A simple mathematical model of bacterial transmission within a hospital was
used to study the effects of measures to control nosocomial transmission o
f bacteria and reduce antimicrobial resistance in nosocomial pathogens. The
model predicts that: (i) Use of an antibiotic for which resistance is not
yet present in a hospital will be positively associated at the individual l
evel (odds ratio) with carriage of bacteria resistant to other antibiotics,
but negatively associated at the population level (prevalence). Thus infer
ences from individual risk factors can yield misleading conclusions about t
he effect of antibiotic use on resistance to another antibiotic. (ii) Nonsp
ecific interventions that reduce transmission of all bacteria within a hosp
ital will disproportionately reduce the prevalence of colonization with res
istant bacteria. (iii) Changes in the prevalence of resistance after a succ
essful intervention will occur on a time scale of weeks to months, consider
ably faster than in community-acquired infections. Moreover, resistance can
decline rapidly in a hospital even if it does not carry a fitness cost. Th
e predictions of the model are compared with those of other models and publ
ished data. The implications for resistance control and study design are di
scussed, along with the limitations and assumptions of the model.