineer the gut community to resist K. pneumoniae colonisation have potential as an alternative to antibiotics. After initially screening 120 human gut symbionts for their individual ability to inhibit K. pneumoniae, I am currently working with a selection of the best- and worst-performing strains and testing combinations of up to ten strains to find out how species diversity influences community performance. I am also interested in whether K. pneumoniae inhibition is due simply to the additive effect of member strains, or if the combination of strains also plays a role, implying an effect of interactions between symbiont strains. My lab work takes place in the Department of Biochemistry and in the germ-free mouse facility at the Kennedy Institute of Rheumatology.