Editors' ChoiceTuberculosis

Delving Deeper into the Persistent Mysteries of Tuberculosis

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Science Translational Medicine  19 Oct 2011:
Vol. 3, Issue 105, pp. 105ec170
DOI: 10.1126/scitranslmed.3003305

Mycobacterium tuberculosis (Mtb), the bacterium that causes tuberculosis, remains a global scourge causing more than one million deaths annually. In some individuals with tuberculosis, Mtb enters a latent, clinically asymptomatic phase that may become reactivated years later as an infectious, symptomatic disease. Because latently infected individuals act as a reservoir for this pathogen, understanding how Mtb successfully survives in a hostile host environment is of central importance, particularly if powerful new drugs are to be developed. Now, Griffin and colleagues, using high-density mutagenesis and deep sequencing, have identified several new key bacterial genes whose products are involved in the sterol catabolic pathway that is critical for Mtb growth. Targeting these genes may provide a new therapeutic strategy for drug development.

During chronic infection, Mtb must adapt to the host microenvironment to maintain viability. In this regard, the procurement of carbon for energy is critical for the persistence of Mtb and is achieved by scavenging host cholesterol. Consonant with this observation, Griffin et al. sought to identify critical genes in the microbial sterol catabolic pathway that metabolize host cholesterol, and that when interrupted, affect mycobacterial viability. To do this, the investigators developed a transposon library in an annotated Mtb strain (H37RV) using the transposon himar1. The selection of this particular transposon allowed for high-density mutagenesis corresponding to one insertion every 100 base pairs of Mtb DNA. The library was then grown on media with different carbon sources, and DNA was extracted and amplified by using a deep sequencing approach. This strategy showed greater sensitivity for the identification of essential genes compared with more traditional approaches such as transcriptional profiling by microarray hybridization.

Using their method of transposon mutagenesis, deep sequencing, and a robust statistical approach to quantify the contribution of differentially expressed mutants, the investigators identified 96 genes that were important for growth of Mtb on the cholesterol medium. Although some of these genes mapped to the known region for cholesterol catabolism (Cho region), the majority (over 60%) were unexpectedly found in other parts of the mycobacterial genome. These new genetic regions have predicted catabolic functions in intermediary metabolism and sterol degradation.

Thus, with the latest technical and analytical approaches Griffin and colleagues have identified additional genes in the cholesterol catabolic pathway that may be required for the viability of Mtb during its latent phase. Although all of this work was done in vitro, the findings suggest that the principal metabolic adaptations required for persistence of Mtb are to be found in the sterol pathway. If these findings are confirmed in vivo, then there is clear translational potential for developing drugs that target these sterol-catabolizing enzymes and hopefully eradicate the latent form of Mtb.

J. E. Griffin et al., High-resolution phenotypic profiling defines genes essential for mycobacterial growth and cholesterol catabolism. PLoS Pathogens, 29 September 2011 (10.1371/journal.ppat.1002251). [Abstract]

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