Flashcards in mycobacterial dz Deck (45)
1. Describe the unique properties of mycobacteria and how they create special problems for the isolation and identification of these organisms.
Aerobic rod shaped. Slow growing (require 3-6 weeks for primary isolation). Resistant to desiccation (M. tuberculosis is viable after 6-8months in dried sputum) and resistant to disinfectants, but can be killed by UV.
Why is mycobacteria resistant to disinfectants
high lipid content of their cell wall. Clinical samples containing M. tuberculosis are often treated with alkali, which kills most normal flora but not the mycobacteria, prior to culturing.
mycobacterial cell wall
Attached to the peptidoglycan matrix is a polysaccharide (arabinogalactan) that anchors the long chain mycolic acids. Also associated with this basic structure are lipoproteins and glycoproteins. Cell wall is rich in lipids (made of mycolic acids)
What does the tuberculin skin test test for
lipoproteins and glycoproteins attached to the cell wall of TB
What is acid fastness
Hallmark of mycobacteria- The mycolic acids of mycobacteria are long chain lipids of 60-90 carbons, that make the bacteria hardy and very difficult to stain, but once stained resist de-staining with acidified alcohol. Ziehl-Neelson stain stains mycobacteria red and others do not retain the stain.
Which mycobacterial strains can cause human TB
M. tuberculosis, M. africanum, M. bovis, M. pinnipedii, M. microti, and M. canettii. They are all from a common ancestor
What proportion of the world population is infected with TB
1/3 of the world. Most persons infected with M. tuberculosis become tuberculin skin test positive and harbor viable bacteria but show no signs of active disease and are considered latently infected
2. Describe how M. tuberculosis is transmitted and the odds of developing disease.
airborne droplet nuclei (size 1-5 mm) expelled when a person coughs, sneezes, or speaks who has infectious (not latent) TB. Rarely from ingestion. Infected contacts develop a positive delayed-type hypersensitivity reaction to the tuberculin test between 2-10 weeks after initial infection. 5-10% progress to active TB within the first 2 years of infection, another 5% develop active TB within their lifetime.
Who is at increased risk of developing active TB
HIV infected persons with a latent TB infection have a 7-10% chance of developing active TB disease each year. Persons with other medical conditions such as diabetes, end-stage renal disease, and immunosuppressive therapy are also at an increased chance for developing active TB.
forms of TB
Pulmonary, lymphatic, pleural, miliary (disseminated). Infants, HIV positive and immunosuppressed are more likely to get extrapulmonary and miliary forms
3. Describe the development of immunity to M. tuberculosis.
Cell mediated immunity develops at 2-6weeks (TH1). Cell mediated immunity controls infection. Th cells activate macrophages which kill intracellular bacteria. When the macrophages die, bacilli are released and spread to lymph nodes and other tissues/organs. Also IFNgamma and alpha control TB infection.
TB symptoms/ signs
During the initial infection symptoms are absent or produce a mild influenza-like disease. In healthy people, TB lesions will heal and become fibrotic or calcified (visible on CXR). Some mycobacteria may persist within granulomas for decades causing continued antigenic stimulation and possible reactivation later in life. Latent TB can not be spread
4. List the immune factors known to control M. tuberculosis.
Th1 cells (and macrophages), IFNg and IFNa.
The bacteria are confined in “tubercles”, granulomas (see figure) consisting of epithelioid cells, giant cells, and lymphocytes. With time the tubercles grow and their centers may become necrotic (caseous necrosis).
The combination of a (usually single)TB lesion in the lung and in the draining bronchial lymph node
5. Differentiate between primary, latent, and reactivation tuberculosis.
Primary: 5-10% chance in first 2 years post infection. In immunocompromised often spreads to all organs leading to potentially fatal miliary TB. Secondary: aka reactivation. Often associated with impaired immune function (AIDs, measles, corticosteroids, chemo, anti-TNFalpha therapy). Disease manifestations are in large part the result of hypersensitivity to tubercular antigens and are not the result of a specific bacterial toxin
Most common site of TB reactivation and pathogenesis
Apex of the lung (most highly oxygenated), ), but can occur in any tissue harboring latent bacteria from a primary infection. Lesions slowly become necrotic, caseous, and eventually liquefy. Adjacent lesions may coalesce to form larger lesions and eventually break through to the bronchi. Organisms actively grow both intra- and extracellularly, and may reach very high densities in these lesions. The bacteria-laden material is discharged into the bronchi resulting coughing and transmission the bacilli to other susceptible individuals.
