Mycobacterium tuberculosis is a slim, non-motile, non-spore forming, Gram-positive, obligate aerobe, and acid-fast bacillus (rod) with a waxy cell wall. It is found in the genus Mycobacterium and family Mycobacteriaceae. Aside M. tuberculosis, M. bovis (cattle/animal pathogen), M. avium and M. leprae (causative agent of leprosy/Hansen’s disease) are the other important species of the genus Mycobacterium that causes disease in humans. Other non-tuberculous Mycobacteria are members of the human normal microflora, found in water surfaces, are non-contagious and are generally referred to as atypical Mycobacteria. Consumption of unpasteurized (raw) milk and close contact with infected cattle can cause M. bovis infection in humans.
The cell wall of all bacteria in the Mycobacterium genus is different and very unique in that their cell wall is made up of high concentrations of lipids containing long chain fatty acids known as mycolic acid (which makes the cell surface of the bacterium hydrophobic). M. tuberculosis and other related species in the genus Mycobacterium are mostly called acid-fast bacilli because they show a tendency of acid-fastness upon staining. The reason for their acid-fastness is attributed to the high mycolic (fatty) acid content of their cell wall which makes them difficult to be stained readily; but once stained, Mycobacterium species resist decolourization by alcohol or acid – a phenomenon that aids in their detection and differentiation from other non-mycolic bacteria. M. tuberculosis is an airborne pathogen responsible for causing an infection known as tuberculosis (TB) in humans, and it is one of the leading causes of death in the world.
PATHOGENESIS OF MYCOBACTERIUM TUBERCULOSIS INFECTION
M. tuberculosis is an airborne pathogen, and thus the main route of transmission or acquiring the disease is through the upper respiratory system/airways. An infection with the tubercle bacillus, M. tuberculosis, is usually initiated following an inhalation of microscopic particles in aerosols that originate from an active pulmonary TB disease; and the incubation period of the disease after infection is usually 4-12 weeks. An exposure to M. tuberculosis may lead to infection, but most infections do not lead to a TB disease depending on the immune state of the affected individual. People with an active pulmonary TB disease expel plethora of infectious microscopic tubercle bacilli into the atmosphere when they sneeze, cough, expectorate/spit or speak. Because the infectious dose of TB is very low (about 5-10 bacteria), inhaling as little as 10 bacteria in an aerosol, droplets or dust particles can cause an infection.
The human respiratory system showing portal of entry and possible sites of M. tuberculosis infection in the body.
However, individuals with prolonged, frequent, or intense contact with TB disease persons are at particularly high risk of becoming infected than people with lesser contact. The immune system of the human host and the strain of the tubercle bacilli are critical in determining the pathogenesis of the disease – in that people with active immunity tends to resist and contain the disease more than those with a weakened resistance. After inhalation, the bacterium M. tuberculosis is carried past the upper and middle airways system until it reaches the alveolar surfaces of the lungs where they are deposited. Adjacent lymph nodes are also infected by the bacterium, and small inflammatory lesion is formed around these sites as well. Inside the lungs, alveolar macrophages surround the bacterium by the process of phagocytosis, and this leads to hypersensitivity reaction that forms tubercles (small and hard nodules characteristic of the TB disease). This is known as primary infection, and in this scenario; the host immune system is able to restrict and contain the tubercle bacillus within the pulmonary system– thus preventing it from spreading and leading to a disease state.
At this stage, TB infection is limited, and the lesions formed are self-healing even though all the tubercle bacilli may not be destroyed. People with weakened immune system experience active pulmonary infection, and this leads to the destruction of the lungs and the spread of the pathogen to other parts of the body leading to death. However, most cases of primary TB infections are usually handled well by the host immune system and the Mycobacterium continues to multiply intracellularly (but under the watch of macrophages) without any significant damage to the host cells.
