Alberta

As the intensity of livestock production increases, so too does the importance of identifying possible relationships between common industry practices and factors that pose a risk to human health. Rumensin (monensin sodium) is the ionophore most commonly fed to cattle to improve feed efficiency (1). Because the implementation 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 material 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 survival, 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 μg/mL of monensin, and viable numbers were determined after 0, 6, 12, 24, and 48 h of incubation at 37°C. Survival was expressed as percentage of survival in respective controls. Data were fit to a randomized block design and evaluated by analysis of variance (General Linear Model, SAS for Windows; SAS Institute, Cary, North Carolina, USA). Treatment effects (P < 0.05) were identified by using the least significant difference (LSD) test.

At concentrations of 5, 15, or 25 μg/mL, monensin did not affect growth or survival of E. coli O157:H7 strain 3081 as compared with the 0 μg/mL controls; 98.2% to 100% survival was exhibited after 48 h (P > 0.05). At 50 μg/mL, monensin significantly reduced the growth of E. coli O157:H7 strain 3081. After 24 and 48 h, survival rates were 77.8% and 58.0%, respectively (P < 0.05). Monensin did not affect growth of E. coli 25922 at any of the levels tested (P > 0.05). Monensin fed to cattle at recommended levels (5.5 to 33 mg/kg diet dry matter (DM)) would attain ruminal concentrations of 5 to 10 μg/mL, assuming 70 kg ruminal content and DM intake of 12 kg/d (4).

It has been suggested that the inhibitory effect of ionophores on gram-positive populations in the rumen allows gram-negative populations to flourish when these compounds are fed (5). The common hypothesis is that the bacterial outer membrane dictates the pattern of ionophore resistance, but conflicting observations have been reported. Some studies have indicated that some gram-negative species are not resistant to high levels of ionophores (4), as was evident in the present study. Adaptation to ionophores by initially susceptible gram-negative bacteria has also been reported, as well as development of resistance by gram-positive bacteria (5). These findings imply that the presence of an outer membrane is not the only criterion for ionophore resistance among bacteria.

Monensin-mediated inhibition of gram-positive organisms in the rumen may provide a selective advantage, but this may or may not carry through to include E. coli O157:H7. Other gram-negative organisms, such as Fibrobacter succinogenes, have been reported to grow well in the presence of monensin (6) and may outcompete organisms such as E. coli O157:H7. The survival of E. coli O157:H7 in the rumen and its subsequent introduction into the hindgut depend upon successful competition for nutrients with the commensal anaerobes in the rumen (5). While it may be possible that ionophores alter the microflora of the rumen in ways that may give E. coli O157:H7 a selective advantage, these effects may be subtle and indirect.

Monensin did not enhance the growth of E. coli in pure culture settings. This study revealed monensin to affect E. coli O157:H7 only at 50 μg/mL, at which concentration survival of the organism was impaired. However, the study does not consider the effects on ruminal microbial ecology, particularly how monensin can change community dynamics in a manner that enhances toxigenic E. coli. Thus, E. coli O157:H7 populations in the rumen would likely not be directly affected by monensin included as a feed supplement, but the indirect effect(s), positive or negative, of this ionophore inhibiting gram-positive ruminal bacteria should not be dismissed. Mixed cultures studies are necessary to evaluate interspecies effects of monensin on the numerous strains of E. coli O157:H7 that occur in the ruminal microflora. Subsequent controlled in vivo studies would then be necessary to determine any association between the feeding of monensin and fecal shedding of E. coli O157:H7 by cattle.