Mycobacterium tuberculosis and Other Mycobacteria | Kevin P. Fennelly Jerrold

The genus Mycobacterium is the only member of the family Mycobacteriaceae and consists of slow-growing, rod-shaped, non–spore-forming, nonmotile organisms that are acid fast; that is, once stained, they resist destaining by acid alcohol treatment. This characteristic is attributed to their cell walls that have unusually high content of lipids (25%), in contrast to 0.5% for gram-positive bacteria and 3% for gram-negative bacteria. Mycolic acids are the principal components of the complex cell walls (1), some of which contribute to virulence. Wax D, a complex of peptides, polysaccharides, and mycolic acid, enhances cell-mediated immune responses against mycobacterial proteins. Cord factor is lethal to mice and inhibits polymorphonuclear leukocyte migration. Sulfolipids inhibit activation of macrophages and, thus, production of microbicidal molecules (2).

Mycobacteria are straight or slightly curved bacilli, 0.2 to 0.6 µm wide by 1.0 to 10 µm long, sometimes with branching (3). They are more strongly acid fast than organisms that are weakly acid fast, such as Nocardia, Rhodococcus, Corynebacterium, Tsukamurella, and Gordona species. Acid fastness is partly or completed lost at some stages of growth in some species and under conditions of starvation in Mycobacterium tuberculosis (4). On cultivation, colony morphology is variable. Some species grow on simple substrates, others are fastidious, and some are not cultivable (e.g., Mycobacterium leprae). Species requiring light to form pigment are called photochromogens, and those forming pigment with or without light are scotochromogens. Growth rates are slow, with generation times ranging from 2 hours to more than 20 hours and with grossly visible colonies appearing at 2 days to 8 weeks. Optimal growth temperatures vary from 30° to 45°C. Previously considered an obligate aerobe, M. tuberculosis has been found to survive anaerobic conditions (5).

There are now more than 100 species of mycobacteria identified, and the number continues to grow, largely because of the contribution of molecular genetics to taxonomic studies (6,7). Before these developments, only about 30 culturable species had been previously identified as “mycobacteria other than tuberculosis,” owing to the limitations imposed by biochemical and culture-based tests (8). Mycobacteria are ubiquitous and range from organisms that have no pathogenicity for humans to the highly virulent species of M. tuberculosis and M. leprae. Between these extremes are species increasingly recognized as opportunistic pathogens in patients with preexisting lung disease or immunosuppressed patients. Although cumbersome, mycobacteria other than tuberculosis (MOTT) is probably the most appropriate nomenclature because “nontuberculous mycobacteria” (NTM) still form tubercles (9), although M. leprae remains a special case. These organisms are also sometimes referred to as “atypical mycobacteria” causing the diseases of “atypical tuberculosis” to differentiate them from “typical” tuberculosis (TB) or as “environmental mycobacteria” to reflect their ecological niches in soil or water.

MYCOBACTERIUM TUBERCULOSIS

Characteristics of the Pathogen

Robert Koch described M. tuberculosis as “The Etiology of Tuberculosis” in 1882 (10). “They are rod-shaped bacilli…often inside cells” (10). Some taxonomists now consider the five members of the M. tuberculosis complex to be so closely related that they should be regarded as subspecies of M. tuberculosis rather than distinct species (6). M. tuberculosis and Mycobacterium bovis cause the most disease in humans. Mycobacterium africanum may cause 18% to 49% of TB in Africa (11,12). Mycobacterium microti is a pathogen of rodents. Mycobacterium canetti is the most recently described member of the complex. Humans are the only natural reservoir for M. tuberculosis, despite its ability to infect other primates, elephants, and experimental animal models (6).

M. tuberculosis grows slowly with a doubling time of 12 to 18 hours. On complex solid media, colonies are visible in 3 to 6 weeks. Growth is enhanced by 10% carbon dioxide. Virulent strains form strands or cords in liquid media. M. tuberculosis is differentiated from other mycobacteria by its production of niacin, reduction of nitrates, usual production of a heat-labile catalase, and characteristic susceptibility to isoniazid (INH). A radiometric method using Middlebrook 7H12 culture medium with radiolabeled fatty acid (palmitate labeled with carbon 14) as substrate (Bactec) has reduced the time required for isolation and identification of M. tuberculosis by half, from 4 weeks to 2 weeks on average. Tubercle bacilli can be distinguished from NTM by their sensitivity to p-nitro-o-acetylamino-3-hydroxypropiophenone or by nucleic acid amplification tests. A major advance opening the field of molecular epidemiology is DNA fingerprinting by restriction fragment length polymorphism (RFLP), which has been successfully applied in outbreaks of TB (13). The most widely used system is based on RFLP with

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detection of the insertion element IS6110, which is present only in isolates of M. tuberculosis complex and in high copy numbers (14). The sequencing of the entire genome of M. tuberculosis is a landmark achievement that opens the door for major scientific advances in genomics and proteomics in the near future (15).

