The innate ability of microorganisms to respond rapidly to changes in the growth environment has been termed ‘reactivity’ and is a property of most microbial species. This feature of microbial physiology is of importance in the culturing of microbial cells on a large scale where environmental heterogeneity is unavoidable and gradients of temperature, pH, oxygen and nutrient concentration can occur. To assess the consequences of such gradients, a study was made of the influence of a discontinuous addition of glucose to a glucose-limited chemostat culture of Klebsiella aerogenes. Results revealed a marked and unexpected shift in metabolism with an increased capacity for glucose uptake and respiration. This increased reactivity enhanced the ability of K. aerogenes to compete with other species (e.g. Escherichia coli) in chemostat culture.