Researchers have discovered that a mutation in the cellulose making machinery of E. coli bacteria allows them to cause severe disease in people -- 'good' bacteria make cellulose and 'bad' bacteria can't. The mutations stopped the E. coli making the cell-surface carbohydrate cellulose and this led to increased inflammation in the intestinal tract of the host, resulting in a breakdown of the intestinal barrier, so the bacteria could spread through the body. Understanding how bacteria spread from intestinal reservoirs to the rest of the body is important in preventing infections and tackling antibiotic resistance.