Epidemiology

There were an estimated 8 million (range, 6.3 to 11.1 million) incident cases of TB in the world in 1997, with 1.9 million (range, 1.4 to 2.8 million) deaths attributed to TB annually (16). Eighty percent of the global burden of TB lies in 22 countries in Southeast Asia, sub-Saharan Africa, and Eastern Europe. Ninety-five percent of all TB cases and 98% of TB deaths occur in developing countries, largely because of the global human immunodeficiency virus (HIV) epidemic as well as to poverty, war, migration, and poor political commitment to TB control. Nearly one third of new TB cases in sub-Saharan Africa now occur in HIV-infected individuals, and 9 of the 10 countries with the highest incidence rates are in Africa. Although only 5% to 10% of individuals with latent infection with tuberculosis (LTBI) develop active disease, one third of the world’s population, or 1.86 billion people, is estimated to be infected, an enormous reservoir of potential disease.

Multidrug-resistant tuberculosis (MDR-TB), defined as in vitro drug resistance to at least isoniazid and rifampin, continues to be a global public health problem, despite recent significant decreases in prevalence in France and the United States (17). Although the median prevalence among all new cases was only 1.0%, “hot spots” of MDR-TB in Eastern Europe and Asia continue to have rates among new cases of 5 to 14.1%. In Estonia, the prevalence of MDR-TB among all cases increased from 11.7% in 1994 to 18.1% in 1998. Although directly observed short-course chemotherapy has decreased morbidity and mortality in TB (18), it alone does not cure drug-resistant TB (19).

Case rates in the United States had been declining at a rate of 6% per year, but there was a resurgence of TB from 1986 to 1992 attributed to increased immigration, waning of the public health infrastructure, but probably most importantly the HIV epidemic. The principal increase in the number of TB patients occurred in the 25- to 44-year-old age group, and most cases were reported from locations with high HIV prevalence, such as New York City, New Jersey, and Florida. Nosocomial outbreaks involving multidrug-resistant strains of M. tuberculosis with transmission to patients and health care professionals (20) were alarming and led to new action and investment in controlling TB (21).

Case rates in the United States have declined steadily since 1992; in 2000, there were 16,377 reported cases for a rate of 5.9 per 100,000, the lowest on record in the United States (22). Racial and ethnic minorities remain disproportionately affected, accounting for 75% of all reported cases. Nearly one third of cases occur in non-Hispanic blacks, 23% in Hispanics, 21% in Asians and Pacific Islanders, and 1% in Native Americans and Alaskan Natives. In 2000, the proportion of TB cases in foreign-born persons increased from 24% to 46% of all cases, owing to increased immigration and improved control of TB in U.S.-born persons. Similar changes have been noted in other high-income countries, suggesting the need for developing a more global approach to TB control. The impact of highly active antiretroviral therapy (HAART) for HIV infection on the epidemiology of TB has been generally favorable in the United States (23), Europe (24), and Brazil (25).

Transmission

After decades of debate about the mode of transmission, Wells and Riley confirmed that most TB is transmitted by the airborne route (26,27). Close contacts of a smear-positive patient are at maximal risk for being infected. Roughly 25% to 50% of those exposed become infected, and the attack rate may be as high as 80% if a sustained, heavy exposure occurs (28,29,30). The rate of infection for contacts of smear-negative, culture-positive patients (5%) had previously been thought to be similar to the background rate in the community (31). A more recent study in San Francisco using molecular epidemiology (i.e., DNA fingerprinting of isolates using RFLP to assess clustering) found that smear-negative cases accounted for about 17% of recent transmission (32). Transmission by other means is rare. Infection can occur by dermal inoculation of bacilli, occasionally seen in hospital personnel handling infected tissues and cultures. In the developed countries, infection with M. bovis through milk from infected cows has been eliminated by pasteurization of milk. Gastrointestinal transmission may still affect those in developing countries, including cross-border travelers (33).

Spread of TB can be rapid in settings of overcrowding. Case rates among prison inmates are among the highest reported prevalence rates (34). In the ex-USSR state of Georgia, there were 448 confirmed TB cases among 7,473 inmates, for a prevalence of 5,995 per 100,000 (35). Nursing home residents also have higher case rates than those of age-matched control subjects who live at home, probably owing to cohabitation with infectious patients with TB and to waning of immunocompetence secondary to aging (36). The prevalence of TB in homeless persons is substantially higher than in the general population, and shelters have been identified as sources of TB outbreaks (37,38).

