Mycobacterium Tuberculosis.docx

Mycobacterium Tuberculosis

Species The Mycobacterium

tuberculosis complex

(MTC)

consists

of Mycobacterium

africanum, Mycobacterium bovis, Mycobacterium canettii, Mycobacteri microti, Mycobacterium tuberculosis.

Description and significance Mycobacterium tuberculosis is a acid fast bacteria, which can form acid-stable complexes when certain arylmethane dyes are added. All species of mycobacteria have ropelike structures of peptidoglycan that are arranged in such a way to give them properties of an acid fast bacteria. Mycobacteria are abundant in soil and water, but Mycobacterium tuberculosis is mainly identified as a pathogen that lives in the host. Some species in its Mycobacterium tuberculosis complex have adapted their genetic structure specifically to infect human populations. M. tuberculosis can be isolated in labs and stored at –80 degrees to be studied extensively, and the most commonly used strain of M. tuberculosis is the H37Rv strain. One way to study M. tuberculosis in culture is to collect samples of mononuclear cells in peripheral blood samples from a healthy human donor and challenge macrophages with the MTC. M. tuberculosis has very simple growth requirements and is able to grow slowly in harsh conditions. Their acid-fast property is the strongest when there is glycerol around. However, when glucose is the main source of nutrient, the utilization of glycerol by M. tuberculosis is inhibited. Therefore, it’s been shown that glutatmate, and not glucose, is actually the main source of nutrient for initiating growth. Since as many as 32% of the human population is affected by Tuberculosis (TB), an airborne disease caused by infection of M. tuberculosis in one way or another, and about 10% of them becomes ill per year , it is not hard to imagine the significance in understanding the genome of the pathogen to develop and improve strategies for treatment by developing specific drugs that target the gene products of M. tuberculosis.

Genome structure Mycobacterium tuberculosis has circular chromosomes of about 4,200,000 nucleotides long. The G+C content is about 65%. The genome of M. tuberculosis was studied generally using the strain M. tuberculosis H37Rv. The genome contains about 4000 genes. Genes that code for lipid metabolism are a very important part of the bacterial genome, and 8% of the genome is involved in this activity. The different species of the Mycobacterium tuberculosis complex show a 95-100% DNA relatedness based on studies of DNA homology, and the sequence of the 16S rRNA gene are exactly the same for all the species. So some scientists suggest that they should be grouped as a single species while others argue that they should be grouped as varieties or subspecies of M. tuberculosis. Plasmids in M. tuberculosis are important in transferring virulence because genes on the plasmids are more easily transferred than genes located on the chromosome. One such 18kb plasmid in the M. tuberculosis H37Rv strain was proven to conduct gene transfers.

Cell structure and metabolism M. tuberculosis has a tough cell wall that prevents passage of nutrients into and excreted from the cell, therefore giving it the characteristic of slow growth rate. The cell wall of the pathogen looks like a Gram-positive cell wall. The cell envelope contains a polypeptide layer, a peptidoglycan layer, and free lipids. In addition, there is also a complex structure of fatty acids such as mycolic acids that appear glossy. TheM. tuberculosis cell wall contains three classes of mycolic acids: alpha-, keto- and methoxymycolates. The cell wall also contains lipid complexes including acyl glcolipids and other complex such as free lipids and sulfolipids. There are porins in the membrane to facilitate transport. Beneath the cell wall, there are layers of arabinogalactan and peptidoglycan that lie just above the plasma membrane. The M. tuberculosis genome encodes about 190 transcriptional regulators, including 13 sigma factors, 11 two-component system and more than 140 transcription regulators. Several regulators have been found to respond to environmental distress, such as extreme cold or heat, iron starvation, and oxidative stress. To survive in these harsh conditions for a prolonged period

in the host, M. tuberculosis had learned to adapt to the environment by allowing or inhibiting transcription according to its surroundings.

