RAPPORT DES MALADIES

Effect of monensin on survival and growth of Escherichia coli O157:H7 in vitro A s the intensity of livestock production increases,so too does the importance of identifying possible relationships between common industry practices and fac-tors that pose a risk to human health. Rumensin (mon-ensin sodium) is the ionophore most commonly fed to cattle to improve feed efficiency (1). Because the imple-mentation of ionophore feeding occurred just prior to the first human cases of Eschericha coli O157:H7 foodborne illness in 1982, concerns arose that ionophores may select for E. coli O157:H7 in the bovine gut (1). Transference of E. coli O157:H7 to beef most likely arises from contamination of carcasses with fecal mate-rial at slaughter and processing (2). A recent survey of feedlot management practices reported a tentative association between the prevalence of E. coli O157:H7 and the use of ionophores in cattle rations (3), but no controlled studies in cattle to date have established a direct cause-and-effect relationship between ionophore use and fecal shedding of E. coli O157:H7. Moreover, the effect of monensin on the growth, or survival, or both of E. coli O157:H7 has not been determined. A pure culture study was conducted to investigate whether monensin might be enhancing the growth, or sur-vival, or both of the pathogen E. coli O157:H7. An E. coli O157:H7 strain (3081) and a non-O157:H7 strain (ATCC 25922) were inoculated (1 105 cfu/mL) separately into triplicate tubes of brain-heart infusion broth containing 0, 5, 15, 25, or 50 g/mL of monensin, and viable num-bers were determined after 0, 6, 12, 24, and 48 h of incu-bation at 37°C. Survival was expressed as percentage of survival in respective controls. Data were fit to a ran-domized block design and evaluated by analysis of variance (General Linear Model, SAS for Windows