A new report by Krašovec et al. has shown that the rate of mutation in Escherichia coli to rifampicin resistance is inversely correlated with population density, and that lowering population density can increase mutation rates by up to threefold. The ability of E. coli to sense surrounding population densities is dependent on the luxS gene, and bacteria with deletions of this gene show no change in mutation rate even when exposed to variations in surrounding population densities. This study provides insights into how mutation rates are controlled in bacteria and has implications for studies of antibiotic resistance.