Tuberculosis Mycobacteria Are Acid Fast Bacilli Biology Essay

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Mycobacteria are acid fast bacilli which is due to the high lipid content of their cell wall.They are neither gram positive or gram negative. Mycobacteria is a genus within the order Actinomycetales that comprise a large number of well characterised species. The major pathogens are Mycobacterium Tuberculosis the cause of tuberculosis and mycobacterium leprae, the cause of leprosy .Atypical mycobacteria such as Mycobacterium avium-intracellular complex and Mycobacterium kansaii, can cause tuberculosis like disease but are less frequent pathogens (Brooks,2007)

M.tuberculosis is slow growing, it has a doubling time of 18 hours in contrast to most bacteria in which it is usually an hour or less. Because growth is slow, specimens are usually held for up to 8 weeks until deemed to be negative. It is an obligate aerobe which explains the predominance of disease in the upper lobe of the lung and in the kidneys as they are highly oxygenated areas. M.tuberculosis is carried in airborne particles called droplet nuclei of 1-5 microns in diameter depending on the environment the particles can remain suspended in the environment for several hours (Brooks,2007). The likelihood of transmission depends on the infectivity of the source of the case, the degree of exposure to the case(e.g proximity,ventilation and length of exposure) and susceptibility of the person in contact with an infected case(BMJ,2013).

The development of TB requires infection by M tuberculosis and inadequate containment by the immune system. Patients infected with m tuberculosis who have no clinical, bacteriological or radiographic evidence of Active TB are said to have a latent TB infection. Active TB may occur from re activation of previously latent infection or from progression of primary infection. Transmission of TB occurs from individuals infected with pulmonary disease(BMJ,2013).

Pathogenesis of disease

Infection occurs by aerosol transmission, droplet nuclei containing tubercle bacilli which reach alveoli of the lungs. The organism deposits in the alveoli M. tuberculosis preferentially engulfs alveolar macrophages and other reticuloendothelial cells .It multiplies and survives within a phagosome. m.tuberculosis produces a protein called "exported repetitive protein" that prevents the phagosome from fusing with the lysosome, so the organism escapes the degradative enzymes in the lysosome(lange) this allows proliferation of tuberculosis within the macrophages. Cell Mediated Immune response Development of an activated lymphocyte response to m.tuberculosis antigen produces a type IV hypersensitivity response to the organism which results in the emergence of activated macrophages that can ingest and destroy the bacilli.(rubins) Exposure may lead to clearance of M.tuberculosis,persistent latent infection or progression to primary disease.T cells and macrophages form a granuloma with a centre that contains necrotic material ,m.yuberculosis, and peripheral granulation tissue consisting mainly of macrophages and lymphocytes, the granuloma serves to prevent furthrer growth and spread of m. tuberculosis.Theses patients are non infectious and have latent TB .Active TB occurs through a process of reactivation.Approximately 10% of patients with latent infection will go on to develop active disease.

Mycobacterium does not produce exotoxins nor does it have any endotoxins in its cell wall. The organism has developed mechanisms that allow for its survival in macrophages. Properties that contribute to its virulence include Mechanism of cell entry ,the tubercle bacillus can bind directly to mannose receptors on macrophages via cell wall associated mannosylated glycolipid. M.Tuberculosis can grow intracellularly .When TB has been phagocytosed it can inhibit the fusion of the lysosome and phagosome by secreting a protein that modifies the membrane of the phagosome. The slow generation time of TB may lead to the immune system not recognising the bacteria therefore the immune system is not triggered adequately enough. The high lipid content in the cell wall explains the reason for the impermeability and resistance to antimicrobial agents(text book bacteriology).Cord factor(trehalose dimycolate) is associated with virulence .Virulent strains grow in a cordlike pattern unlike avirulent strains which do not. M.tuberculosis contains several proteins that when combined with waxes provoke delayed hypersensitivity.These proteins are antigens in the tuberculi skin test.It is resistant to hydration and survives in dry sputum this may be important I aerosol transmission(lange)

Symptoms include cough,fever anorexia weight loss and malaise which are common .

Pulmonary TB locates in the lungs most cases are pulmonary. Extrapulmonartry TB occurs in places other than the lungs such as the larynx, lymph nodes, pleura, brain, kidneys, bones and joints.This is found more ofet in HIV infected or other immunosuppressed people and young children. Miliary TB is carried out to all parts of the body through the bloodstream however this is the more rare form.

