Caseum: a Niche for Mycobacterium tuberculosis Drug-Tolerant Persisters

Abstract

Caseum, the central necrotic material of tuberculous lesions, is a reservoir of drug-recalcitrant persisting mycobacteria. Caseum is found in closed nodules and in open cavities connecting with an airway. Several commonly accepted characteristics of caseum were established during the preantibiotic era, when autopsies of deceased tuberculosis (TB) patients were common but methodologies were limited. These pioneering studies generated concepts such as acidic pH, low oxygen tension, and paucity of nutrients being the drivers of nonreplication and persistence in caseum. Here we review widely accepted beliefs about the caseum-specific stress factors thought to trigger the shift of Mycobacterium tuberculosis to drug tolerance. Our current state of knowledge reveals that M. tuberculosis is faced with a lipid-rich diet rather than nutrient deprivation in caseum. Variable caseum pH is seen across lesions, possibly transiently acidic in young lesions but overall near neutral in most mature lesions. Oxygen tension is low in the avascular caseum of closed nodules and high at the cavity surface, and a gradient of decreasing oxygen tension likely forms toward the cavity wall. Since caseum is largely made of infected and necrotized macrophages filled with lipid droplets, the microenvironmental conditions encountered by M. tuberculosis in foamy macrophages and in caseum bear many similarities. While there remain a few knowledge gaps, these findings constitute a solid starting point to develop high-throughput drug discovery assays that combine the right balance of oxygen tension, pH, lipid abundance, and lipid species to model the profound drug tolerance of M. tuberculosis in caseum.

Keywords: caseum; foamy macrophages; granuloma; hypoxia; intracellular lipophilic inclusions; necrosis; persistence; phenotypic drug resistance; tuberculosis.

FIG 1

Bacterial burden and growth kinetics of M. tuberculosis in ex vivo rabbit caseum. Bacterial burdens on day 1 and day 8, 22, or 29 after excision of caseum from 13 rabbit cavities are shown. M. tuberculosis in caseum appears to be nonreplicating over the whole incubation period. Adapted from the work of Sarathy et al. (13).

FIG 2

The caseous compartments of necrotic granulomas contain extracellular M. tuberculosis. This subpopulation adapts to local stresses such as low oxygen tension and high lipid content, resulting in the accumulation of intrabacterial lipid inclusions, a shift to a state of nonreplication, and phenotypic drug resistance. Created with BioRender.