The slow-growing NTM can be subdivided into three groups based on the phenotypic characteristics of the species. Mycobacterium spp. synthesize carotenoids (a group of yellow to red pigments) in varying amounts and thus can be categorized into three groups based on the production of these pigments: photochromogens, scotochromogens, and nonphotochromogens. Some of these NTM are considered potentially pathogenic for humans, whereas others are rarely associated with disease.

Photochromogens

The photochromogens (Table 1) are slow-growing NTM that produce colonies that require light to form pigment.

Table1.  Characteristics of Nontuberculous Mycobacteria—Photochromogens

Scotochromogens

 The scotochromogens (Table 2) are slow-growing NTM that produce pigmented colonies whether grown in the dark or the light. The epidemiology of the potentially pathogenic scotochromogens has not been definitively described. In contrast to potentially pathogenic nonphotochromogens, these agents are rarely recovered in the clinical laboratory.

Table2. Characteristics of Nontuberculous Mycobacteria—Scotochromogens

Nonphotochromogens

The nonphotochromogens (Table 3) are slow-growing NTM that produce unpigmented colonies whether grown in the dark or the light. Of the organisms in this group, M. terrae complex (M. terrae, M. triviale, and M. nonchromogenicum) and M. gastri are considered nonpathogenic for humans. The other nonphotochromogens are considered potentially pathogenic, and many are frequently recovered in the clinical laboratory. The nonphotochromogens belonging to Mycobacterium avium complex are frequently isolated in the clinical laboratory and are able to cause infection in the human host.

Table3.Characteristics of the Nontuberculous Mycobacteria Nonphotochromogens and Species Considered Potential Pathogens

Mycobacterium avium Complex (MAC). Largely because of the increasing populations of immunosuppressed patients, the incidence of infection caused by M. avium complex spp., as well as these organisms’ clinical significance, has changed significantly since they were first recognized as human pathogens in the 1950s. The introduction of highly active antiretroviral therapy (HAART) has dramatically reduced the infections caused by these organisms in patients with acquired immunodeficiency syndrome (AIDS).

General Characteristics. Taxonomically, M. avium complex comprises M. avium, M. intracellulare, M. avium subsp. avium, M. avium subsp. paratuberculosis, M. avium subsp. silvaticum (wood pigeon bacillus), M. vulneris, M. marseillense, M. bouchedurhonense, and M. timonense. The name M. avium subsp. hominissuis has been proposed for another subspecies capable of infecting humans. Unfortunately, the nomenclature is somewhat confusing. Although M. avium and M. intracellulare are clearly different organ isms, they so closely resemble each other that the distinction cannot be made by routine laboratory determinations or on clinical grounds. As a result, these organisms some times are referred to as M. avium-intracellulare. Furthermore, because isolation of M. avium subsp. paratuberculosis in a routine laboratory setting is exceedingly rare, the term M. avium complex is most commonly used to report the isolation of M. avium-intracellulare.

Epidemiology and Pathogenesis. MAC is an important pathogen in both immunocompromised and immunocompetent populations. These are among the most commonly isolated NTM species in the United States. MAC is particularly noteworthy for its potentially pathogenic role in pulmonary infections in patients with AIDS and also in patients who are not infected with HIV. The organisms are ubiquitous in the environment and have been isolated from natural water, soil, dairy products, pigs, chickens, cats, and dogs. As a result of extensive studies, it is generally accepted that natural waters serve as the major reservoir for most human infections.

Infections caused by MAC are acquired by inhalation or ingestion. The pathogenesis of MAC infections is not clearly understood. The organisms are commonly associated with respiratory disease clinically similar to tuberculosis in adults, lymphadenitis in children, and disseminated infection in patients with HIV. However, these organisms and other environmental NTM have extraordinary starvation survival. They can persist well over a year in tap water, and MAC tolerates temperature extremes. In addition, similar to legionellae, M. avium can infect and replicate in protozoa. Amoebae-grown M. avium is more invasive toward human epithelial and macrophage cells.

MAC cultures can have an opaque, a translucent, or a transparent colony morphology. Studies suggest that transparent colonies are more virulent because they are more drug resistant, are isolated more frequently from the blood of patients with AIDS, and appear more virulent in macrophage and animal models.

M. avium subsp. paratuberculosis is known to cause an inflammatory bowel disease (known as Johne’s disease) in cattle, sheep, and goats. It also has been isolated from the bowel mucosa of patients with Crohn’s disease, a chronic inflammatory bowel disease of humans. The organism is extremely fastidious, seems to require a growth factor (mycobactin, produced by other species of mycobacteria, such as M. phlei, a saprophytic strain) and may take as long as 6 to 18 months for primary isolation. Whether these and other mycobacteria actually contribute to development of Crohn’s disease or are simply colonizing an environmental niche in the bowel of these patients remains to be elucidated.

Clinical Spectrum of Disease. The clinical manifestations of M. avium complex infections are summarized in Table 3.

Other Nonphotochromogens. Several other mycobacterial species that are considered nonphotochromogens are potentially pathogenic in humans. The epidemiology and spectrum of disease for these organisms are summarized in Table 3. In addition to the species in this table, other, newer species of mycobacteria that are non photochromogens have been described, such as M. celatum and M. conspicuum. These newer agents appear to be potentially pathogenic in humans.

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قسم الشؤون الفكرية يعلن مشاركة ممثلين من 18 دولة إفريقية في خدمة زائري الإمام العسكري (عليه السلام)

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العتبة العباسية المقدسة تختتم مجالس العزاء بذكرى شهادة الإمام الحسن العسكري (عليه السلام)

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المزيد

الآخبار الصحية

دون آثار جانبية.. زيت عشبة طبيعية قد يعالج القلق

"ليكانيماب".. علاج الزهايمر يطرح رسميا في السوق الألمانية

متى يجب تنظيف الأسنان في الصباح.. قبل أم بعد الإفطار؟

ارتفاع الوفيات.. منظمة الصحة تدق جرس الإنذار بشأن الكوليرا

المزيد

الآخبار التكنلوجية

أكبر مشروع طاقة رياح في نصف الكرة الجنوبي يحقق إنجازين جديدين

5 أمور لا تفعلها أثناء تشغيل مكيف سيارتك

ضوء إدارة المحرك: أسئلة مهمة لفهم عمله ومعانيه

"واتساب" يطلق خاصية جديدة لكتابة الرسائل بالذكاء الاصطناعي

المزيد

مواضيع ذات صلة

Rapidly Growing Nontuberculous mycobacteria (RGM)

Mycobacterium Tuberculosis Complex

Anaerobic Gram-Positive and Gram-Negative Cocci

The Salmonellae

The shigellae

Diseases Caused by Enterobacteriaceae Other than Salmonella and Shigella

Antigenic Structure of Enterobacteriaceae

Gram-Negative Rods

Gram-Positive, non–Spore- Forming Bacilli

Gram-Positive, Spore Forming Bacilli

Morphology and Identification of Enterobacteriaceae

Clinical Findings of Enterococci