6. Describe M. tuberculosis pathogenesis
Intracellular survival- inhaled bacilli are phagocytosed and multiply in alveolar macrophages. M. tuberculosis interferes with membrane controlled trafficking and arrests phagosome maturation at a stage when no harm can be done to the pathogen (marked by prevention of phagosome acidification), while delivery of nutrients by membrane bound vesicles is unimpeded.
How does M. tuberculosis arrest phagosome maturation
PIM: Stimulates fusion between phagosomes and early endosomes ensuring continual nutrient supply to the phagosomal compartment. Man-LAM: Inhibits phagosomal maturation. SapM: protein that cleaves the late endosomal vesicular marker PI3P in the phagosome membrane, which in turn prevents fusion of the M. tuberculosis phagosome with lysosomes
Antimycobacterial mechanisms of activated macrophages
The antimicrobial peptide cathelicidin, that requires vitamin D for its expression in macrophages, kills intracellular M. tuberculosis in human macrophages. The production of NO by activated macrophages is critical for control of M. tuberculosis infection in animal models. Its role in human infection is less clear. Superoxide production does not appear to be important for control.
7. Discuss the primary goal of tuberculosis control.
The primary goal of tuberculosis control is to identify active infectious cases and treat them in order to save the individual and stop transmission to others. Next step is treat latent infections to prevent progression. Finally, vaccination with BCG prevents disseminated forms in children
8. Describe the symptoms of active tuberculosis and two methods for detecting latent infection.
Symptoms of pulmonary TB include cough, chest pain, and hemoptysis. Systemic symptoms of TB include fever, chills, night sweats, appetite loss, weight loss, and fatigue.
8. Describe methods for detecting latent TB infection.
1. Mantoux tuberculin skin test: The primary method for testing but is not used to diagnosis TB because up to 20% of pulmonary TB cases will have a negative skin test and up to 50% of disseminated cases will have a negative test result. 2. Interferon gamma release assays (IGRAs): measure the release of interferon-gamma (IFN-γ) in whole blood in response to stimulation by various antigens.
describe the PPD test and what is considered positive results
A preparation of tubercular protein is injected intradermally under skin. A positive rxn is due to cell mediated immune response to the protein. Positive results after 2-3 days. Low risk: >15mm induration. Intermediate risk: >10mm induration. High risk: >5mm induration
describe the IFN gamma release assay and its advantages over PPD
Incubate pts lymphocytes O/N with M. tuberculosis antigens and measure IGN gamma release. Advantages: less cross-reactivity from vaccination with BCG and non-tuberculosis mycobacteria. They are less susceptible to reader variability. They require only one patient visit to obtain results, and may be more specific
9. Describe a typical antimicrobial regimen for treatment of active M. tuberculosis (drug sensitive).
Isoniazid, rifampin, ethambutol, and pyrazinamide are given for 2 months. Then pyrazinamide is stopped and ethambutol is stopped (when isolate is shown to be drug susceptible). INH and rifampin are continued for an additional 4 months for a total of 6 months. For cases slow to respond, 9 months
describe treatment of latent TB
INH alone for 9 months. Rifampin for 4 months. OR INH plus rifapentine for 3 months (given once weekly)
isoniazid MOA and adverse rxns
Inhibits mycolic acid cell-wall synthesis; specific for mycobacteria and activated by catalase; bactericidal; can cause drug-induced hepatitis.
rifampin MOA and adverse rxns
Inhibits RNA synthesis by binding RNA polymerase; turns body fluids red-orange (which is effective in determining patient compliance); bactericidal; can cause drug-induced hepatitis and hypersensitivity reactions