Ecology The Mycobacterium tuberculosis forms a complex with other higher related bacteria called the M. tuberculosis complex that consists of 6 members: Mycobacterium tuberculosis and Mycobacterium africanmum, which infect humans; Mycobacterium microti, which infects vole;Mycobacterium bovis, which infects other mammalian species as well as humans; M. bovis BCG, a variant of Mycobacterium bovis andMycobacterium canettii, a pathogen that infects humans. M. tuberculosis first infected humans 10,000-15,000 years ago. It has been found in early hominids originating in East-Africa. Therefore, studying the population structure of the species might provide insights about Homo sapiens' migratory and demographic history. Once inside the human host cell, Mycobacterium tuberculosis inflicts a contagiousinfectious disease called tuberculosis (TB), although the disease could either be latent or active depending on the ability of the person’s immune system to defend against the pathogen. In 1993, the World Health Organization declared TB a global public health emergency. It is estimated that one third of the world population is infected byMycobacterium tuberculosis, which leads to 8 to 10 million new cases, and 3 million deaths each year. The disease especially affects those in developing countries, those of the aging population and those who have HIV/AIDs because of their weak immune system. Because TB is an infectious disease for humans, it is important to sequence the genome of the Mycobacterium tuberculosis in order to find drugs fight against the bacteria by developing potential drug targets. Especially since Mycobacterium tuberculosis is multi-drug resistance and could cause latent infection, it is especially hard to treat and prompts scientists to research for new drug targets by looking through the Mycobacterium tuberculosis genome and gene products.

TUBERCULOSIS The Disease Tuberculosis (TB) is a disease caused by infection from the bacteria M. tuberculosis. If not treated properly, TB can be fatal . Currently, the World Health Organization estimates that over 13 million people have TB and about 1.5 million die each year from the disease. Tuberculosis most commonly affects the lungs (pulmonary TB). Patients with active pulmonary TB usually have a cough, an abnormal chest x-ray, and are infectious. TB can also occur outside of the lungs (extrapulmonary), most commonly in the central nervous, lymphatic, or genitourinary systems, or in the bones and joints . Tuberculosis which occurs scattered throughout the body is referred to as miliary TB. Extrapulmonary TB is more common in immunosuppressed

persons

and

in

young

children

.

When a person with active pulmonary TB coughs, sneezes, or talks, the bacteria that cause TB may spread throughout the air. If another person breathes in these bacteria, there is a chance that they will become infected with tuberculosis. Repeated contact is usually required for infection . However, not everyone infected with TB bacteria becomes sick. Roughly 5% of people infected with M. tuberculosis actually develop TB. People who are infected but not sick have latent TB infection. Those who have a latent infection are asymptomatic, do not feel sick, and are not contagious.

Symptoms Early symptoms of active pulmonary TB can include weight loss,fever, night sweats, and loss of appetite .Due to the vague initial symptoms of TB, an infected person may not feel that there is anything wrong. The infection can either go into remission or become more serious with the onset of chest pain and coughing up bloody sputum . The exact symptoms of extrapulmonary TB vary according to the site of infection in the body.

Transmission TB is spread through the air from one person to another. Microscopic droplets that contain the bacteria may be expelled when a person who has infectious TB coughs or sneezes. They can remain suspended in the air for several hours, depending on the environment. When a person breathes in M. tuberculosis, the bacteria can settle in the lungs and begin to grow. From there, they can move through the blood to other parts of the body. TB in the lungs can be infectious because the bacteria are easily spread to other people. TB in other parts of the body, such as the kidney or spine, is usually not infectious . If a person has confirmed TB or is suspected of having TB, the best way to stop transmission is through immediate isolation. Therapy should begin immediately. Contagiousness declines rapidly after a standard treatment regimen has begun, provided the patient adheres to the course of therapy.

Risk Groups Anyone can get TB. However, some groups are at higher risk to get active TB disease. People at high risk include those : 1. with HIV infection 2. in close contact with those known to be infectious with TB 3. with medical conditions that make the body less able to protect itself from disease (for example: diabetes, or people undergoing treatment with drugs that can suppress the immune system, such as long-term use of corticosteroids) 4. from countries with high TB rates 5. who work in or are residents of long-term care facilities (nursing homes, prisons, some hospitals) 6. who are malnourished 7. who are alcoholics or IV drug users

Infection Infection can develop when a person breathes in tubercle bacilli from expelled droplets from an infected individual. The droplets reach the alveoli of the lungs where the bacilli can be deposited (4). Alveolar macrophages ingest the tubercle bacilli and destroy most of them. Some can multiply within the macrophage and be released when the macrophage dies. From there, the bacilli can spread to other regions of the body through the bloodstream. The areas in which TB is most likely to develop are: the apex of the lung, the kidneys, the brain, bones and lymph nodes. This

process

of

dissemination

prepares

the

immune

system

for

a

reaction

.

