Nat. Med. https://doi.org/10.1038/s41591-019-0469-4 (2019)

In the midst of a rising tide of asthma, growing up on a rural farm is known to offer a strong protective effect, due in large part to the distinctive environmental microbiota there that alters lung immunity. In Nature Medicine, Kirjavainen et al. undertake a prospective non-interventional study to determine whether a ‘farm-like’ microbiota in an urban environment can also be protective. They develop an index, FaRMI, that describes how ‘farm-like’ the indoor environmental microbiota is during an early childhood window. In two urban cohorts from Finland and Germany, a ‘farm-like’ microbiota in an indoor setting also significantly protects children from developing asthma. The protective effect is independent of absolute bacterial richness or load, and ‘farm-like’ fungi do not seem to offer protection. Instead, alterations in select, potentially modifiable bacterial species are probably responsible for the protective effect.