Ten weeks after treatment started the children’s Prevotella levels had multiplied 712-fold. In addition, those of another species, Bifidobacterium, had quadrupled. Bifidobacterium is what is known as a “probiotic” organism—something that acts as a keystone species in the alimentary ecosystem, keeping the mixture of gut bacteria healthy. Now, two years later, although levels of Prevotella have fallen back somewhat, they are still 84 times higher than they were before the experiment started. Levels of Bifidobacterium, meanwhile, have gone up still further—being five times higher than they had been at the beginning of the study. This, says Dr Krajmalnik-Brown, suggests the children’s guts have become healthy environments that can recruit beneficial microbes by themselves.

Crucially, these changes in gut bacteria have translated into behavioural changes. Even 18 weeks after treatment started the children had begun showing reduced symptoms of autism. After two years, only three of them still rated as severe, while eight fell below the diagnostic cut-off point for ASD altogether. These eight thus now count as neurotypical.