CAUSATIVE AGENTS OF TUBERCULOSIS. BIOLOGICAL PROPERTIES OF REPRESENTATIVES MYCOBACTERIUM TUBERCULOSIS. PATHOGENESIS, LABORATORY DIAGNOSTICS, CONTROL AND PROPHYLAXIS OF TUBERCULOSIS.
Tuberculosis (TB) is the most common cause of infectious disease–related mortality worldwide. TB is transmitted by airborne droplet nuclei, which may contain fewer than 10 bacilli. TB exposure occurs by sharing common airspace with an individual who is in the infectious stage of TB. When inhaled, droplet nuclei are deposited within the terminal airspaces of the lung. Upon encountering the bacilli, macrophages ingest and transport the bacteria to regional lymph nodes.
Taxonomy and classification of mycobacteria:
Genus Mycobacterium includes about 40 species
• Species causing tuberculosis are M.tuberculosis and M.bovis
• Species causing leprosy is M.leprae
Conditionary pathogenic mycobacteria (atypical mycobacteria) may provoke different diseases of immune-compromised persons
Saprophytic mycobacteria are isolated from smegma (M.smegmatis
M.tuberculosis is a rod, arranged singly or at small clumps.
M.bovis is short, thin rod
• It is acid fast bacillus
• It is non-sporeforming, non-capsulated, non-motile
• Its cell wall is resemble more to Gram-positive cell wall but it is poorly stained by Gram's
• It is revealed with Ziehl-Neelsen staining technique (once stained with hot fuchsine it resists decolourization with alcohols, acids, alkalis )
Cultivation of M.tuberculosis:
• It is obligate aerobe
• Optimum temperature is 370C, growth range is between 250C and 400C
• Optimum pH is 6.4-7.0
• It does not grow on the ordinary media
• Colonies of M.tuberculosis arise after 4-6 weeks of incubation, but growth of M.bovis appears within 6-10 weeks
Culture media for tubercle bacilli:
• Solid media
Containing egg: Lowenstein-Jensen, Petragnini, Dorset
Containing blood: Trashis
Containing serum: Loeffler
Containing potato: Pawlowsky
• Liquid media: potato-glycerol broth, Dubos, Middlebrook, Proskauer and Beck, Sauton, nutrient broth for L-form of tubercle bacilli, citrate blood
Cultural characteristics of tubercle bacilli:
On solid media:
M.tuberculosis forms creamy white, dry, rough, raised, irregular colonies with a wrinkled surface which become yellowish in old cultures
M.bovis grows producing white, flat, smooth, moist and easily emulsified colonies
On liquid media:
mycobacteria produce yellowish surface tuberous-wrinkled hydrophobic pellicle which may extend along the sides above the medium. After several days pellicle may sick on the bottom. The broth remains transparent.
Resistance of tubercle bacilli:
Due to high amount of lipids, waxes, fatty acids (hydrophobic properties and low permeability of the cell wall ) mycobacteria are :
Relatively resistant to phenol disinfectants, alcohols in routine concentrations, but sensitive to aldehydes, iodine solutions
Relatively resistant to desiccation and ultraviolet radiation (in dried sputum they survive within some weeks)
Enough sensitive to heating and dye at 600C after 15-20 min and at boiling after 5-7 min
Group specific antigens are polysaccharides and phosphatides
Type specific antigens are proteins
Cell walls of mycobacteria include a lot amount of allergens which induce hypersensitivity reactions.
Protein fraction (tuberculin) provokes delayed cell-mediated hypersensitivity which may be revealed with skin test (Mantoux test )
Virulent factors of tubercle bacilli:
Cord–factor is a major virulent factor.
Toxic for tissues fatty acids (phtioid, mycolic acids) provoke multiplying of epithelioid cells; fats and waxes provoke polymorphic reactions in tissues and giant cells formation
Antiphagocytic action of lipid components
Protein allergens induce delayed type of hypersensitivity
Epidemiology of tuberculosis:
M.tuberculosis causes chronic respiratory infections in humans
M.bovis is pathogenic for cattle, humans, carnivores.
Depending on time of infection, type of response and localization of infecting bacilli tuberculosis may be primary (after recent exogenous infection) and post-primary (more often by endogenous infection)
The source of tubercle bacilli may be:
1. Humans with open case of either pulmonary or extra pulmonary tuberculosis
2. Animals, especially ill cattle, shedding tubercle bacilli with urine, milk and sputum
Tuberculosis is air-borne infection.
The mode of transmission is by direct inhalation of aerosolized bacilli contained in droplet nuclei of sputum
But the more often humans are infected by inhalation of dried sputum contained in the dust
Infection with M.bovis arises by ingestion of contaminated non-pasteurized milk and dairy products
Post-primary tuberculosis is due to reactivation of tubercle bacilli localized in the lung of have infected person. According to world Health Organization (WHO) report indicated that major progress has been made on TB control, more than 20 million patients had been treated in DOTS programs worldwide and more than 16 million of them had been cured. Mortality of TB has been declining and incidence diminishing or stabilizing in all Worlds except sub- Saharan Africa and to some Eastern Europe. The global treatment success rate among new smear- positive TB cases had reached 83% by 2003 and in 2004 the case detection rate which has accelerated globally since 2001.
