Can you improve your gut flora by picking up bacteria while playing outdoors?

None of the available studies show that your gut flora concretely improves from playing outside or from contact with outdoor soil. The idea that you pick up 'good bacteria' while gardening or walking outdoors is popular, but it is not supported by the current literature.

House dust and outdoor soil do contain a rich mix of micro-organisms, sometimes 500 to 1,000 different species. Whether that turns out to be a net benefit or a net harm for your gut microbiome remains unclear. The available studies are observational: they identify associations but cannot establish cause and effect.

There are also clear risks. Outdoor soil and house dust are confirmed sources of Clostridium botulinum spores, the bacterium that causes infant botulism. This is a serious, life-threatening condition. For babies, exposure to outdoor bacteria is therefore certainly not harmless.

Air pollution adds further nuance to the picture. Exposure to polluted outdoor air is associated in both animal and human studies with a disrupted gut microbiome, although findings in humans are not yet consistent. 'Being outside' is therefore no guarantee of a healthier microbiome; how clean the air is plays a major role.

Early exposure to a varied microbial environment does appear to have some effect on the development of the gut microbiome and on the risk of allergies and asthma. But here too the evidence is indirect and observational, and it concerns the early years of life, not the deliberate 'inhaling of bacteria' as an adult.