Pathogenesis

Infectious particles of experimentally nebulized M. tuberculosis were estimated to be 1 to 5 µm by Wells (26,39), a size range well suited to deposition of particles in the alveoli (40). Riley and co-workers used infections of guinea pigs exposed to the air from an experimental TB ward to estimate the concentrations of aerosols of M. tuberculosis from infectious patients (41), and other exposures have been estimated by modeling data based on those studies (42,43). These data suggest that the production of infectious aerosols in pulmonary TB varies over 3 orders of magnitude from 1.25 to more than 1,500 infectious units per hour (44).

Bacilli that are deposited in the alveoli and that escape clearance mechanisms then encounter a number of innate immune mechanisms, including surfactant proteins (45), toll-like receptors (46), granulysin (47), defensins (48), and other antimicrobial peptides. Tubercle bacilli escaping these defense mechanisms undergo phagocytosis by macrophages. After phagocytosis, mycobacteria are contained within phagosomes. Although killed mycobacteria are found in lysosomes, some mycobacteria appear to block the maturation of the phagosome and to resist delivery to the lysosome (49).

The immune response to M. tuberculosis contributes considerably to the pathology and is characterized by a paradoxical mix of immune activation and suppression (50). Acquired cellular immunity develops within 4 to 6 weeks. Subsets of T lymphocytes producing macrophage-activating cytokines, in particular interferon-γ (IFN-γ), enable the phagocytes to destroy intracellular bacilli. Macrophage production of cytokines, such as tumor necrosis factor (TNF), proteases, and lipases, enhances liquefaction of caseous foci, favoring extracellular bacillary growth and cavity formation. Other cytokines, such as transforming

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growth factor, may favor local bacillary growth, suppress T-cell responses, and promote fibrosis. Although pulmonary infection remains restricted anatomically, organisms spread to the draining hilar lymph nodes and then through the thoracic duct to the bloodstream. On pathologic examination, inflammation, caseation necrosis, cavity formation, and fibrosis occur sequentially (51). However, there may be a mixed response, with various stages occurring in different areas of the lungs at the same time. Spread within the lungs is by bronchial channels (bronchogenic spread) or directly to neighboring alveoli. Progression from minimal disease to advanced disease may occur within months but is accelerated to weeks with HIV co-infection. Extrapulmonary disease occurs when bacillemia results in the seeding of other tissues of the body. Such dissemination occurs more commonly among the immunosuppressed, notably in HIV infection. Bacilli are usually not totally eradicated because a lifelong potential for reactivation remains.

The mechanisms underlying reactivation of latent TB are not well understood. However, a critical role for TNF-α regulation is now recognized. In an experimental murine model of chronic persistent TB, neutralization of TNF-α resulted in fatal reactivation of TB with an increased bacillary burden in the lungs and in severe lung pathology (52). These data are consonant with clinical observations of 70 reported cases of reactivation TB after treatment with infliximab, an antibody against TNF-α used in the treatment of Crohn’s disease and rheumatoid arthritis (53). Similarly, the importance of IFN-γ in host defense is supported by increased susceptibility to mycobacterial infections in those with inherited deficiencies of IFN receptors, as discussed later regarding NTM infections.

The pathogenesis of latent infection is even less well understood, but it appears that the bacillus is capable of survival through at least two stages of “nonreplicating persistence,” one of which is an anaerobic stage (5). Persistence of M. tuberculosis is facilitated by isocitrate lyase, an enzyme essential for fatty acid metabolism, which is expressed more with activation of host macrophages (54). Exposure to chemotherapy induces the sigF gene, which is similar to genes involved in bacterial stress responses (55), reflecting the ability of the tubercle bacillus to respond not only to the host response to infection but also to therapy.

Prevention

The most effective methods to prevent TB are (a) rapid, appropriate drug treatment of active pulmonary disease to prevent further transmission; and (b) treatment of latent infection to prevent reactivation. These interventions are discussed in Chapter 156.

Vaccination

Although bacille Calmette Guérin (BCG) vaccine has been used worldwide for decades except in a few developed countries, the value of BCG vaccination as protection against TB remains unclear, with protection rates varying from 0% to 80%. Genomic analyses of BCG strains reveals numerous differences among strains, but their effects on the variability of effectiveness are unknown (56). BCG vaccination may be beneficial in reducing miliary and meningeal disease in children. BCG should not be administered to children or adults who have symptomatic HIV infection because it may lead to disseminated BCG infection. However, the World Health Organization recommends BCG vaccination for asymptomatic HIV-infected children who are at increased risk for tuberculous infection. The tuberculin skin test (TST) is the mainstay of assessing exposure in contact investigations in developed countries; therefore, BCG vaccination is usually not recommended because it may confuse the interpretation of the TST. Intense efforts are underway to develop better TB vaccines.