In secondary infection (which may be active pulmonary or extra-pulmonary infection), M. tuberculosis in the lungs becomes reactivated after several months or years due to malnutrition, impaired immunity or poor health condition of the individual. Secondary (post-primary) infection can also occur when primary infections do not heal effectively and this is usually experienced in about 10% of the primary TB cases. Tubercle bacilli that survived the primary lesion or infection processes are mainly responsible for sparking up a post-primary (reactivation) types of TB. The further course of the TB disease (i.e. from primary to secondary infection) depends on the outcome of the encounter between the host’s specific cell-mediated immunity (CMI) and the tubercle bacilli itself.
CMI is usually protective and lifelong in some TB infected individuals. But in some other cases, the tubercle bacilli or particles become dislodged from their containment by the mechanisms of the CMI into the host’s airways later in their lifetime, and this occurs when there is a significant reduction in the individual’s T-cell immune response. At this stage, the pathogen multiplies sporadically in the host’s body, dissemination to vital body begins and the TB disease symptoms start to emanate. The clinical signs and symptoms of TB (i.e. in secondary infection) which only appears in pulmonary TB or extra-pulmonary TB (i.e. when the disease becomes active) and may resemble other lung/respiratory diseases include loss of appetite, chest pain, prolonged and productive coughs with blood, fever, unexplained weight loss, poor growth in children, and fatigue. The disseminated forms of TB which emanate from a secondary infection include military TB (which affect the spleen, lymph glands and liver due to dissemination of the pathogen via blood), tuberculosis meningitis (which affect the brain and meninges), renal and urogenital tuberculosis (which affect the GIT and kidney) and bone and joint tuberculosis (which affect the spinal cord or vertebrae).
DIFFERENCES BETWEEN A TB INFECTION AND A TB DISEASE
It is noteworthy that an infection with M. tuberculosis does not necessarily connote that someone has a TB disease. Exposure to aerosols, dust particles and respiratory droplets (e.g. sputa, sneeze and cough from an infectious TB patient) containing sufficient amount or dosage of tubercle bacilli is the first basis for the acquisition of M. tuberculosis. Whether the exposure that led to an infection will consequently result into a TB disease is dependent on so many factors including the strain of the infecting pathogen, the state of the host’s immunity, host’ health condition amongst others. TB infection and TB disease are quite two different phenomenons.
While the former (i.e. TB infection) presents no clinical symptom, is not infectious, has a normal chest X-ray results and shows a negative sputum smear and culture test results; the latter (i.e. TB disease) presents with clinical symptoms (cough, fever, and weight loss), is infectious, chest X-ray reveals lesions, and sputum smear and culture test results are positive. In both cases of a TB infection and TB disease, the bacterium M. tuberculosis is always present and skin (tuberculin) test results are always positive in these individuals. However, people with a TB infection are not infectious (i.e. they cannot spread the infection to other people) but people with a TB disease can easily transmit the causative agent of TB to susceptible non-infected individuals.
LABORATORY DIAGNOSIS OF MYCOBACTERIUM TUBERCULOSIS INFECTION
The primary specimen for the laboratory diagnosis of a potential TB disease is sputum that is collected in a screw-cap, leak-proof sample container. The reason for using a screw-cap, leak-proof sample container in collecting sputum instead of the usual snap-closing containers is to minimize the spread of the disease or pathogen through aerosols which non-screw-capped, leak-proof sample containers are capable of initiating. Blood, laryngeal swab, bronchoscopic specimens, pleural and peritoneal fluids, gastric lavage and CSF can also be collected from infected patients and analyzed for other TB types. Culturing and microscopic technique are usually the two main methods used for the detection of M. tuberculosis from sputum. Mantoux (tuberculin) skin test, DNA probe testing kits for TB, PCR and chest X-ray are other diagnostic tools or measures used for the clinical diagnosis of the disease.
