Associations of faecal microbiota with influenza-like illness in participants aged 60 years or older: an observational study

People aged 60 years or older are at high risk for respiratory infections, one of the leading causes of mortality worldwide. Vaccination is the main way to protect against these infections; however, vaccination is less effective in older adults than in younger adults due to ageing of the immune system, so innovative strategies that improve vaccine responses could provide a major public health benefit. The gut microbiota regulates host immune homoeostasis and response against pathogens, but human studies showing the effects of the gut microbiota on respiratory infections in older adults are sparse. We aimed to investigate the composition of the microbiota in relation to respiratory infections and local and systemic immune markers in older adults during an influenza season.In this observational study, participants were selected from an influenza-like illness (ILI) prospective surveillance cohort in which community-dwelling adults aged 60 years and older in the Netherlands were recruited through their general practitioner or the Civil Registry. Inclusion criteria have been described elsewhere. Participants completed questionnaires and self-reported symptoms. To measure microbiota composition, faecal samples were collected from participants registering an ILI event, with a follow-up (recovery) sample collected 7-9 weeks after the ILI event, and from asymptomatic participants not reporting any event throughout the season. We tested associations between microbiota profiles and a set of health-related variables, patient characteristics, and local and systemic immune markers. We cultured identified bacterial biomarkers for ILI with CaCo-2 cells in an in vitro intestinal epithelial model and measured the induced immune response. This study is registered with http://www.trialregister.nl, NL4666.Between Oct 1, 2014, and April 30, 2015, 2425 older adults were recruited into the ILI surveillance cohort. From Oct 1, 2014, to June 15, 2015, faecal samples were collected from 397 participants, of whom 213 (54%) reported an ILI event once throughout the season and 184 (46%) did not. 192 ILI participants recovered and provided follow-up samples. Microbiota composition was altered during an ILI event. The Bacteroidetes (mean relative abundance 17·51% [SD 11·41] in the ILI group and 14·19% [10·02] in the control group; adjusted p=0·014) and the Proteobacteria (3·40% [8·10] in the ILI group and 1·57% [3·69] in the control group; adjusted p=0·015) were more abundant in the ILI group than in the control group. The abundance of Ruminococcus torques was positively associated with ILI and the abundance of Escherichia/Shigella, negatively correlated with alpha diversity, and negatively co-occurred with beneficial taxa, including butyrate producers. R torques was associated with pro-inflammatory profiles, both locally in faeces and systemically in blood. ILI-associated taxa (R torques and Escherichia coli) had symbiotic effects on the cellular immune response when cultured together in an in vitro model.The abundances of specific bacteria could be used as potential biomarkers for susceptibility to respiratory infections and as targets for intervention in the ageing population.The Dutch Ministry of Health, Welfare and Sport, and the Strategic Program of the National Institute for Public Health and the Environment.