Mycobacteria-Vishy Flashcards
What are the characteristics of mycobacteria?
oRod-shaped bacteria.
oObligate aerobes.
oAcid-fast organisms (Once stained w/ carbol fuschin, they resist destaining by ethanol-hydrochloric acid.)
oDon’t secrete exotoxins or endotoxins.
oSlow growing bacteria and require special media.
What makes mycobacteria acid-fast?
Acid fastness is due to a thick cell wall made up of complex lipids.
•Arabinoglycan
•Mycolic acid: long chain fatty acids (give the acid fastness)
Since they don’t secrete exotoxins or endotoxins, what do they use to induce pathogenesis?
Have cell wall components that can induce pathogenesis.
•Chord factor: gives a serpentine appearance when grown in liquid media.
•Wax D: adjuvant enhancing immunogenic effects of antigens.
•Purified protein derivative (PPD): used in the TB skin test.
•Phthiocercol mycocerosate is required for pathogenesis in the lungs
•Phosphatides induces caseating necrosis in the lungs
What are the special growing requirements for mycobacteria?
Protein w/ malachite green.
Generation times:
•M. tb = 18hrs. (colonies take 4-5 weeks)
•M. leprae = 14 days.
*Only grown in footpads of mouse and armadillos, never in vitro.
What are some quick facts about Tuberculosis?
- Eight million new cases of TB and three million deaths annually
- One third of the world’s population is latently infected with Mycobacterium tuberculosis (LTB1)
•Have normal chest X-rays.
•Can only confirm with TB skin test. - Only 10% of infected individuals come down with the active disease.
What are the bacterial characteristics of TB?
- Tuberculosis (TB) is caused by Mycobacterium tuberculosis
* Can produce catalase and niacin.
What are the steps of infection with TB?
- Infection originates in the lung thru inhaled droplets.
- The particles are phagocytosed by alveolar macrophages.
- Macrophages release TNF-alpha which induces granuloma formation.
- Cell mediated immunity is initiated (CD4 T cells) because the bacterium is intracellular.
- Formation of a granuloma (90% of people)
- If no granuloma, the bacterium can spread to anywhere in the body leading to miliary tuberculosis.
- HIV patients w/ CD4+ T cells of less than 200/microliter have increased risks for TB infection and reactivation of latent TB infection.
What main factor is implicated in granuloma formation? What is the granuloma made of? What does granuloma formation do? What role do Th1 cells release? What do CD8+ T cells and NK cells release? What is the significance of caseum formation?
TNF-alpha: involved in formation and maintenance of granuloma.
•Granuloma is made up of monocytes, macrophages, giant cells and epithelioid cells.
•Granuloma restricts further infection.
•Chemokines attract neutrophils, monocytes and lymphocytes to the granuloma.
•Th1 subset of CD4+ T cells release IL-2 which induce T-cell expansion.
oThey also release IFN-gamma which activates monocytes to differentiate into macrophages.
oMacrophages can make ROS.
oIncrease NO production.
•CD8+ T cells and NK cells release perforin and granulysin which help.
•Antibodies do not help.
•The granuloma gets converted to a caseum (cheese-like structure) which is a hostile environment for the bacteria.
oHas low pH and oxygen tension.
oThe caseum heals by fibrosis
oThis doesn’t always kill the bacteria and it can stay latent.
oIn those 10% of people who can’t fight the disease for some reason, the caseum undergoes liquefaction which the bacteria like. They end up growing and multiplying leading to cavity formation in the lung.
What are the clinical findings for pulmonary, miliary and gastrointestinal TB?
- Fever, cough, weight loss, chest pain and night sweats (pulmonary tuberculosis).
- Lesions or millets in various organs, scrofula (swollen lymph nodes) (miliary tuberculosis).
- Diarrhea, abd. pain, fever and weight loss (gastrointestinal TB).
What are the lab techniques used to diagnose TB?
- Sputum specimens stained with carbol fuschin or auramine-rhodamine (more sensitive, used w/ fluorescent microscope).
- BEST way to confirm TB is to inoculate the sputum sample in bacterial agar and observe for colonies.
-Use Lowenstein-Jensen (LJ) media which has malachite green that prevents the growth of contaminating bacteria.
-BACTEC: quicker technique that uses radioactive metabolites (carbon). Determines mycobacteria. Can then use catalase and niacin assays to confirm M. Tb. - Tuberculin skin test:
•PPD is a combination of several M. tb proteins that can elicit hypersensitive reactions in individuals who were previously infected w/ TB.
•Used to determine if an individual has latent TB.
•Induration observed after 48 hours.
-Type IV delayed type hypersensitivity reaction.
•Positive findings:
-Person w/ low risk = 15mm or above.
-High risk groups = 10mm and above.
-HIV-infected subjects = 5mm and above. - QuantiFERON TB Gold kit: Involves one blood draw which is treated w/ M. tb antigens and the supernatants are collected and measured for IFN-gamma production.
How is TB treated?
•Before antibiotics, patients were kept in santoria and give nutritional food and sun bath.
•Vitamin D enhances host immune functions against TB infection.
Antibiotics:
•Normal patient: isoniazid, rifampicin and pyrazinamide.
oPyrazinamide for 2 months. The other two for 6 months
•HIV-positive patient: isoniazid, rifampicin, pyrazinamide and ethambutal for 9 months.
•Multi-drug resistant strain (MDR): resistant to rifampicin and isoniazid. Treat w/ ciprofloxacin, ethionamide and amikacin.
•Extreme drug resistant strains (XDR): resistant to fluoroquinolones.
•Biggest problem with treatment is patient compliance due to the long treatment times leading to drug resistance. Directly operated short course (DOTS) has been developed to combat this. (people make sure you take the medicine)
How is TB prevented in developing countries?
BCG: attenuated or avirulent strain of M. bovis. Protective efficacy = 0-80%. Given in countries where TB is endemic. (not U.S.A.)
What are the 3 major types of atypical mycobacteria? What is group IV?
Usually only affect immuno-compromised people.
3 Types:
•Group I (Photochromogens): Produce yellow-orange pigments in the presence of light.
•Group II (Scotochromogens): Produce yellow-orange pigments in the absence of light.
•Group III (Non-chromogens): Doesn’t produce pigments (or very little).
•Group IV: Include rapidly growing mycobacteria w/ short generation time.
What are 2 examples of Group I atypical mycobacteria? What do they cause?
Group I (Photochromogens): Produce yellow-orange pigments in the presence of light.
•Examples: M. kansasii and M. marinum.
oMarinum causes fish tank granuloma and is acquired when cleaning fish tanks or swimming in pools w/ the bacteria and you have skin ulcers.
What is an examples of Group II atypical mycobacteria? What does it cause?
Group II (Scotochromogens): Produce yellow-orange pigments in the absence of light.
•Examples: M. scrofulaceum
oEnters oropharynx and infects draining lymph nodes.
