Essential genes are the hubs of cellular networks, but lack of high-throughput methods for titrating gene expression has limited our understanding of the fitness landscapes against which their expression levels are optimized. We developed a modified CRISPRi system leveraging the predictable reduction in efficacy of imperfectly matched sgRNAs to generate defined levels of CRISPRi activity and demonstrated its broad applicability. Using libraries of mismatched sgRNAs predicted to span the full range of knockdown levels, we characterized the expression-fitness relationships of most essential genes in Escherichia coli and Bacillus subtilis. We find that these relationships vary widely from linear to bimodal but are similar within pathways. Notably, despite ∼2 billion years of evolutionary separation between E. coli and B. subtilis, most essential homologs have similar expression-fitness relationships with rare but informative differences. Thus, the expression levels of essential genes may reflect homeostatic or evolutionary constraints shared between the two organisms. Display omitted •A universal bacterial system for titrating CRISPRi using partially mismatched sgRNAs•Determined expression-fitness relationships of E. coli and B. subtilis essentialome•Expression-fitness relationships are shared within pathways and between homologs•Shared homeostatic constraints underlie the optimization of essential gene expression Hawkins and Silvis et al. develop a system for predictably titrating gene expression in bacteria by introducing specific mismatches into CRISPRi sgRNAs. Mismatched sgRNAs enable multiple knockdown levels across many genes in a single experiment. They use this technique to determine the expression-fitness curves of all essential genes in Escherichia coli and Bacillus subtilis, finding that they are shared within pathways and between homologs diverged by ∼2 billion years.