Biology of Mtb in non-replicating states. Most studies of bacterial antimicrobial resistance (AMR) deal with heritable resistance. We also study phenotypic AMR, sometimes called phenotypic tolerance-- a context-dependent, non-heritable form of AMR. We have distinguished two broad classes of phenotypic AMR. These pertain when the majority of bacteria in the population under study are replicating (class I) or non-replicating (class II). We have further distinguished two types of class II phenotypic AMR. In class IIa phenotypic AMR, the non-replicating bacteria, when removed from growth-suppressive conditions, can grow as colony-forming units (CFU) on agar plates—the gold standard in microbiology for counting viable bacteria. In class IIb phenotypic AMR, the bacteria cannot grow as CFU, but their viability and potential for lethality can be proved in other ways. Such “differentially detectable” (DD) Mtb confound efforts to design and monitor antimicrobial therapy. We are working to characterize DD Mtb and identify compounds that can kill them.