Pigs from these cleaner environments also had much smaller proportions of bacteria from the Lactobacillaceae.
Kelly's team also found that the differences in gut microbes affected the expression of genes associated with the piglets' immune systems. Animals raised in the isolated environment expressed more genes involved in inflammatory immune responses and cholesterol synthesis, whereas genes linked with infection-fighting T cells were expressed in the outdoor-bred pigs.
Glenn Gibson, a food microbiologist at the University of Reading, UK, says that previous studies have suggested that immune responses are linked to organisms in the gut. "This study takes a step forwards by tallying the gene-expression response into this," he says. However, he adds, because the study was carried out in pigs, there is no way to be certain that the results are relevant to humans.
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Jonathan Rhodes, a gastroenterologist at the Royal Liverpool University Hospital in Liverpool, UK, points out that people with chronic inflammation of the digestive tract, known as Crohn's disease, have reduced numbers of Firmicutes, as did the cleaner piglets. But, he adds, patients with Crohn's also have reduced overall bacterial diversity, similar to the outdoor pigs, suggesting that the results might not extrapolate directly to human disease.
Kelly argues, however, that the comparable organ sizes of humans and pigs, and the similarities between the microorganisms found in their guts, makes pigs good model animals for such studies