Conventional wisdom holds that exposure to dirt and germs is bad for one's health: after all, 'cleanliness is next to godliness.' More recently, the high prevalence of immune-mediated diseases in industrialized countries (with relatively high standards for cleanliness and sanitation) has called this notion into question. The 'hygiene hypothesis' proposes that the increasing incidence of immune-related disorders results from reduced exposure to microbes in early childhood. Indeed, previous research has shown that animals that are susceptible to autoimmune disease are more likely to develop disease (and their disease is more severe) when they are raised in germ-free conditions than when they are exposed to bacteria. Now, a new study by Denise Kelly (University of Aberdeen, UK) and colleagues establishes a direct link between microbe exposure and immune health.

Credit: Eric Isselée

Kelly's study assessed the effects of early life environment on gut microbes and immune health in piglets. Piglets were housed in one of three environments: outdoors, indoors or in isolators. Piglets housed in isolators were also treated daily with antibiotics. When the piglets were 5, 28 and 56 days old, their gut microbial communities were evaluated. In piglets that were housed outdoors, the gut microbiota was dominated by bacteria of the phylum Firmicutes, especially Lactobacillus (BMC Biol. published online 20 November 2009; doi:10.1186/1741-7007-7-79). Lactobacillus bacteria have health-promoting effects and are known to limit the proliferation of potentially pathogenic microbes like Escherichia coli and salmonella in the gut. In contrast, piglets that were housed indoors in hygienic conditions had fewer Lactobacillus and more potentially pathogenic bacteria.

The researchers also examined the expression of genes associated with immune function in the three groups of piglets. In piglets housed in isolators, expression of genes involved in inflammatory immune responses and cholesterol synthesis was elevated. In piglets housed outdoors, however, expression of genes associated with T cells was elevated. These results suggest that the piglets housed outdoors had a more stable and tolerant immune system than did those housed indoors.

This study is the first to demonstrate a causal link between living environment and immune response. Although it is not known how closely the piglets' physiology represents human physiology, Kelly believes that the similarities in the microbial communities of pigs and humans and their comparable sizes make pigs a good model.