Expression profiles of genes related to stress responses, substrate assimilation, acetate metabolism, and biosynthesis were obtained by monitoring growth of Escherichia coli MG1655 in Luria-Bertani (LB) medium with transcriptional microarrays. Superimposing gene expression profiles on a plot of specific growth rate demonstrates that the cells pass through four distinct physiological states during fermentation before entering stationary phase. Each of these states can be characterized by specific patterns of substrate utilization and cellular biosynthesis corresponding to the nutrient status of the medium. These data allow the growth phases of the classical microbial growth curve to be redefined in terms of the physiological states and environmental changes commonly occurring during bacterial growth in batch culture on LB medium.