Caseum: a Niche for Mycobacterium tuberculosis Drug-Tolerant Persisters

Abstract

SUMMARY 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.

One Size Fits All? Not in In Vivo Modeling of Tuberculosis Chemotherapeutics

“…Rabbits, with their large lungs that can host a large number of lesions without significant health effects, have proven to be highly useful in evaluating drug distribution, lesion penetration and cellular accumulation of drugs in different lesion types using laser microdissection and high sensitivity MALDI-MRM-MS imaging methods ( Hudock et al., 2014 ; Sarathy and Dartois, 2020 ). Through these technologies, we have learned the location of the bacteria in the lesions though their lipid content and that MOX may preferentially accumulate in foamy macrophages ( Blanc et al., 2018a ; Blanc et al., 2018b ).…”

“…After decades of little activity, numerous compounds, new drug candidates, and possible combinations are being pushed forward for assessment: the question is how to best evaluate the compounds that advance into in vivo testing ( ) ( Libardo et al., 2018 ; Tornheim and Dooley, 2019 ; Huszár et al., 2020 ). Additional factors to be considered are the complex pathology characteristics of human disease and that successful treatment may rely both on compound distribution into regions of disease and the differential abundance and susceptibility of the bacteria in these diverse regions ( Kaplan et al., 2003 ; Prideaux et al., 2015 ; Hunter, 2016 ; Hunter, 2018 ; Strydom et al., 2019 ; Sarathy and Dartois, 2020 ). The characteristics of useful in vivo models for evaluating new compounds include: development of relevant disease through a low dose Mycobacterium exposure; development of a persistent infection with both intracellular and extracellular bacteria; development of heterogenous lesion pathology, including granulomas and pneumonia with regions of necrosis with relatively low bacterial loads and cavities with highly heterogenous bacterial loads ( Kaplan et al., 2003 ; Hunter, 2016 ); as well as being amenable to methods for assessing the state of the infection in situ .…”

Antibiotic tolerance

“…M . tuberculosis ’ high tolerance is likely a consequence of its extremely slow growth rate in the lung [ 36 ]. Antibiotic tolerance also appears to determine infection outcomes in other bacterial species.…”

Pathogenesis of Human Immunodeficiency Virus-Mycobacterium tuberculosis Co-Infection

“…As Mtb infection progresses, granulomas can undergo necrosis, creating a cheese-like substance called caseum that is extremely difficult to penetrate with antibiotics [43]. The intracellular conditions of lipid-laden foamy macrophages provide both nutrition and an environment that is suitable for Mtb bacilli survival [44].…”

“…Interestingly, HIV+ Mtb patients consistently have lower frequencies of cavitation suggesting CD4+ T cells likely mediate this process [2]. While the hypoxic environment of caseum inhibits Mtb proliferation, cavitation expansion into the lung's airways creates an oxygen-rich environment that favors Mtb growth and dissemination from the primary granuloma [43,48].…”