In the 1970's, I developed methods to detect the penicillin-sensitive enzymes of peptidoglycan synthesis as penicillin-binding proteins (PBPs) and identified the individual target enzymes inhibited by penicillin and other beta-lactam antibiotics when they kill bacteria. Subsequently, I worked on mechanisms of resistance to penicillin and identified the role of interspecies recombination and the formation of mosaic genes in the emergence of resistance in pneumococci, gonococci and meningococci, and became interested more generally in the extent of homologous recombination in bacteria and in their population and evolutionary biology.  We developed new tools for identifying strains of bacterial pathogens - particularly multi-locus sequence typing (MLST) - and have been interested in the concept of species in bacteria.