Epidemiological aspects

Worldwide m.tuberculosis causes more deaths worldwide than any other single microbial agent(lange review)

Close contact for example with family or exposure from a hospital make transmission easier by droplet nuclei. Susceptibility to tuberculosis is the risk of acquiring the infection and then the risk of disease after infection. A tuberculin negative person depends on the source of the positive bacilli in order for them to be affected especially when there are sputum positive patients. Risk is shown to be proptrional to the rate of active infection in the population, crowding ,socio economic disadvantage and inadequacy of medical care. In More likely to acquire the infection at an earlier age in urban areas rather than rural populations. However disease only occurs in a small population of those affected with infection. Epidemiological patterns have been shown from population in the United States in which some individuals are more at risk than others. Minorities, predominantly African American and Hispanics ,HIV infected patients, homeless persons and the very young and very old.

Kills about 3 million people and affects around 9 million per year. Affects apparent healthy individuals as well as immunocompromised, particularly obvious with patients with AIDs(mims)

MDR TB caused by organisms resistant to both isoniazid and Rifampin which are the two most effective anti-TB drugs. These drugs are considered first line drugs. Foreign born persons increased from 325% in 1993 to 88% in 2009 (CDC web)

XDR TB is a relatively rare type of drug resistant TB that is resistant to isoniazid and rifampin plus any fluoroquinolone and at least one of three injectable second line drugs.(CDC website)No apparent trend in the number of cases over time in the USA. Greatest number of cases was 10 in 1993 no cases reported in 2003 or 2009.

The resurgence of TB disease began in the mid 1980s with increasing cases until it peaked in 1992 .The number of cases began declining in 1993.In 2009 marked the seventeenth year of decline in total number of reported cases from the United States. From 1993 until 2002 the total number of cases declined around 5%-7% annually. Further to this rates continued to decline from 2003 to 2008 but at a much slower pace. In 2009 50 states reported 11,545 cases. This represents a decline of 10.5% from 2008(cdc change wording)

Diagnostic strategy

Because of drug resistance especially to INH susceptibility test should be performed. Because the organism grows very slowly the tests usually takes several weeks which is too long to guide the choice of drugs. The luciferase assay can detect drug resistant organsims within a few days

First line test for active investigation include CXR, 3 sputum samples nucleic acid amplification test ,FBC and electrolytes.


sputum smear the ezaminer looks for AFB at least three samples should be collected minimum 8 hours apart including early morning specimen best to detect m tuberculosis. If sputum is positive graded from 1+ to 4+ depending on number of organisms seen .This may help to estimate the degree of infectiousness.

A sputum culture should always be performed as it is the most sensitive and specific test .It is required for precise identification and drug susceptibility testing.Growth may take 4to 8 weeks and is reoted on the same scale as the sputum smear.The patient will have sputum cultures performed at least monthly until two consecutive cultures are negative.

A full blood count conducted shows leukocytosis and anemia, this has been shown in 10% of pateints. Other abnormalities include elevated monocyte and eosinophil counts.

Nucleic acid amplification test(NAAT) DNA or RNA amplification tests can be carried out for rapid diagnosis on sputum or any sterile body fluid. Results are available in less than 8 hours. This is useful in smear positive disease to confirm that observed mycobacteria are M tuberculosis and in smear negative disease for rapid diagnsosis.

Other tests to consider include gastric aspirate used usually in patients unable to produce sputum, would show positive result positive for AFB. Bronchoscopy is useful when other diagnoses considered in patients in whom pulmonary TB is still suspected after other methods proved non diagnostic.

Tuberculin skin test,interferon gamma release assays measure response of T cells to TB antigen in order to diagnose prior exposure. genotyping, susceptibility testing. (best prac)

Progression nad monitoring of disease

Liver function shoud be checked prior to staring therapy .Monthly LFTs should be obtained in pateints with abnormal basleine liver function , underlying liver disease HIV co infection pregnancy and other risk factors for hepatitis. In general patients should have a CXR at the completion of treatment for pulmonary TB .In patients with multi drug resistance close follow up two years after completion of treatment is recommended.

