The influence of melatonin on growth of E. coli O157:H7 in pure culture and exogenous melatonin on faecal shedding of E. coli O157:H7 in experimentally infected wethers.

Aims: To determine if exogenous melatonin (MEL) influences growth of Escherichia coli O157:H7 in pure culture and if MEL affects faecal shedding patterns of E. coli O157:H7 or total leucocyte counts in sheep.
Methods and Results: Two strains of E. coli O157:H7 were cultured in the presence of varying concentrations of MEL. Maximal specific growth rates of E.coli O157:H7 strains were not affected by MEL addition in pure culture. Wethers (n = 16) received either 0 (CONT) or 25 mg MEL hd(-1) day(-1) for 21 days. Daily shedding patterns of E. coli O157:H7 were not different (P > 0.10) between groups with faecal populations of E. coli O157:H7 decreasing daily (P < 0.01) in both groups. However, shedding tended to differ between the control and treated group by the end of the experiment. Total WBC and differential leucocyte counts were not affected by treatment.
Conclusions: Melatonin had no affect on specific growth rates in pure culture nor did the administration of exogenous MEL alter bacterial shedding patterns or immune response indicators in experimentally infected wethers exposed to a long photoperiod.
Significance and Impact of the Study: Although MEL did not affect shedding patterns or gastrointestinal populations of E. coli O157:H7, the tendency for MEL-treated sheep to shed less E. coli O157:H7 towards the end of the experiment warrants further research. Providing MEL for a longer period of time, or at greater concentrations, may elucidate a potential role that MEL plays in the seasonal shedding patterns of E. coli O157:H7 in livestock.