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TB is an airborne disease caused by the bacterium Mycobacterium tuberculosis (M. tuberculosis) (Figure 2.1). M. tuberculosis and seven very closely related mycobacterial species (M. bovis, M. africanum, M. microti, M. caprae, M. pinnipedii, M. canetti and M. mungi) together comprise what is known as the M. tuberculosis complex. Most, but not all, of these species have been found to cause disease in humans. In the United States, the majority of TB cases are caused by M. tuberculosis. M. tuberculosis organisms are also called tubercle bacilli | Chapter 2 Transmission and Pathogenesis of Tuberculosis Table of Contents Chapter Objectives.19 Introduction.21 Transmission of TB.21 Pathogenesis of TB.26 Drug-Resistant TB MDR and XDR .35 TB Classification System.39 Chapter Summary.41 References.43 Chapter Objectives After working through this chapter you should be able to Identify ways in which tuberculosis TB is spread Describe the pathogenesis of TB Identify conditions that increase the risk ofTB infection progressing to TB disease Define drug resistance and Describe the TB classification system. Chapter 2 Transmission and Pathogenesis of Tuberculosis 19 Introduction TB is an airborne disease caused by the bacterium Mycobacterium tuberculosis M. tuberculosis Figure 2.1 . M. tuberculosis and seven very closely related mycobacterial species M. bovis M. africanum M. microti M. caprae M. pinnipedii M. canetti and M. mungi together comprise what is known as the M. tuberculosis complex. Most but not all of these species have been found to cause disease in humans. In the United States the majority of TB cases are caused by M. tuberculosis. M. tuberculosis organisms are also called tubercle bacilli. Figure 2.1 Mycobacterium tuberculosis Transmission of TB M. tuberculosis is carried in airborne particles called droplet nuclei of 1-5 microns in diameter. Infectious droplet nuclei are generated when persons who have pulmonary or laryngeal TB disease cough sneeze shout or sing. Depending on the environment these tiny particles can remain suspended in the air for several hours. M. tuberculosis is transmitted through the air not by surface contact. Transmission occurs when a person inhales droplet nuclei containing M. tuberculosis and the droplet nuclei traverse the mouth or nasal passages upper respiratory tract and bronchi to reach the alveoli of the lungs Figure 2.2 . M. tuberculosis is carried in airborne particles called droplet nuclei of 1-5 microns in diameter. Infectious droplet nuclei are generated when persons who