Tuberculosis Infection Control

In response to outbreaks of nosocomial MDR-TB involving fatalities in both patients and health care workers, the Centers for Disease Control and Prevention issued new “Guidelines for Preventing the Transmission of Mycobacterium tuberculosis in Health-Care Facilities, 1994” (57). These guidelines are being revised, but the hierarchy of administrative, engineering, and personal respiratory protection control measures remain the basis for TB infection control. The most important strategy to prevent transmission of M. tuberculosis in congregate settings is the rapid identification, isolation, and appropriate treatment of infectious patients, which is the goal of administrative controls.

The foundation of engineering controls is the negative-pressure “airborne infection room.” Negative pressure within the room prevents infectious aerosol from being carried outside, whereas dilution ventilation of 6 to 12 air changes per hour (ACH) removes the infectious particles. Adjunctive measures include ultraviolet germicidal irradiation (UVGI) of either upper room air or the inside of ducts and high-efficiency particulate air (HEPA) filtration. Both require expertise in installation and periodic maintenance to be efficacious. The adjunctive measures are probably most effective in high-risk areas where there may be infectious TB patients but where there is inadequate dilution ventilation.

Personal respiratory protection refers to the use of masks designed to protect the wearer from inhalation hazards, such as infectious aerosols. This continues to be the most controversial aspect of the Centers for Disease Control and Prevention guidelines. The most commonly used masks are disposable particulate respirators now certified as N95, meaning that the filter material is 95% efficient at removing test particles. Better markers of infectiousness are needed to prioritize more confidently and to implement such resources, especially in crowded facilities in high-incidence areas.

M. tuberculosis has been transmitted not only by the infectious aerosols generated during bronchoscopies also but directly by contaminated bronchoscopes (58). Pseudoinfections due to poor disinfection have also been reported (58) and can be costly. Improved disinfectants have become available, and rigorous policies and procedures are required to clean and disinfect such equipment (59).

MYCOBACTERIA OTHER THAN MYCOBACTERIUM TUBERCULOSIS

Mycobacteria other than M. tuberculosis and M. leprae are ubiquitous in the environment, usually in water or soil, and have a varied spectrum of pathogenicity for humans. With the greater frequency of disease due to some NTM and the increasing numbers of immunocompromised patients, recognition of these infections is important. As mycobacterial infections other than TB are not reportable to public health departments, the best available data are from national surveys. The most recent data in the U.S. are from 1981 to 1983, before the HIV epidemic, when clinical and laboratory data were collected on 5,469 patients; almost 90% of the isolates were from respiratory tract specimens (60). Prevalence rates were calculated using a diagnostic algorithm, with most disease associated with Mycobacterium avium complex (MAC; 1.3 per 100,000), Mycobacterium kansasii

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(0.3 per 100,000), and Mycobacterium fortuitum-chelonae (0.2 per 100,000). The prevalence rate of all NTM infections was estimated to be 1.78 per 100,000, corresponding to 4,201 prevalent cases in the United States. After the onset of the HIV epidemic, the number of infections due to disseminated infections with MAC and other MOTT increased tremendously in the United States and other developed countries (61).

The amount of disease due to MOTT in the developing world is even less well understood, and detection is limited by laboratory capacity and by the high prevalence of TB. In contrast to the United States, there is little to no disease due to M. avium in Ugandan patients with acquired immunodeficiency syndrome (AIDS) (62). This is not explained by lack of environmental exposure because M. avium has been isolated frequently in soil and water samples in Uganda and other African sites (61). A decreased risk for disseminated MAC infection has been observed among HIV-infected persons with a history of active TB (relative risk, 0.52; p < 0.001) (63), suggesting a protective role of TB in preventing disseminated MAC infection. In northern Malawi, IFN-γ responses were highest for M. avium, M. intracellulare, and M. scrofulaceum (the MAIS complex), suggesting prior exposure to these MOTT (64). The investigators found concordance between these results and skin test results, with an additional observation that delayed hypersensitivity to rapidly growing mycobacteria was associated with a decreased risk for TB and leprosy (65).

