31 Jul 2003 19:7 AR AR195-MI57-26.tex AR195-MI57-26.sgm LaTeX2e(2002/01/18) P1: GCE10.1146/annurev.micro.57.030502.091033Annu. Rev. Microbiol. 2003. 57:641–76doi: 10.1146/annurev.micro.57.030502.091033Copyrightc° 2003 by Annual Reviews. All rights reservedFirst published online as a Review in Advance on June 4, 2003THE SECRET LIVES OF THE PATHOGENICMYCOBACTERIAChristine L. Cosma,1David R. Sherman,2andLalita Ramakrishnan1,3Departments of1Microbiology,2Pathobiology, and3Medicine, University ofWashington, Seattle, Washington 98195; email: [email protected];[email protected]; [email protected] Words tuberculosis, latency, granuloma, virulence, macrophage■ Abstract Pathogenic mycobacteria, including the causative agents of tuberculo-sis and leprosy, are responsible for considerable morbidity and mortality worldwide. Ahallmark of these pathogens is their tendency to establish chronic infections that pro-duce similar pathologies in a variety of hosts. During infection, mycobacteria reside inmacrophages and induce the formation of granulomas, organized immune complexesof differentiated macrophages, lymphocytes, and other cells. This review summarizesour understanding of Mycobacterium–host cell interactions, the bacterial-granulomainterface, and mechanisms of bacterial virulence and persistence. In addition, we high-light current controversies and unanswered questions in these areas.CONTENTSINTRODUCTION ..................................................... 642THE CAST .......................................................... 643M. tuberculosis Complex .............................................. 643M. marinum and M. ulcerans ........................................... 644M. leprae .......................................................... 646M. avium Complex ................................................... 646FIRST ENCOUNTERS—THE MACROPHAGE ............................ 647Mycobacteria and Macrophages ........................................ 647Mycobacteria and Other Host Cells ..................................... 647Mycobacteria and Epithelial Cells ....................................... 648Entry into Macrophages ............................................... 648The Mycobacterium Phagosome ........................................ 649Host Proteins Associated with the Mycobacterium Phagosome ................ 649Mycobacterium Phagolysosome Fusion: In Vitro and In Vivo Studies ........... 650GRANULOMA FORMATION: HOST VERSUS PATHOGEN ................. 651Features of Mycobacterium Granulomas .................................. 651Caseous Necrosis .................................................... 6510066-4227/03/1013-0641$14.0064131 Jul 2003 19:7 AR AR195-MI57-26.tex AR195-MI57-26.sgm LaTeX2e(2002/01/18) P1: GCE642 COSMA¥SHERMAN¥RAMAKRISHNANLymphocytes in Mycobacterium Granulomas ..............................652Influence of Specific Mycobacterium Virulence Determinants onGranuloma Composition ............................................. 653BACTERIAL PERSISTENCE ........................................... 653Latent Tuberculosis, Reactivation, and Reinfection .........................654Location of Bacteria During Latency .................................... 655State of the Bacteria During Latency .................................... 655Animal Models .....................................................657In Vitro Models ..................................................... 658BACTERIAL FACTORS INVOLVED IN PATHOGENESIS ................... 659Expression Screens .................................................. 660Virulence Screens ................................................... 660Virulence Determinants ............................................... 661Regulatory Mutants .................................................. 661Mycobacterial Cell Envelope and Virulence ............................... 663Surface and Secreted Proteins .......................................... 664Use of Comparative Genomics in Virulence Determination ...................665CONCLUDING REMARKS ............................................ 666INTRODUCTIONWe dance around in a ring and supposeThe Secret sits in the middle and knowsRobert FrostThe majority of the >50 species that comprise the genus Mycobacterium are non-pathogenic environmental bacteria related closely to the soil bacteria Streptomycesand Actinomyces. However, a few species are highly successful pathogens, includ-ing Mycobacterium tuberculosis, M. leprae, and M. ulcerans, the causative agentsof tuberculosis, leprosy, and Buruli ulcers, respectively. Their key to success liesat least in part with their ability to establish residence and proliferate inside hostmacrophages despite the antimicrobial properties of these cells (41, 52). The hostmounts a complex immune response involving both innate and adaptive compo-nents that often sequesters the pathogen in organized structures called granulomas.However, the pathogenic mycobacteria are extraordinarily adept at establishinglong-term infections that can manifest as acute or chronic disease or be clinicallyasymptomatic with the potential to resurface later. Understanding the factors thatcontribute to this long and complex relationship between pathogen and host isessential to our ability to modulate its clinical outcomes.At least eight Mycobacterium genome-sequencing projects are at or nearcompletion (http://www.tigr.org/; http://www.sanger.ac.uk/). Comparison of thesegenomes will likely reveal important clues regarding the evolution and pathogenicmechanisms of mycobacteria. This review highlights some of the interesting fea-turesandprevalent beliefsabout tuberculosisand othermycobacterioses. Ourcom-parative approach focuses on the importance of in vivo studies, and we discuss31 Jul 2003 19:7 AR AR195-MI57-26.tex AR195-MI57-26.sgm LaTeX2e(2002/01/18) P1: GCEMYCOBACTERIUM-HOST INTERACTIONS 643mycobacterial diseases in the context of their host range, pathology, and pathogen-esis. We highlight commonalities and differences that potentiate understanding oftheir pathogenic mechanisms, describe recent studies of mycobacterial virulencefactors that illuminate mechanisms of pathogenesis, and concentrate on recentadvances and current outstanding issues.THE CASTM. tuberculosis ComplexThe M. tuberculosis complex consists of M. tuberculosis, M. africanum, M. canet-tii, M. bovis, and M. microti, which are closely related organisms sharing >99%identity at the nucleotide level