BACTERIAL colonies must often cope with unfavourable environmental conditions^1,2. To do so, they have developed sophisticated modes of cooperative behaviour^3–10. It has been found that such behaviour can cause bacterial colonies to exhibit complex growth patterns similar to those observed during non-equilibrium growth processes in non-living systems¹¹; some of the qualitative features of the latter may be invoked to account for the complex patterns of bacterial growth^12–18. Here we show that a simple model of bacterial growth can reproduce the salient features of the observed growth patterns. The model incorporates random walkers, representing aggregates of bacteria, which move in response to gradients in nutrient concentration and communicate with each other by means of chemotactic 'feedback'. These simple features allow the colony to respond efficiently to adverse growth conditions, and …