Although it appears that most NTM are acquired from the environment, there are at least 10 reports in the literature of familial clustering of cases, most often with M. kansasii (66,67,68). In only one of these was there microbiologic testing of the home tap water, which yielded negative results (68). Thus, it is not clear whether these clusters were due to a shared environmental exposure, such as to tap water, or to person-to-person transmission. If the latter occurs, it probably does so rarely given the small number of clusters reported over the past 40 years. The most recently reported family cluster was clearly due to a common environmental source: an indoor hot tub contaminated with multiple mycobacterial species (69). NTM appear to have become part of the ecology of indoor living; 90% of biofilm samples from water-treatment plants, home plumbing, and aquaria were found to contain M. chelonae, Mycobacterium flavescens, M. fortuitum, Mycobacterium gordonae, M. kansasii, and Mycobacterium terrae-nonchromogenicum (70).

Mycobacteria have been implicated with uncertainty for years as causative agents in both sarcoidosis and Crohn’s disease. It appears that Propionibacterium species are more likely associated with sarcoidosis than the mycobacteria (71). An association between cell wall–deficient forms of M. avium subspecies paratuberculosis and Crohn’s disease has been suggested (72), but the role of mycobacteria in Crohn’s disease remains controversial (61,73).

The most clinically important NTM and the clinical infections associated with them are listed in Table 265.1 and discussed

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later. Nosocomial transmission and pseudoinfections have been reported most often due to contaminated water supplies, and iatrogenic infections have been associated with contaminated surgical materials. Because of the environmental sources of many MOTT, it is likely that additional species will be associated with nosocomial disease in the future. More detailed coverage of treatment can be found in Chapter 34, and additional clinical information is in Chapter 157.

TABLE 265.1. Infections Associated with Mycobacteria

Mycobacterium species Lungs Skin, soft tissue Skeletal, tendon Lymphadenitis Disseminated M. tuberculosis complex M. africanum + + + M. bovisa ++ + + + M. tuberculosisa +++ + + ++ ++ M. lepraea +++ Slowly growing mycobacteria M. avium complexa (M. avium and M. intracellulare) ++ ++ ++ ++ M. branderi + + M. celatum + + + M. genavense + M. gordonaeb + + + M. haemophiluma + + + M. interjectum + + M. intermedium + M. kanasiia ++ + ++ ++ M. lentiflavuma,b + + + + M. malmoense + + + M. marinum ++ ++ + M. scrofulaceum + + + + M. simiae + + M. smegmatis + + + + M. szulgaia + + + + + M. terrae/chromogenicum complex + + M. ulcerans + M. xenopia + + Rapidly growing mycobacteria M. abscessusa ++ ++ ++ + ++ M. chelonaea ++ ++ ++ M. fortuitum complex (M. fortuitum, M. peregrinum)a + ++ ++ + + M. mucogenicum + M. neoarum + +++, more common; ++, less common; +, infrequent/rare. a Nosocomial or iatrogenic infections reported. b Contamination/pseudoinfection most common.

Pathogenesis

The pathogenesis of mycobacterial infections other than TB is less well understood, although there are some common features. Because these organisms are ubiquitous in the environment but rarely cause disease, host susceptibility factors may be more important determinants of infection than the degree of exposure or the virulence of the organism. The most striking genetic susceptibilities are conferred by deficiencies of IFN-γ receptor, involving both the complete IFN-γ receptor ligand-binding chain (IFN-γR1) and the signaling chain (IFN-γR2). These children are unable to form mature granulomas and are susceptible to severe mycobacterial and salmonella infections. Milder clinical presentations are associated with partial deficiencies of these factors. These deficiencies highlight the essential role of IFN-γ in defense against mycobacterial infections, also supported by the symptomatic and radiographic improvement of seven patients with refractory, disseminated infection with NTM in a small trial of therapeutic IFN-γ given subcutaneously (74).

Acquired deficiency of cell-mediated immunity, most commonly due to HIV infection, is also associated with disseminated MAC infection. Other host factors that predispose to pulmonary mycobacterial infections are preexisting lung disease, including underlying chronic lung diseases such as chronic obstructive pulmonary disease (COPD) and pneumoconioses (75,76); bronchiectasis (77); and cystic fibrosis (78). Pulmonary infections other than MOTT occur in women without recognized risk factors (79) or with pectus excavatum, thoracic scoliosis, and mitral valve prolapse (80). However, the mechanisms underlying the susceptibility in these women are not known.

