School of Mathematics, University of Birmingham, Edgbaston, Birmingham, United Kingdom.|Department of Surgery, University of Texas Southwestern Medical Center, Dallas, Texas, United States of America.|Institute of Microbiology and Infection, School of Biosciences, University of Birmingham, Edgbaston, Birmingham, United Kingdom.|Department of Microbiology and Molecular Genetics, University of Texas McGovern Medical School at Houston, Houston, Texas, United States of America.|School of Mathematics, University of Birmingham, Edgbaston, Birmingham, United Kingdom.