Lifestyle can be viewed as the environment surrounding an organism and the relationships that it establishes with other species. It is one of the driving forces that contribute to the final shape of bacterial genomes. To assess how these forces affect global cellular functions, we investigated the fraction of the genome devoted to transcription-related proteins, small-molecule metabolism enzymes, and transport, for 60 bacterial genomes classified by lifestyle. Larger genomes were found to harbour more transcription factors per gene than smaller ones. In addition, free-living bacteria (with a few exceptions) are clearly enriched for transcription factors, beyond the expected proportion based on their genome size. This suggests that under complex conditions, gene expression regulation and signal integration have been strongly selected for to enable rapid adaptation to environmental conditions.