Mycobacterium avium Complex

Mycobacterium avium and Mycobacterium intracellulare, previously differentiated by their virulence for chickens and rabbits, respectively, are similar biochemically and genetically and are referred to as MAC. Growth is slow and optimal at 37°C. Colonies are usually not pigmented and are either opaque and domed or transparent and flat. MAC, like many other MOTT, has been isolated from both water and soil (81,82). In addition, MAC has been isolated from bronchoscopes, raw milk, and cigarettes (61). The source of MAC infection is usually unknown, although cases have been linked to isolates from hospital and municipal tap water (83). Hot tubs are another source of MAC aerosols causing both infection and hypersensitivity pneumonitis in immunocompetent patients (84,85). About 90% of MAC isolates from AIDS patients have been identified as M. avium by DNA probe, whereas about 40% of isolates from non-AIDS patients were M. intracellulare (61). Infection with more than one strain or species in AIDS patients appears common (61). This is an important consideration because different strains from the same patient may have different drug susceptibility patterns (86).

MAC is the most common MOTT associated with human disease, and its ability to cause human disease has only been recognized since 1943 (87). In a U.S. national survey from 1981 to 1983, the prevalence of MAC disease was estimated to be 1.3 per 100,000. In a study of 670,000 U.S. Navy recruits, delayed hypersensitivity responses to purified protein derivative from MAC (PPD-B or the “Battey” antigen) were 70% or greater in those from the Southeast compared with 10% to 20% in those from the North and West (88). This suggests at least greater exposure to MAC in the southeastern United States, probably from environmental sources.

Risk factors for MAC infection are as suggested earlier: (a) patients with preexisting lung disease, especially COPD, (b) thin middle-aged and elderly women, and (c) patients with HIV infection. The epidemiology may be changing because there are anecdotal reports among clinicians that there appear to be fewer male patients with COPD and more thin women among their non–HIV-infected patients with pulmonary MAC infections. These patients present with symptoms similar to those of persons with underlying lung disease, often with an insidious onset and indolent progression. Disseminated MAC disease affects immunocompromised hosts, specifically AIDS patients (89). Cervical lymphadenitis (especially in children), bone and joint infection, urinary tract involvement, and cutaneous infections, including panniculitis, have been reported.

The diagnosis of pulmonary MAC requires the presence of pulmonary symptoms with chest radiograph abnormalities of infiltrates, nodules, or cavities. MAC may occasionally present as a chest mass. The microbiologic diagnosis requires three positive acid-fast bacilli (AFB) cultures (or two positive cultures and one positive smear) if sputum alone is obtained. If a bronchoscopy is done, one positive culture (2 to 4+) is sufficient on washing or biopsy, or if histopathology demonstrates AFB or granulomatous inflammation, any growth on another specimen is adequate (90).

Chemotherapy for MAC is similar to that for TB in that it requires multiple drugs to avoid the emergence of resistance, except that MAC is usually resistant to isoniazid. Currently recommended therapy for pulmonary MAC without HIV co-infection includes daily treatment with clarithromycin or azithromycin; rifampin or rifabutin; and ethambutol (90). Sputa should be examined monthly for AFB smears and cultures, and treatment should continue for 1 year after conversion to culture negativity. Streptomycin given 2 to 3 times per week may be added for the first 2 months. Treatment of either pulmonary or disseminated MAC infection in the HIV-infected patient is similar, but rifabutin is the preferred rifamycin. AIDS patients with disseminated MAC should receive lifelong therapy with (1) clarithromycin or azithromycin, and (2) ethambutol, with or without rifabutin, unless immune reconstitution occurs due to HAART (91). For HIV-infected patients with CD4 cell counts below 50 cells/µL, the preferred primary prophylaxis regimen is clarithromycin or azithromycin (91). Because of increasing evidence that anti-MAC prophylaxis can usually be stopped safely in AIDS patients responding to HAART (92,93), current recommendations suggest discontinuation if the CD4 count remains above 100 cells/µL for 3 months or more (91). See Chapter 34 for drug dosages and additional recommendations. The utility of drug susceptibility testing for MAC and other NTM has been debated in the literature. Although current recommendations do not suggest routine drug susceptibility testing for all NTM, they do suggest its utility as baseline testing in case the patient does not respond to therapy or in the event of relapse (90). Emergence of drug-resistant mutants in patients with AIDS is likely due to the large numbers of bacilli present, and drug-resistant isolates have been recovered in patients receiving both single- and multiple-drug therapy. Relapse of symptoms has been associated with the emergence of drug resistance (61).

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Mycobacterium kansasii

Initially isolated in 1953, M. kansasii is most commonly isolated from tap water but not from natural water (90). M. kansasii antigens cross-react with PPD-S (from M. tuberculosis); hence, delayed hypersensitivity skin testing has little utility. Reports suggest that M. kansasii disease is more common in the central United States, England, and Wales. In the United States, the prevalence of M. kansasii disease is greatest in the metropolitan areas of the Southeast and Midwest; the male-to-female ratio is 4:1. Eighty percent of patients are white; their median age is 50 years. Chronic obstructive lung disease and pneumoconiosis predispose to infection. Miners, welders, sandblasters, and painters are at highest risk for development of disease. Guinea pigs exposed to M. kansasii developed progressive lung disease only if the inoculum contained dust aerosols (94).

