THE TRANSMISSION DYNAMICS OF ANTIBIOTIC-RESISTANT BACTERIA - THE RELATIONSHIP BETWEEN RESISTANCE IN COMMENSAL ORGANISMS AND ANTIBIOTIC CONSUMPTION

We propose a mathematical model of the transmission dynamics of coloni zation by commensal bacteria within a human community subject to varyi ng levels of antibiotic use designed to control morbidity induced by p athogenic strains of the normally commensal organisms. Colonization is assumed not to induce morbidity in the majority of cases, and antibio tic use is assumed to be related to the arrival and growth of pathogen ic strains that give rise to infections including clinical symptoms of disease. In the absence of antibiotic resistance, the model shows how the pattern of antibiotic prescription and use can eliminate the non- pathogenic commensal strains from the host community if the fraction o f people taking antibiotics with a defined efficacy exceeds some criti cal level. The model is extended to take account of the evolution of a ntibiotic resistance in the commensal population. We assume resistance may be either plasmid-mediated or conferred by selection of low-level pre-existing mutants, and that resistant organisms may experience red uced reproductive fitness. Invasion of the host community by drug-resi stant commensals is possible ii. certain antibiotic prescribing patter ns pertain. We calculate these conditions in terms of the transmission parameter of the organism and the level of antibiotic prescription an d use. The model is employed to address the issues of how best to use antibiotics in populations harbouring resistant organisms, and when re sistant bacteria will out-compete sensitive strains.