Mycobacterial Acid Tolerance Enables Phagolysosomal Survival and Establishment of Tuberculous Infection In Vivo.

The blockade of phagolysosomal fusion is considered a critical mycobacterial strategy to survive in macrophages. However, viable mycobacteria have been observed in phagolysosomes during infection of cultured macrophages, and mycobacteria have the virulence determinant MarP, which confers acid resistance in vitro. Here we show in mice and zebrafish that innate macrophages overcome mycobacterial lysosomal avoidance strategies to rapidly deliver a substantial proportion of infecting bacteria to phagolysosomes.

Mycobacteria manipulate macrophage recruitment through coordinated use of membrane lipids.

The evolutionary survival of Mycobacterium tuberculosis, the cause of human tuberculosis, depends on its ability to invade the host, replicate, and transmit infection. At its initial peripheral infection site in the distal lung airways, M. tuberculosis infects macrophages, which transport it to deeper tissues.

An in vivo platform for rapid high-throughput antitubercular drug discovery.

Treatment of tuberculosis, like other infectious diseases, is increasingly hindered by the emergence of drug resistance. Drug discovery efforts would be facilitated by facile screening tools that incorporate the complexities of human disease. Mycobacterium marinum-infected zebrafish larvae recapitulate key aspects of tuberculosis pathogenesis and drug treatment.

Drug tolerance in replicating mycobacteria mediated by a macrophage-induced efflux mechanism.

Treatment of tuberculosis, a complex granulomatous disease, requires long-term multidrug therapy to overcome tolerance, an epigenetic drug resistance that is widely attributed to nonreplicating bacterial subpopulations. Here, we deploy Mycobacterium marinum-infected zebrafish larvae for in vivo characterization of antitubercular drug activity and tolerance. We describe the existence of multidrug-tolerant organisms that arise within days of infection, are enriched in the replicating intracellular population, and are amplified and disseminated by the tuberculous granuloma.

Trafficking of superinfecting Mycobacterium organisms into established granulomas occurs in mammals and is independent of the Erp and ESX-1 mycobacterial virulence loci.

Although tuberculous granulomas, which are composed of infected macrophages and other immune cells, have long been considered impermeable structures, recent studies have shown that superinfecting Mycobacterium marinum traffic rapidly to established fish and frog granulomas by host-mediated and Mycobacterium-directed mechanisms. The present study shows that superinfecting Mycobacterium tuberculosis and Mycobacterium bovis bacille Calmette-Guérin similarly home to established granulomas in mice. Furthermore, 2 prominent mycobacterial virulence determinants, Erp and ESX-1, do not affect this cellular trafficking.

Zebrafish and frog models of Mycobacterium marinum infection.

Mycobacterium marinum infection of poikilothermic animals, such as fish and frogs, results in chronic granulomatous diseases that bear many similarities to mycobacterioses in mammals, including tuberculosis. This unit describes three animal models of M. marinum infection that can be used to study basic aspects of Mycobacterium-host interactions and granuloma development, as well as trafficking of immune cells in host tissues.

Mycobacterium marinum Erp is a virulence determinant required for cell wall integrity and intracellular survival.

The Mycobacterium tuberculosis exported repetitive protein (Erp) is a virulence determinant required for growth in cultured macrophages and in vivo. To better understand the role of Erp in Mycobacterium pathogenesis, we generated a mutation in the erp homologue of Mycobacterium marinum, a close genetic relative of M. tuberculosis.

Superinfecting mycobacteria home to established tuberculous granulomas.

A central paradox of tuberculosis immunity is that reinfection and bacterial persistence occur despite vigorous host immune responses concentrated in granulomas, which are organized structures that form in response to infection. Prevailing models attribute reinfection and persistence to bacterial avoidance of host immunity via establishment of infection outside primary granulomas. Alternatively, persistence is attributed to a gradual bacterial adaptation to evolving host immune responses.

The secret lives of the pathogenic mycobacteria.

Pathogenic mycobacteria, including the causative agents of tuberculosis and leprosy, are responsible for considerable morbidity and mortality worldwide. A hallmark of these pathogens is their tendency to establish chronic infections that produce similar pathologies in a variety of hosts. During infection, mycobacteria reside in macrophages and induce the formation of granulomas, organized immune complexes of differentiated macrophages, lymphocytes, and other cells.