Pathogenesis of primary tuberculosis:
After inhalation the most part of bacilli are arrested in the upper respiratory tract. The bacilli which reach to alveoli will be ingested by alveolar macrophages.Tubercle bacilli withstand phagocytosis (due to a lot amount of lipids into the cell wall) and multiply into the macrophages. Accumulating mycobacteria stimulate an inflammatory focus and cell-mediated hypersensitivity. Activated macrophages release cytokines which are responsible for specific tissue lesion, named tubercle. Tubercle is an avascular granuloma, composed of a central zone with giant cells and peripheral zone with lymphocytes and fibroblasts (epithelioid cells).
In immune compromised person mycobacteria multiply in such lesion resulting in formation of Ghon focus.
From Ghon focus mycobacteria spread to hilar lymph nodes and cause their specific inflammation and enlargement (they are named together “primary complex”)
At best case, primary complex heals spontaneously in 2-6 months leading to a calcified nodule (a few bacteria may survive in such nodule)
At worst case, Ghon focus :
undergoes caseation with development of specific pneumonia and even generalised infection due to spreading of bacilli from lesion
Primary tuberculosis occurs in children from endemic region. The single tubercle lesion is localized in the middle or lower right lobe with enlargement of the draining lymph nodes
Secondary tuberculosis is diagnosed in adults.
It appears due to reactivation of latent infection or rarely due to exogenous reinfection It affects mainly upper lobes of the lungs.
The multiple lesions are necrotized resulting in either cavity formation or miliary pneumonic focuses. Also tubercle bacilli spread from lungs via lymph and blood to other organs such as spleen, liver, kidneys, bones, joints and others (extrapulmonary hematogenous dissemination)
Humans, as a rule, possess a high level of natural defense against tubercle bacilli.
Only 1-10% of infected persons fall ill with tuberculosis.
It depends on number and virulence of the infecting bacilli and host factors such as
nutrition, immunocompetence, genetic factors, presence of coexisting illness, stress and others In infected persons cell-mediated immunity with delayed hypersensitivity (allergy) develops Immunity is not sterile and disappears after elimination of mycobacteria from the host
Direct microscopy of smears from sputum stained with Ziehl-Neelsen technique (acid fast bacilli are bright red, other cells are blue)
It is reliable method of presumptive diagnostics when the number of shedding bacilli is about 10 000 per ml. Microscopy of smears from sputum after homogenisation and flotation (enriched methods).Homogenisation is carried out with alkali solutions, flotation is used with hydrocarbons. Fluorescent microscopy of smears stained with auramine or rodamine fluorescent dyes
allows to reveal from 10 to 100 bacilli per ml (high sensitive)
Before culture collected sputum is homogenisated and concentrated with alkali and acid
Material is inoculated into two media as following :
glycerol-potato broth and LJ medium
Cultures are observed for visible growth within 8-12 weeks .
Identification is based on:
Microscopy and speed of growth
Niacin test: (+) for M.tuberculosis and (-) for M.bovis
Nitrate reduction test: (+) for M.tuberculosis and (–) for M.bovis)
Laboratory diagnostics (experimental):
Animal inoculation (biological or experimental method)
The concentrated material is inoculated both into the guinea pig
(M.tuberculosis revealing) and rabbit (M.bovis)
Infected animals dye within 2-4 weeks
Biological method may used for detection of L-form in the sputum
Serological testing :
is rarely used, Complement fixation test, ELISA, precipitation tests,
mmunobloting and other have low diagnostic significance
Allergic test (Mantoux test) :
is carried out by intradermal inoculation of PPD-tuberculin
(purified protein derivate). It is used for detection recent infected persons ,
5 TU is injected intradermally on the flexor aspect of the forearm raising a wheal
The site of injection is examined after 48-72 hrs
Induration is measured and depending on size a conclusion is made as following: more than 10 mm will be positive result and less than 5 mm is negative results
After BCG vaccination allergic test will be positive some years but size of induration is decreasing. After infection size of lesion is more than in previous year (conversion of tuberculin test)
Convert persons must be examined to diagnose primary tuberculosis
Infected persons without any signs of tuberculosis should be prophylactic treated with phtivasid (isoniasid) during 3-6 months in endemic region
Persons with negative test must be vaccinated with BCG vaccine
Immunoprophylaxis is with alive attenuated BCG vaccine (Bacillus Calmette-Guerin is attenuated by serial subcultures of M.bovis on the glycerol-bile- potato broth over a 13 years)
It is administered to babies at third day after birth, Post-vaccinal immunity lasts some years (5-7 years),Buster vaccination is employed according to results of Mantoux test (negative persons), Infected persons are not revaccinated. Improving access treatment measures should be undertaken to indentify and address physical, financial social and cultural barriers – as well as health system – barriers to accessing treatment services. Particularly attention should be given to the poorest and most vulnerable population groups, particularly in the poorest rural area and urban settings, involving providers who practices close to where patients live, ensuring that services are free or heavily subsidized.
Specific antituberculous chemotherapeutic drugs used include Isoniasid, Ethambutol, Pyrazinamide,Rifampicin,Streptomycin. Therapy should be complex with at least two or more antituberculous preparations because in recent years a great number of multiresistant forms of tubercle bacilli cause infections in humans.