A recent paper describes how the mutation of a single gene is sufficient to turn a harmless bacterium found in our gut into an invasive pathogen. Taken alone, this isn’t terribly surprising; many genes regulate the expression of other genes and some (“master” genes) act as switches that control a whole host of other genes.  The gene mutated in this study isn’t one of these “master” genes, though; it’s a structural gene and there’s a much more  intriguing reason for its broad impact on the bacterium.