Host acute stress responses and the regulation of C. jejuni virulence in the avian gut

Food producers strive to minimise the risks of food poisoning for consumers. The most important food-associated bacterium, which infects humans, is Campylobacter, which is naturally present in large numbers of raw poultry products. If Campylobacter could be eliminated from chickens on the farm it would have a significant impact on human health by preventing many thousands of cases of food poisoning each year. If control is to be achieved it is important that we better understand how Campylobacter infects chickens, how the animal host and the pathogen interact, and the impact that modern production methods have on this interaction. Environment and farming practices all influence the microbiology of the farm. Modern chicken production is highly intensive and even the presence of a visitor can cause stress to the animals. Our hypothesis is that increased levels of stress in chickens may lead to higher levels of Campylobacter in the birds and cause the bacterium to behave in a very different way from normal. The reasons for this are not clear, but may involve changes in the populations of protective bacteria in the gut, alterations in immunity, and increased pathogen fitness compromising the animal's ability to resist infection. Animals stressed or in pain release neurotransmitters throughout their body. For example, a stressed chicken being caught and transported to the slaughterhouse will have higher levels of neurotransmitters in its gut. The main neurotransmitter in this respect is noradrenaline and levels of this can increase rapidly during stress. Noradrenaline will affect the animal by altering the permeability of the gut cells and by changing fluid balance, but it will also change the nature and behaviour of pathogenic bacteria in the gut. For example, recent work at the University of Bristol has shown that Campylobacter grow much more quickly and are more motile in the presence of noradrenaline. Other research on Salmonella and E. coli showed that bacterial cell surface structures, important in animal infection, are also influenced by noradrenaline. The purpose of this work is to understand the effects of noradrenaline on Campylobacter, particularly on how the bacterium acquires iron, interacts with animal tissues, and how this affects the expression of genes important for infection. The work will explore the hypothesis that noradrenaline allows translocation of Campylobacter from the gut to other tissue and that it also makes the bacteria better able to survive in the farm environment. A better understanding of the interaction between Campylobacter and the stressed host could eventually enhance animal welfare and improve food safety.

Campylobacter jejuni is regarded as being a commensal in the chicken gut but work at the University of Bristol challenges this assumption. We have shown that broiler flocks with pre-existing enteric infections and birds subject to stress by either transport and/or thinning are significantly more likely to become infected with Campylobacter and to excrete higher numbers of the pathogen. What is not understood is why stress or infection should have such a marked impact on the susceptibility of chickens to Campylobacter and on the behaviour of the bacterium. The effects of stress on animals are multi-factorial but what is common to all is that their neurotransmitter levels will be raised. We have shown that one neurotransmitter, noradrenaline (NA), markedly stimulates the growth of C. jejuni, particularly in low iron environments. In fact, the stimulatory effects seen with C. jejuni can exceed those seen with either Salmonella or E. coli, which may reflect the very different behaviour of C. jejuni compared to other enteric pathogens. Campylobacter may be better able to grow in the intestine of an animal under stress because of raised intestinal NA levels. Greater pathogen numbers would result in increased bacterial translocation across the gut epithelium, although other factors may also be important. In low iron conditions, NA increases the number of cells of C. jejuni that are flagellated and this may affect virulence. NA also increases invasion of epithelial cells in vitro. This could be particularly important because ~5% of chickens on retail sale contain C. jejuni in deep muscle tissues, which could enhance survival during cooking. Gut permeability is also changed by stress in the animal, which may increase translocation of bacteria from the gut. In animals compromised by either stress and/or infection, C. jejuni may behave more like an avian pathogen in that cells become more tissue-invasive. The proposed work will identify the molecular basis for the effects of NA on C. jejuni, particularly on iron metabolism, interaction with gut tissues and the expression of genes important in the infection of humans and animals. We will identify the bacterial determinants that allow NA to change Campylobacter behaviour. The work will explore the hypothesis that invasion from the gut is more likely when NA is present because the bacteria are better equipped to cross the intestinal epithelium and survive in the submucosa. A better understanding of the interaction between Campylobacter and the stressed host could eventually enhance animal welfare and improve food safety. Improved understanding of the reasons for the NA-mediated switch from 'commensal' to 'pathogen' may also provide insights into virulence mechanisms in humans. The contaminated farm environment is the main source of Campylobacter entry into poultry flocks. The better bacteria survive in the environment the more likely they are to be transmitted between animal hosts. With this in mind, studies will also be carried out to determine whether Campylobacter cells, pre-exposed to NA, show increased environmental fitness because changes in iron metabolism increase the potential to resist oxidative stress, a major factor in environmental survival.