The most common form of disease due to M. kansasii is a chronic pneumonia, generally milder than TB but associated with more hemoptysis. Extensive unilateral upper lobe disease is more common than with MAC infection, but none of the radiographic features is specific for any of the mycobacterioses, including TB. Cervical lymphadenitis (especially in children), cutaneous abscesses or cellulitis mimicking pyogenic infection, and musculoskeletal involvement including arthritis, fasciitis, tenosynovitis, or osteomyelitis have been reported. Dissemination occurs rarely with far-advanced disease or in immunocompromised hosts such as AIDS patients (95). M. kansasii does not cause as much disease in AIDS patients as does MAC, for unknown reasons.

Trained microscopists can sometimes identify M. kansasii by virtue of the long, beaded bacilli with Ziehl-Neelsen staining, but culture is always needed for confirmation (96). It is the only mycobacterium that produces a bright-yellow pigment on exposure to light. Although in vitro drug resistance may be reported to isoniazid at 0.2 or 1.0 µg/mL and to streptomycin at 1 µg/mL, this may not have clinical relevance as long as rifampin is being used (90). All isolates are resistant to pyrazinamide; thus, it is not recommended for treatment. Although there have been no clinical treatment trials, the current American Thoracic Society (ATS) recommendation is to treat disease due to M. kansasii with isoniazid, rifampin, and ethambutol for 18 months, including at least 12 months during which sputum cultures are negative (90). Clarithromycin has excellent in vitro activity; hence, it is a reasonable alternative drug.

Rapidly Growing Mycobacteria

Early studies classified the rapidly growing mycobacteria (RGM) by phenotypic characteristics into subspecies of M. fortuitum (M. fortuitum biovar fortuitum, M. fortuitum biovar peregrinum, and an unnamed biovariant complex) and of M. chelonae (M. chelonae subspecies abscessus and M. chelonae subspecies chelonae). However, more recent taxonomy studies using molecular biologic tools have classified these as separate species (97). Colonies grow at 25° to 40°C in 7 days on both mycobacterial medium and routine bacteriologic medium. Acid fastness is often negative (98). A number of nosocomial outbreaks have been associated with exposure to tap water (99). This may be due to biofilms, 90% of which may contain mycobacteria (70).

Skin and soft tissue infection, the most common clinical form of disease, is usually secondary to a puncture wound and occurs within 6 weeks to 6 months. Systemic symptoms are absent, and dissemination is extremely unusual. Spontaneous resolution occurs within 1 to 2 years in as many as 20% of cases. Nosocomial infections include skin infections of vaccination sites, surgical wounds (including plastic surgery), and indwelling peritoneal or intravenous catheters. Disseminated disease is uncommon in severely immunocompromised persons and is associated with a high rate of mortality. Chronic pneumonia due to RGM presents in a manner indistinct from disease caused by other mycobacteria (100). Most patients are women who have no preexisting lung disease. Risk factors include achalasia, cystic fibrosis, lipoid pneumonia, and healed TB. On radiographic examination, a reticulonodular pattern is seen, usually bilateral, sparing the upper lobes, and rarely associated with cavities. Clinical exacerbations occur during a period of years, but there is no spontaneous resolution. In general, these mycobacteria are resistant to antituberculous drugs, except amikacin, and are sensitive to cefoxitin, clarithromycin, and imipenem. The rapidly growing mycobacteria have varying susceptibility patterns to drugs. A combination of parenteral antibiotics (e.g., amikacin and cefoxitin), followed by treatment with oral drugs for up to 6 months, has been effective. Surgical resection for infection by drug-resistant species (M. abscessus) and with unilateral localized disease may be considered.

Other Mycobacteria

M. marinum causes “swimming pool granuloma” with sources from water and aquatic organisms. A photochromogen, M. marinum grows optimally at 32°C; colonies appear in 8 to 14 days. Cutaneous disease usually occurs as a result of trauma to skin in contaminated, nonchlorinated fresh, or salty water. Two to 3 weeks after inoculation, a single papulonodular lesion develops, which may later ulcerate. A sporotrichoid pattern, with abscess formation and secondary nodules along the lymphatics, can also occur. Dissemination in immunocompromised hosts, synovitis, and osteomyelitis have been reported.

