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Tuberculosis part-2

الكلية كلية الطب     القسم  الاحياء المجهرية     المرحلة 3
أستاذ المادة حبيب صاحب نهر المزيداوي       4/24/2011 10:04:11 PM

Tuberculosis: Part-2

 

Introduction:

 

The genera Mycobacterium and Nocardia have been grouped into the family Mycobacteriaceae within the order Actinomycetales based upon similarities in staining and motility, lack of spore formation, and catalase production. These genera are characterized by the presence of long-chain fatty acids, called mycolic aids.

 

Mycobacterium tuberculosis complex:

 

The first sign of a new infection is often conversion of the intradermal skin test with purified protein derivative (PPD) to positive or detection of a lesion by chance on a chest x-ray in an otherwise asymptomatic individual.

 

Clinical signs and symptoms develop in only a small proportion (5-10 percent) of infected healthy people. These patients usually present with pulmonary disease; prominent symptoms are chronic, productive cough, low-grade fever, night sweats, easy fatigability, and weight loss. Tuberculosis may present with or also exhibit extrapulmonary manifestations including lymphadenitis; kidney, bone, or joint involvement; meningitis; or disseminated (miliary) disease. Lymphadenitis and meningitis are more common among normal infants with tuberculosis, and all extrapulmonary manifestations are increased in frequency among immunocompromised individuals such as patients on chronic renal dialysis and elderly, malnourished, or HIV-infected individuals.

 

Structure:

 

Mycobacteria are slender, curved rods in stained clinical specimens. The cell wall is composed of mycolic acids, complex waxes, and unique glycolipids. The mycolic acids containing extremely long (C60 to C90) side chains are joined to the muramic acid moiety of the peptidoglycan by phosphodiester bridges and to arabinogalactan by esterified glycolipid linkages. Species variations are characterized by variation in sugar substitutions in the glycolipids or peptidoglycolipids. The mycobacterial cell wall is acid-fast (i.e., it retains carbolfuchsin dye when decolorized with acid-ethanol). This important property allows differential staining in contaminated clinical specimens such as sputum. Other important wall components are trehalose dimycolate (so-called cord factor, as it is thought to induce growth in serpentine cords on artificial medium) and mycobacterial sulfolipids, which may play a role in virulence. Another unique constituent which may contribute to pathogenesis is lipoarabinomannan (LAM). Purified LAM from virulent and attenuated strains of mycobacteria may differ structurally, and these differences may contribute to their varying abilities to stimulate cytokine production in mononuclear cell cultures.

 

 This unusual cell wall structure endows mycobacteria with resistance to dehydration, acids, and alkalis. The resistance to acids and alkalis is useful in the isolation of mycobacteria from contaminated clinical specimens such as sputum. Treatment of sputum with dilute solutions of sulfuric acid or sodium hydroxide will allow mycobacteria to survive and grow on culture medium in the absence of the members of the respiratory flora.

 

Another important consequence of the unique cell wall structure of mycobacteria is the adjuvant action of whole cells when mixed with a wetting agent (e.g., Tween) in an oil-water emulsion. Such a mixture is called Freund s complete adjuvant.

 

Even under ideal culture conditions M tuberculosis and M bovis grow very slowly, with doubling times on the order of 18 to 24 hours. This extremely slow growth, even in vivo, has two consequences of clinical significance: (1) the infection is an insidious, chronic process, which may take several weeks or months to become clinically patent, and (2) on solid media inoculated with clinical material, identifiable mycobacterial colonies may not appear for 4 to 6 weeks. When they do appear, the colonies are irregular, waxy, and buff colored, with bacteria piled up into clumps or ridges.

 

Classification and Antigenic Types:

 

 With the exception of M leprae, the mycobacteria are classified into two broad categories members of the M.  tuberculosis complex (M.  tuberculosis, M. bovis, M. microtii, M. africanum) and nontuberculous mycobacteria (virtually all other species), which often are described based on their growth rate and pigmentation with and without exposure to light. Although there are antigenic differences among species on the basis of serologic reactions to carbohydrate moieties in the glycolipids, such determinations are not clinically useful. Modern molecular biologic techniques have revealed a remarkable conservation of genes coding for the immunodominant antigens of all mycobacteria, including M. leprae.

 

Pathogenesis:

 

 In this country, virtually all M. tuberculosis infections occur by airborne transmission of droplet nuclei containing a few viable, virulent organisms produced by a sputum-positive individual. The bacilli are deposited in the alveolar spaces of the lungs, where they are engulfed by alveolar macrophages. A portion of the infectious inoculum resists intracellular destruction and persists, eventually multiplying and killing the macrophage. The ability of virulent mycobacteria to survive within phagocytes justifies their designation as facultative intracellular pathogens. The mechanisms of intracellular survival are not clear and may vary from species to species. There is some evidence that M. tuberculosis can prevent phagosome-lysosome fusion. Other studies have demonstrated that virulent mycobacteria can prevent acidification of the phagolysosome, perhaps by modulating the activity of a membrane proton pump. In addition, some of the components of the mycobacterial cell wall (e.g., cord factor) may be directly cytotoxic to macrophages. Although hemolysins and lipases are produced by M. tuberculosis, their role in escape of tubercle bacilli from the phagosome, and the importance of extravacuolar organisms in pathogenesis are unknown. Most of the tissue destruction associated with tuberculosis results from cell-mediated hypersensitivity, rather than direct microbial aggression.

