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Bronchiolitis and fecal flora

As the bronchiolitis season hits our wards, we will again be wondering why it is that some infants are severely affected while others, exposed to the same virus and same environment, get away with just a sniffle. A rather surprising answer might be: the bacteria in their gut. Researchers from the US gathered stool samples from 40 infants with bronchiolitis bad enough to need admission, and used sophisticated molecular techniques (16S rRNA gene sequencing) to characterise the profile of predominant bowel bacteria (Hasagawa K, et al. Pediatrics 2016. doi:10.1542/peds.2016-0218). These were compared with samples from 115 healthy age-matched controls. They identified four different profiles occurring in about equal quarters overall, dominated respectively by E. coli, Bifidobacterium, Enterobacter/Veiillonella, and Bacteroides species. Amongst the bronchiolitis group they found a significantly greater prevalence of the Bacteroides–dominant profile (44%; OR 4.6: 95% CI 1.6 to 15.5), and fewer with the Enterobacter-dominant profile (15%). Prevalences for the other two profiles did not differ. So can gut bacteria really influence what goes on in the lungs? In mice, studies have shown that disrupting the gut microbiota can alter responses to respiratory viruses through mechanisms involving CD4 and CD8 T cells. We already know that breastfeeding can protect against bronchiolitis, and part of this might be the effect of breast milk on the gut flora. These findings may be important in finding ways to prevent bronchiolitis, as prebiotics and probiotics can change infant gut flora substantially. However this study does not prove cause and effect: although they attempted to control for possible confounding factors, it remains possible that the association is simply a manifestation of some innate characteristic that both influences both gut flora and susceptibility to respiratory viruses.