Summary Many of the substrates available as energy sources for microorganisms in the human colon, including dietary plant fibre and secreted mucin, are insoluble. It seems likely that such insoluble substrates support a specialized microbiota, and in order to test this hypothesis, faecal samples from four healthy subjects were fractionated into insoluble (washed particulate) and liquid fractions. Analysis of 1252 PCR‐amplified...