David Whitlock has not showered or bathed for 15 years, yet he does not have body odour. "It was kind of strange for the first few months, but after that I stopped missing it," he says. "If I get a specific part of my body dirty, then I'll wash that specific part" – but never with soap. As well as germs, soap gets rid of the skin's protective oils and alters its pH level. Although Whitlock appreciated gaining an extra 15 minutes a day from soap-dodging, his primary motivation was to encourage friendly microbes to live on him in symbiotic harmony. The bacteria get to feast on the ammonia from his sweat and he gets low-maintenance, balanced skin.

Just as awareness of the importance of the gut microbiome has led to a boom in probiotic and fermented foods and supplements, there is increasing interest in our skin microbiome: the trillions of microbes that protect us from pathogens and keep us healthy by making vitamins and other useful chemicals.