Mycobacterium simiae is a slow-growing species that may cause pulmonary disease mimicking TB or pulmonary MAC. In some laboratories relying on biochemical testing, it may be confused with M. tuberculosis because M. simiae is the only NTM that is niacin positive. It is unique in having 16S ribosomal RNA sequences similar to those found in both rapidly growing and slowly growing mycobacteria. Drug resistance is common; thus, in vitro susceptibility testing should guide therapy (90).

Mycobacterium scrofulaceum is a cause of cervical lymphadenitis in children 1 to 5 years of age; it must be differentiated from M. tuberculosis and MAC. Because of antigenic and drug susceptibility similarities with MAC, some authors have classified it in a group with M. avium and M. intracellulare as the MAIS complex.

Mycobacterium ulcerans is endemic to Australia, Africa, and Mexico. It causes a chronic necrotizing skin infection known as Bairnsdale ulcer in Australia and Buruli ulcer in Uganda, usually on the distal aspect of an extremity following trauma. Satellite lesions and extensive ulceration requiring wide surgical débridement may develop. Treatment options include dapsone and streptomycin with or without ethambutol (87).

Unique among mycobacteria in its absolute requirement for iron, Mycobacterium haemophilum is a fastidious bacillus that grows at temperatures below 32°C. It most often causes nodular, popular, or cystic skin lesions that may progress to painful ulcers in individuals immunocompromised by HIV infection (101), organ or bone marrow transplantation, or lymphoma (102). Osteomyelitis, arthritis, sinusitis, pneumonitis, lymphadenitis, and bacteremia may also occur. Successful treatment regimens have included ciprofloxacin, clarithromycin, and rifampin (103).

Mycobacterium genavense is a slowly growing fastidious organism that fails to grow on conventional solid media but grows slowly in liquid culture. M. genavense infection causes a syndrome indistinguishable from MAC and is estimated to be

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responsible for 10% of disseminated NTM infections (104,105). Fever, weight loss, diarrhea, hepatomegaly, splenomegaly, and anemia are common. Response to antimycobacterial therapy directed against MAC and survival in patients with M. genavense infection are similar to those with MAC infection and are dependent on absolute CD4⁺ cell levels in AIDS patients.

Mycobacterium xenopi grows optimally at 43°C; hence, its isolation from clinical specimens may require prolonged incubation at 37°C or at higher temperatures. It is commonly isolated from tap water and has been implicated in nosocomial pulmonary infections (106) and bronchoscopy-associated pseudoinfections (107). It has been increasingly recognized in Western Europe as a cause of pulmonary disease (108). Extrapulmonary and disseminated disease has been reported. Initial therapy should include a macrolide, a rifamycin, and ethambutol (90).

Mycobacterium szulgai causes lung and extrapulmonary disease indistinguishable from TB (109). It occurs worldwide, but its source is unknown. Most patients respond to treatment with antituberculous therapy (90). Mycobacterium, malmoense is slow growing, often requiring at least 6 weeks of incubation (103). It is rare in the United States but increasingly reported from northern Europe, England, and Wales. It may cause pulmonary disease in patients with underlying lung disease as well as extrapulmonary disease in children or immunosuppressed patients. Most isolates are susceptible to ethambutol, rifampin, and streptomycin.

Members of the slowly growing M. terrae complex may cause infections of the lungs, skin, bones, and joints. Drug susceptibility testing should be done because patterns vary widely. Surgical excision often is needed. Although the rapidly growing Mycobacterium smegmatis is often used in laboratories as an avirulent environmental species, it may cause human disease (110). M. gordonae has been called the “tap-water bacillus.” Although it may rarely cause infection in patients with and without immunosuppression (111), it is more commonly a source of pseudoinfections (103). Mycobacterium celatum may infect patients with AIDS and rarely immunocompetent individuals (112), and it may cause false-positive results on nucleic acid amplification tests for M. tuberculosis (113,114).

Largely due to the HIV epidemic and the increasing use of immunosuppressive therapies, infections have been found to be caused by a growing number of mycobacteria that are uncommon, unidentified, or previously unrecognized as pathogens, including the following Mycobacterium species: M. alvei, M. asiaticum, M. bohemicum, M. branderi, M. brumae, M. confluentis, M. conspicuum, M. gastrii, M. flavescens, M. goodi, M. hassiacum, M. heckeshornense, M. heidelbergense, M. intermedium, M. interjectum, M. kubicae, M. lentiflavum, M. mageritense, M. mucogenicum, M. nvocastrense, M. peregrinum, M. phlei, M. septicum, M. shimodei, M. thermoresistibile, M. triplex, M. tusciae, and M. wolinskyi (115).