In most infected individuals, the response from the immune system kills most of the bacilli. At this stage, a latent TB infection has been created, which may be detected by using the Mantoux tuberculin skin test (see below). Within weeks after infection, the immune system is usually able to halt the multiplication of the tubercle bacilli, preventing further progression. Most people recover completely from an initial infection, and the bacteria eventually die off . In some people, the tubercle bacilli overcome the defenses of the immune system and begin to multiply, resulting in the advancement to active TB disease. This process may occur shortly after infection or several years later .

Diagnosis The Mantoux tuberculin skin test, also known as the PPD (purified protein derivative) test, is used to detect TB infection. It is performed by injecting a small amount of tuberculin, a complex of purified M. tuberculosis proteins, into the skin of the arm. The reaction formed on the arm determines the result of the test. A positive reaction for TB infection only reports that a person has been infected with TB bacteria. It does not tell whether or not the person has active disease. Other tests, inclluding a chest x-ray and a sample of sputum, are needed to determine whether the person has active disease. In pulmonary TB, lesions on x-rays are often seen in the apical segments of the upper lobe or in the upper segments of the lower lobe. However, lesions may appear anywhere in the lungs, especially in HIV-positive and other immunosuppressed persons .

Stained slides of sputum smears ready for microscopy. World Lung Foundation 2006

An abnormal chest x-ray may indicate TB infection which is then confirmed or not by additional tests. Chest x-rays may be used to rule out the possibility of pulmonary TB in a person who has a positive reaction to the tuberculin skin test and no symptoms of disease. Persons suspected of having pulmonary TB generally will have sputum specimens examined by acid-fast bacilli smear and culture. Detection of acid-fast bacilli in stained smears examined microscopically may provide the first clear evidence of the presence of M. tuberculosis. Positive cultures for M. tuberculosis are used to confirm the diagnosis of TB .

Treatment of Tuberculosis M. tuberculosis is a very slow-growing, intracellular organism. Consequently, treatment requires the use of multiple drugs for several months . With appropriate antibiotic treatment, TB can be cured in most people. Treatment usually combines several different antibiotic drugs that are given for at least 6 months, sometimes for as long as 12 months. However, many M. tuberculosis strains are resistant to one or more of the standard TB drugs, which complicates treatment greatly . Currently, there are 10 drugs approved by the U.S. Food and Drug Administration for the treatment of TB. Of the approved drugs, isoniazid (INH), rifampin (RIF), ethambutol (EMB),

and pyrazinamide (PZA) are considered first-line antituberculosis agents. These four drugs form the foundation of initial courses of therapy Drug-resistant TB is major problem for the treatment of the disease. Multidrugresistant TB (MDR-TB), is defined as disease caused by TB bacilli resistant to at least isoniazid and rifampicin, the two most powerful anti-TB drugs . MDR-TB is intrinsically resistant to drugs but its resistance can be exacerbated by inconsistent or partial treatment. When patients do not take all their medicines regularly for the required period because they start to feel better, drugresistant bacteria can arise. While drug-resistant TB is generally treatable, it requires extensive chemotherapy (up to two years of treatment) with second-line anti-TB drugs. These second line drugs produce more severe adverse drug reactions more frequently than the preferred first line drugs. There are six classes of second-line drugs used for the treatment of TB 1. aminoglycosides 2. fluoroquinolones 3. polypeptides 4. thioamides 5. cycloserine 6. p-aminosalicylic acid

Prevention TB is largely a preventable disease. Isolation of patients and adequate ventilation are the most important measures to prevent its transmission in the community. In the United States, healthcare providers try to identify people infected with M. tuberculosis as early as possible, before they have developed active TB.