Risk of developing TB is the highest after 2 years of infection(cdc)


Multi drug therapy is used in treatment to prevent drug resitant mutants emerging during the 6 to 9 month period of treatment. So if the organism becomes resitant to one drug will be inhibited bty the other. There are ten drugs currently approved by the FDA for treatment of Tuberculosis there are five main first line anti TB agents which include;Isoniazid(INH),Rifampin(RIF),Ethambutol(EMB),Pyrazinamide(PZA).Treatment for most patients with pulmonary tuberculsosis is with INH, Rifampin and Pyrazinamide. INH and rifampin are given for 6 months but pyrazinamide treatment is stopped after two months. In immunocompromised patients, and are likely to have INH resitance. ethambutol is also given. The four drugs are given for 9-12 months Although the sputum sample becomes non infectious withi 2-3 weeks the long therapy time is required because of the location of the organism is intracellular , the blocking of the entry of the drug, the organism grows slowly, some metabolically inactive organisms in the lesions remain therefore the drugs do not take effect on these so infection may reoccur at some stage.(cdc website not pdf)

Increasingly resiatance to INH and other drugs used for treatment of tuberculosis is being recognized in the United states this is especially true amongst immigrants from Latin America and South East Asia. Strains have emerged which are multidrug resistant (MDR) seen specifically amongst AIDs patients Mostly shown to be resitant to INH and Rifampin however some isolates have shown to be resistant to three or four drugs.(lange review)

Presently the prevention of spread depends on rapid identification and treatment of patients. The usage of masks and respiratory isolation is required to stop the spread to medical staff. Contact tracing of individuals exposed to a patient who has been coughing. PPD ski test is important tool in prevention this is used to identify recent coverters(?) and to give treatment for latent infections Gropus that should be screened with PPD test include people with HIV infection, close contacts of patients with acive tuberculosis,low income populations and intravenous drug users.As there were some problems associated with the skin test such as the measurement and interpretation of results .A laboratory test to to detect latent infection was developed. This test called quantiFERON-TB, this measure the amount of Ï’ interferon released from pateints lymphocytes afetr exposure to PPD in cell culture.

BCG vaccine can be used to induce partial resistance to tuberculosis. The vaccine conatins a strain of live , attenuated m.bovis called bacillus Calmette-GuerinThe vaccine is effective in preventing the appearance of tuberculosis as a clinical disease, especially in children although it does not prevent infection by M.tuberculosis. However a mjor problem with the vaccine is its variable effectiveness which can range from 0% to 70%.It is used primarily inareas of the world where incidence of the diease is high. (lange)


New methods fpr diagnosis may well eliminate the need for smear and culture are under decelopment. The new methods are nbased on the detection of specific components of the organism or on detection of specific antibodies produced by the patient. Although some methods will require expensive equipment making it leess avalible to those in developing countries. The use of gene probes for diagnosis of TB is in use although on a limited scale. This rticle suggests tha in the near future that nucleic acid techniques will be developed for worldwide use. The techniques will have the highest level of sensitivity and specificity ever achieved (Crawford et al 1989) A study demonstrated the importance of mycobacterial polysaccharide as a potential target for protective antibodies. Several past studies have indicated the importance of antibodies to mycobacterial polysaccharides were associated with protection.

Case study

A 65 year old man was admitted to hospital with a 5 month history of progressive weakness and a weight loss of 13 kg. He also had fever chills and a chronic cough productive eof yellow sputum occasionally streaked with blood.

The patient drank a lot of alcohol anlived in a boarding house next door to the tavern .He had smoked one pack of ci garettes a day for the past 45 years. The patient had no history of tuberculosis, no record of prior skin tests for tuberculosis or abnormal chest radiographs, and no known exposure to tuberculosis(lange)

Clinical Features

Temperature of 39°C, pulse 110/min, blood pressure 120/80 mm Hg.Head and neck examination was normal.During chest examination crackles were heard over the upper lung fields.

Laboratory Findings

Haematocrit was low (30%) and white blood cell count was 9600/µl.Other blood tests were normal.However chest radiograph showed extensive cavity infiltrates in both upper lobes.Tuberculin skin test was negative .A sputum sample obtained immediately and anacid fast stain .Numerous acid-fast bacteria were seen on the smear.Culture of decontaminated and concentrated sputum was positive for acid fast bacteria after 14 days incubation.Mycobacterium tuberculosis was identified by molecular probes after 2 days.Susceptibilty tests of theorganism showed susceptibiltity to isoniazid,rifampin,pyranazamide and streptomycin.


The patient was treated with isoniazid,rifampin,pyranazamide and streptomycin for two weeks followed by directly observed twice weekly administration of isoniazid and and rifampin for 16 weeks .Follow up sputum culture were negative for m tuberculosis.