Experimental Evolution of Drug Resistance for Antibiotic Target Discovery in Chlamydia trachomatis

The recent application of experimental and computational drug discovery workflows has identified hundreds of new molecules that display potent, and in some cases highly selective, activity against bacteria of the genus Chlamydia. The full value of these novel antichlamydial molecules, however, depends critically on our ability to determine their molecular targets and modes of action. A particularly powerful and widely applied strategy for identifying candidate targets involves selecting for bacterial mutants that have acquired resistance to a compound's inhibitory activity, followed by determining the mutations responsible for this resistance. Such mutations often pinpoint the compound's direct target or reveal cellular pathways that modulate target engagement. Here, we present a detailed, step-by-step protocol for applying this approach to the clinically important, human-pathogenic species Chlamydia trachomatis. The workflow includes: (1) determining compound potency by quantitative dose-response analysis, (2) generating a resistant C. trachomatis mutant through serial passaging under progressively increasing selective pressure, (3) confirming the resistance phenotype, and (4) isolating bacterial genomic DNA for whole-genome sequencing to identify resistance-associated mutations and facilitate downstream target discovery.