Stress and microbial endocrinology: prospects for ruminant nutrition

The feed efficiency of ruminant meat and dairy livestock can be significantly influenced by factors within their living environments. In particular, events perceived by the animals as stressful (such as parturition, transport or handling) have been found to affect susceptibility to infection. It has been well documented that even minor stress such as weighing can result in an increase in colonisation and faecal shedding of enteric pathogens such as Salmonella enterica and Escherichia coli O157:H7. Such infections affect both ruminant overall health and therefore performance, and are a particular problem for the meat production industries. Prior explanations for stress enhancing the likelihood of infection is that activation of the sympathetic nervous system under stress leads to the release of neuroendocrine mediators such as the catecholamine stress hormones noradrenaline and adrenaline, which may impair innate and adaptive immunity. More recently, however, another equally compelling explanation, viewed through the lens of the newly recognised microbiological discipline of microbial endocrinology is that the myriad of bacteria within the ruminant digestive tract are as responsive to the hormonal output of stress as the cells of their host. Work from our laboratories has shown that enteric pathogens have evolved systems for directly sensing stress hormones. We have demonstrated that even brief exposure of enteric pathogens to physiological concentrations of stress hormones can result in massive increases in growth and marked changes in expression of virulence factors such as adhesins and toxins. Happy, less stressed ruminants may therefore be better-nourished animals and safer sources of meat. This article reviews evidence that stress, as well as affecting nutrition, in ruminants is correlated with increased risk of enteric bacterial infections, and examines the molecular mechanisms that may be at work in both processes.