 

Eventually, the accumulating mycobacteria stimulate an inflammatory focus which matures into a granulomatous lesion characterized by a mononuclear cell infiltrate surrounding a core of degenerating epithelioid and multinucleated giant (Langhans) cells. This lesion (called a tubercle) may become enveloped by fibroblasts, and its center often progresses to caseous necrosis. Liquefaction of the caseous material and erosion of the tubercle into an adjacent airway may result in cavitation and the release of massive numbers of bacilli into the sputum. In the resistant host, the tubercle eventually becomes calcified.

 

Early in infection, mycobacteria may spread distally either indirectly through the lymphatics to the hilar or mediastinal lymph nodes and thence via the thoracic duct into the blood stream, or directly into the circulation by erosion of the developing tubercle into a pulmonary vessel. Extrapulmonary hematogenous dissemination results in the seeding of other organs (e.g., spleen, liver, and kidneys) and, eventually, reinoculation of the lungs. The resulting secondary lung lesions (as opposed to the initial site of implantation) may serve as the origin of reactivation of clinical disease years or decades later owing to the persistence of viable tubercle bacilli. Primary disease is usually characterized by a single lesion in the middle or lower right lobe with enlargement of the draining lymph nodes. Endogenous reactivation is often accompanied by a single (cavitary) lesion in the apical region, with unremarkable lymph nodes and multiple secondary tubercles.

 

In parts of the world where bovine tuberculosis has not been eliminated and where dairy products are not properly treated, direct infection of the gastrointestinal tract may occur by ingestion of virulent M. bovis organisms. The gut is also exposed occasionally in pulmonary tuberculosis when large numbers of viable M. tuberculosis cells are coughed up and swallowed.

 

In either case, the principal site of involvement is the mesenteric lymph nodes, with subsequent dissemination.

 

Host Defenses:

 

 Innate susceptibility to pulmonary infection with M. tuberculosis is clearly influenced by genetic and/or ethnic variables that have not been defined. Studies of mono- and dizygotic twins and siblings indicate a significant relationship between genotype and resistance to tuberculosis. Studies of inbred experimental animals point to genes both within and outside the major histocompatibility complex that apparently contribute to resistance. In the mouse, at least two genes (Bcg and Tbc) have been implicated in the expression of innate resistance to mycobacteria. A search for human homologues is currently under way. The mechanism for this genetic effect may reflect the ability of macrophages to process and present mycobacterial antigens to the immune system.

 

Acquired immunity following mycobacterial infection usually develops within 4 to 6 weeks and is associated temporally with the onset of delayed hypersensitivity to mycobacterial antigens such as PPD. Successful acquired resistance is mediated by T lymphocytes.

 

Some clinical studies suggest that inappropriately high levels of circulating cytokines, such as tumor necrosis factor alpha, may be responsible for some of the clinical features of tuberculosis (e.g. fever, weight loss). Therapy with cytokine-blocking drugs, such as pentoxyfilline and thalidomide, may prove to be an important adjunct to standard chemotherapy in some tuberculosis patients. Experimental evidence suggests that protection and hypersensitivity may be mediated by distinct subsets of T lymphocytes and may be directed against different mycobacterial antigens.

 

Epidemiology:

 

 Numerous studies of tuberculosis epidemics in closed populations (e.g., on naval vessels and in nursing homes) document the contagious nature of this infection. Fortunately, overt clinical disease actually develops in only a small percentage of those infected. Identification of recently infected individuals is still important, however, because viable mycobacteria persisting in tissues may lead to endogenous reactivation of tuberculosis later in life (see Treatment and Control, below). Reactivation is usually associated with deterioration of the cell-mediated immune response due to aging or to some associated clinical condition. Exogenous reinfection also has been documented, but most cases of so-called "post-primary" tuberculosis in this country are thought to be the result of endogenous reactivation.

 

Tuberculosis epidemiology has been clarified significantly by the development of molecular biological techniques which allow the relatively unambiguous identification of a particular clinical isolate. Recent, active transmission of a single strain of M tuberculosis would result in clinical isolates from several patients which exhibited identical DNA patterns or "fingerprints." In contrast, endogenous reactivation in a group of patients would likely result in isolates which exhibited unique patterns. The most commonly used procedure for comparing isolates involves the use of molecular probes which bind to segments of DNA called insertion sequences (IS) in the mycobacterial genome. Such a sequence, IS6110, has been employed very successfully to track outbreaks of tuberculosis within institutions (e.g. hospitals) associated with recent transmission. This "fingerprinting" procedure, referred to as restriction fragment length polymorphism (RFLP) analysis, is currently being used to detect previously unsuspected transmission between apparently unrelated patients in the community.