Growth of M. tuberculosis on Löwenstein–Jensen (LJ) medium. Notice the colorless rough surface, which are typical morphologic characteristics of M. tuberculosis growth on LJ medium
However, the isolation of the acid-fast bacterium in culture and a positive microscopic staining technique are often the two most reliable methods for detecting the pathogen. Culturing of sputum specimen for the detection of M. tuberculosis is usually performed in Reference Tuberculosis Laboratories due to the expensive nature of this procedure using selective media such as the Löwenstein-Jensen (Middlebrook) culture medium. M. tuberculosis should be cultured for identification purposes and antimicrobial susceptibility testing. Selective media such as the Löwenstein-Jensen medium is used for the isolation of the bacterium. Because the tubercle bacteria are slow growing, culture medium are normally incubated for weeks and at a temperature range of 35-37oC. In summary, the diagnosis of tuberculosis requires the detection of acid-fast bacilli (AFB) in sputum of infected patients through the Ziehl-Neelsen stain or other reliable staining techniques as previously stated. Then this must be followed by culturing in a selective media for the identification of the pathogen and consequently to determine their susceptibility profiles.
Microscopical examination of a Ziehl-Neelsen acid-fast staining of M. tuberculosis. M. tuberculosis appears red or pink under the microscope as seen in this image. The acid fast stain usually depends on the ability of mycobacteria including M. tuberculosis to retain the colour of the dye (e.g. Ziehl-Neelsen) when treated with mineral acid or an acid-alcohol solution such as Ziehl-Neelsen or the Kinyoun stains.
TREATMENT OF MYCOBACTERIUM TUBERCULOSIS INFECTION
The successful treatment of TB disease is usually undertaken using multiple drugs to which the tubercle bacilli are susceptible to due to the possibility of the Mycobacteria in developing resistance to a single anti-tuberculosis drug. Thus, single drug regimens are often ruled out when considering therapy for a TB disease patient. Isoniazid (INH) and rifampin are the two drugs of choice used as first-line TB drugs for the treatment and management of a TB disease. Other first-line TB drugs are ethambutol, pyrazinamide and streptomycin (an injectable TB drug).
The second-line TB drugs include kanamycin (injectable), ofloxacin, capreomycin (injectable), amikacin (injectable), ethionamide, ciprofloxacin and cycloserine. Initial TB therapy is usually started with about three – four TB drugs that include the 2 first-line drugs (INH and rifampin) especially when susceptibility studies are still underway. But when susceptibility results become available, a two-drug therapy is given to the patient except in cases when resistance is anticipated, then the drugs can increase to three or four. The course of drug therapy for TB disease usually spans a period of 9 months in which INH and rifampin are administered concomitantly on a daily basis for about 2 months. TB therapy normally takes a longer period to complete because the pathogen is a slow growing organism, and thus longer time is required to kill them.
The use of two drugs in TB treatment helps to prevent the emergence of tubercle bacilli resistant to each of the drug. Second-line TB drugs are used and included in TB therapy when there is toxicity or resistance associated with any of the first-line drugs. Treatment with multiple anti-TB drugs helps to eradicate the tubercle bacilli and prevent the emergence of resistant strains or spread of the infection to non-TB individuals. In most cases, a direct observed therapy (DOT) in which the patients visits the health center or clinic to take their medication. DOT is anti-tuberculosis program where TB patients are regularly monitored to ensure that they take the full course of their medication so that resistance does not develop and the public health is protected. But when patients fail to comply with their TB regimen, the infection becomes reactivated and the individual becomes infectious again.
PREVENTION AND CONTROL OF MYCOBACTERIUM TUBERCULOSIS INFECTION
People living in high risk regions should be vaccinated with the Bacilli Calmette-Guerin (BCG) vaccine. It should also be administered to infants and children so as to protect them from the infection. BCG is a live attenuated bovine vaccine derived from M. bovis; and the name “BCG” was from the two French scientists (Calmette and Guerin) that developed the vaccine in the early 1920’s.
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