Mycobacterium tuberculosis
Culture Medias
About
Mycobacterium tuberculosis (MTB), the causative agent of tuberculosis in humans, is an obligate aerobe that typically resides in the well-aerated upper lobes of the lungs. MTB is a small, slightly curved rod-shaped bacillus, which may be found individually or in groups. Its characteristic as a facultative intracellular parasite of macrophages and a slow generation time of 15-20 hours are believed to contribute to its virulence. This growth rate is slow compared to other bacteria, such as Escherichia coli, which divides approximately every 20 minutes. MTB has a notable resistance to weak disinfectants and can survive in a dry state for several weeks. This resilience is likely due to its unusual cell wall, rich in lipids such as mycolic acid, which is a key virulence factor. The growth of Mycobacterium tuberculosis is slow, and it may take up to six weeks for visible growth in a culture. The presence of mycolic acid in the cell wall gives the bacterium its acid-fast properties.
Characteristics
Gram-positive, bacilli, acid-fast, growth obligate-aerobic, no growth on blood agar, catalase-positive, nonmotile. Mycobacterium tuberculosis (MTB) is a bacterium that does not conform to traditional Gram-positive or Gram-negative categorization due to its unique chemical properties. Despite having a peptidoglycan layer in its cell wall akin to Gram-positive bacteria, it does not readily take up the Gram stain, often resulting in faintly stained or ghost cells - a telltale sign of the presence of Mycobacteria in a Gram-stained specimen. MTB is a substantial rod-shaped bacillus, measuring 0.2-0.5 x 2-4 µm. To visualize MTB in a sputum specimen under a microscope with 1000X magnification, the sample needs to have a concentration exceeding 10,000 bacilli per ml. A single acid-fast bacillus detected per slide raises suspicion of an MTB infection. Cord factor, also known as trehalose dimycolate, is a glycolipid component of MTB s cell wall. This compound causes the MTB bacilli to arrange themselves into elongated, slender serpentine cords - a characteristic that lends the cord factor its name. Notably toxic to mammalian cells, the cord factor impedes the migration of polymorphonuclear leukocytes and is predominantly produced in virulent strains of MTB.The cell wall of MTB, rich in lipids due to the presence of mycolic acid, prevents it from retaining conventional bacteriological stains. Thus, Ziehl-Neelsen staining or acid-fast staining is employed, in which the acid-fast bacilli are rendered pink against a contrasting backdrop. MTB is a facultative intracellular parasite, primarily of macrophages, and exhibits a slow generation time of 15-20 hours - a trait potentially contributing to its virulence.
Manifestation
Mycobacterium tuberculosis (MTB) is the bacterium responsible for causing tuberculosis (TB) in humans, who are the only reservoir for this pathogen. It is a significant health concern globally, being the leading bacterial infectious disease, affecting about one-third of the world population, approximately 1.8 billion people, annually. TB primarily attacks the lungs but can affect other parts of the body like the kidneys, spine, and brain. It spreads from person to person through the air when an infected individual with active TB in the lungs or throat coughs, sneezes, talks, or sings, releasing bacteria into the environment. It does not spread through physical contact, shared food or drink, or using the same bed linens or toilet seats. There are two TB-related conditions: latent TB and active TB. In latent TB, the bacteria live in the body without causing illness. The infected person does not feel sick, show symptoms, or spread the bacteria. However, if the bacteria become active and multiply, the person progresses from latent TB to active TB disease. In active TB, the bacteria multiply in the body, causing sickness and potential transmission of the bacteria to others. While many with latent TB never develop the disease, others may get sick shortly after infection or years later when their immune system weakens due to another reason. People with weakened immune systems, particularly those with HIV, are at higher risk for developing TB. The complex waxes and cord factor in MTB help it evade destruction by lysosomes and macrophages, which contributes to its pathogenicity. If TB is not properly treated, it can be fatal.
Laboratory Diagnosis
The detection of Mycobacterium tuberculosis (MTB) entails multiple diagnostic procedures: 1. **Microscopy:** Fastest diagnostic approach involving standard light microscopy and fluorescent microscopy. In AFB (acid-fast bacilli) staining, MTB appears as red-pink bacilli. Fluorescent staining techniques, such as rhodamine auramine or acridine orange, make the bacilli appear yellow-orange under UV light. 2. **Culture:** This more sensitive approach involves treating samples with NaOH, followed by centrifugation, and inoculating in a protein-rich medium like Lowenstein-Jensen or Middlebrook medium. After incubation at 37°C for up to 8 weeks, MTB forms dry, creamy colonies. Further identification involves biochemical tests including the niacin test, nitrate reductase test, and urease test, among others.3. **Molecular Diagnosis:** Due to the slow growth rate of MTB, molecular techniques like Nucleic Acid Amplification Tests (NAAT) are crucial for rapid TB detection. NAATs like PCR and others can identify MTB and provide information about drug resistance within a few hours or days. The GeneXpert MTB/RIF assay, for instance, simultaneously detects MTB DNA and resistance to rifampin in less than 2 hours. 4. **Antigen detection:** MTB antigen detection, such as Lipoarabinomannan (LAM), provides direct TB evidence, useful in rapid detection in various body fluids. 5. **Indirect methods:** These include tuberculin skin testing (TST) and interferon-gamma release assays (IGRAs), identifying immune responses to MTB. Both tests can be positive in latent and active TB, necessitating further evaluation for active disease presence. The IGRA assay measures interferon-gamma (IFN-γ) production stimulated by MTB antigens, like Early Secretory Antigen Target 6 (ESAT-6) and Culture Filtrate Protein 10 (CFP-10).