 

Tuberculosis is particularly common in groups such as the elderly, the chronically malnourished, alcoholics, and the poor. The prevalence of clinical tuberculosis among the homeless in the United States may be up to 300 times higher than the national average rate. In recent years, the incidence of disease in racial minorities in the United States has been more than five times that observed in whites. Of particular concern is the very high incidence of tuberculosis among recent immigrants.

 

Perhaps the most significant factor influencing the incidence of mycobacterial disease in the United States since 1984 has been the HIV epidemic. HIV-infected individuals have a high incidence of tuberculosis, characterized by frequent extrapulmonary disease. Both primary, pulmonary infection and endogenous reactivation are seen in HIV-positive individuals. Owing to the loss of T-cell function in these patients, the tuberculin skin test may not be reliable and the chest radiograph may not show the classic well-defined primary tubercle. Both of these observations make the diagnosis of tuberculosis in HIV-infected patients more challenging. Furthermore, tuberculosis often occurs early in the course of HIV infection, before a significant decline in CD4+ T cells has occurred. In this sense, tuberculosis in an apparently healthy young adult may "signal" the presence of underlying HIV infection.

 

Diagnosis:

 

 Infection in an asymptomatic individual can be diagnosed with the help of the intradermal PPD skin test. Intradermal introduction of PPD into a previously infected, hypersensitive person results in the delayed (48-72 hr) appearance of an indurated (raised, hard) reaction with or without erythema. It is impossible to distinguish between present and past infection on the basis of a positive tuberculin test. The Mantoux test requires the intradermal injection of a measured volume (0.1 ml) containing a specified quantity (5 tuberculin units) of PPD. The transverse diameter of induration is measured 48 to 72 hours later. In a person with symptoms suggestive of tuberculosis, clinical specimens (sputum, bronchial or gastric washings, pleural fluid, urine, or cerebrospinal fluid) should be stained and cultured for acid-fast bacilli. Culture and identification of mycobacteria in such specimens are mandatory for diagnosis. Two types of stains are used specifically for detection of mycobacteria: fluorochrome (recommended) and carbol fuchsin. In smears stained with carbol fuchsin, mycobacteria typically appear as red rods (1-10 µm long and 0.2-0.6 µm wide) and often are beaded or banded, but also may appear coccoid or filamentous. In general, the microscopic appearance of the mycobacteria as slightly curved rods does not provide a species identification but may be suggestive for some species.

 

A positive stain and negative culture may be caused by nonviable organisms, such as might occur in persons receiving antituberculosis medication. Culture for mycobacteria involves inoculation of solid and broth media. For specimens such as sputum that are contaminated with normal bacterial flora, a selective medium containing antimicrobial agents should be inoculated. Cultures are incubated at 35° to 37° C in an atmosphere of 5 to 10% CO2. For specimens from cutaneous sites a second set of cultures should be incubated at 30° C. All cultures should be examined weekly for 8 weeks.

 

The major advantage of culture on solid media is that it allows visualization of colony morphology and pigmentation, which is useful diagnostically for distinguishing colonies of M. tuberculosis from those of some nontuberculous mycobacteria. However, they require 3 or 4 weeks. The more rapid broth systems (e.g. Bactec) require only 5 to 12 days, and rely upon the detection of 14C-labeled C02 produced by growing mycobacteria.

 

 Currently, standardized guidelines for susceptibility testing of mycobacteria have been developed only for isolates of M. tuberculosis a positive BACTEC TB vial (indirect test), or on sputum specimens that are smear-positive (direct test). Using a broth system, results are available 5-7 days after bottles are inoculated.

 

Treatment and Control:

 

In individuals with clinical disease, short term (6-9 month) ambulatory therapy with so-called first-line anti-mycobacterial drugs, such as isoniazid, rifampin, pyrazinamide, and ethambutol, results in disappearance of viable tubercle bacilli from the sputum, rendering the patient noninfectious. Prompt therapy, even in the absence of other signs or symptoms, is thought to sterilize the tissues and prevent endogenous reactivation of tuberculosis later in life. Patient compliance is probably the single most important variable affecting treatment outcome. Directly observed therapy (DOT) has been instituted in high prevalence areas, especially among recalcitrant patients, as the only reliable means of ensuring that patients complete their treatment successfully.

 

A viable, attenuated strain of M bovis, called bacille Calmette-Guérin (BCG), after the French microbiologists who developed the strain, has been used in more than 120 countries for many years as a vaccine to prevent clinical tuberculosis. Vaccination is the only feasible approach to controlling this disease in much of the developing world. The efficacy of BCG has varied in field trials from 0 to 85 percent, indicating an influence of unknown local environmental or host factors. BCG is not used in the United States because it results in PPD conversion, thereby interfering with the epidemiological and diagnostic value of the skin test.

 


المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .