Mycobacteria I and II Flashcards
Unique Properties of Mycobacteria
- strictly aerobic (which is why you see TB reactivation in highly aerated upper lung lobes)
- rod shaped
- 0.5-3 microns
- high DNA GC content (61-71%)
- slow growing (3-6 weeks for primary isolation)
- resistant to desiccation, many disinfectants, and alkali
- airborne organisms killed by UV light
Cell wall of mycobacteria
-thought of as Gram-positive type with an inner cytoplasmic membrane covered with a thick peptidoglycan layer but no outer membrane. However, recent studies have shown that the unique lipids on the surface of the cell wall form a structure analogous to the outer membrane of Gram-negative bacteria.
From inside to outside:
- plasma membrane
- peptidoglycan matrix
- polysaccharide (arabinogalactan) that anchors long chain mycolic acids
- lipoproteins and glycoproteins (used for PPD)
What significance does the mycolic acid have?
-long chain lipids, make bacteria hardy and VERY DIFFICULT to STAIN
-once stained, resist destaining with acidified alcohol
= “acid fast”
Use Ziehl-Neelson stain
Virulent M. tuberculosis contain
-cord factor (glycolipid trehalose dimycolate)
What causes human TB?
Members of M. tuberculosis complex (M. tuberculosis, M. africanum, M. bovis, M. pinnipedii, M. microti, and M. canettii)
-1/3 of the world’s pop is infected with M. tuberculosis. Most show no signs of active disease (latently infected)
Risk factors for TB infection
Those with co-morbidity (diabetes, HIV/AIDS, silicosis), malnourished, elderly, poor, smokers, and chronic alcoholics.
Transmission of TB
- almost all due to M. tuberculosis via respiratory droplets when a person w/ infectious (not latent) TB coughs, sneezes, speaks.
- Infected contacts develop +PPD within 2-10 weeks
- 5-10% progress to active TB within first 2 years of infection, another 5% develop active TB within their lifetime.
- HIV patients with latent TB: 7-10% chance of developing active TB each year
Forms of TB
- active TB most commonly affects LUNGS
- can affect almost ANY organ (extrapulmonary TB: most commonly lymphatic or pleural disease)
- disease only lungs in 70%, extrapulmonary only in 20%, and both pulmonary and extrapulmonary in 8% of cases
- Miliary TB: disseminated form of TB, esp in immune compromised
Initial TB infection/immunology
- sx are absent or produce mild influenza-like disease.
- Cell mediated immunity response (mostly TH1) develops at 2-6 weeks
- humoral immunity: antibodies can be generated against several mycobacterial proteins but humoral immunity doesn’t play a big role in recovery/protection
- CD4+ T helper cells activate macrophages (kill intracellular bacteria or slow growth)
- IFN gamma and TNF alpha help control TB
- Bacilli released when macrophages die, spread thru lymphatic channels to lymph nodes, and through bloodstream to other tissues and organs
- On histology: bacteria confined in “tubercles”, granulomas of epithelioid cells, giant cells, and lymphocytes.
- Tubercles grow, centers become necrotic. Combo of lesion in lung and in draining bronchial lymph node = GHON COMPLEX.
What happens to TB lesions of primary infection in most people?
-For most people: lesions heal, calcify (macrophages kill/inhibit bacterial growth).
For others: some organsims persist in granuloma for decades, but there aren’t signs of active disease (latent TB infection)
-In first 2 years: 5-10% chance that TB will overcome immune defenses and progress to primary active disease.
Reactivation of latent TB
- secondary TB
- reactivation of mycobacteria that have been carried in body in latent form; often associated with impaired cellular immune func (AIDS, measles, corticosteroid therapy, cancer chemotherapy, and anti-TNFalpha therapy
Where is the most common site of TB reactivation?
apex of lung (highly oxygenated)
TB pathogenesis: What are the main elements of TB contributing to its survival?
- Bacilli reach alveoli, phagocytosed and multiply in alveolar macrophages.
- M. tuberculosis interferes with membrane controlled trafficking and phagosome maturation:
1. PIM: stimulates fusion between phagosomes and early endosomes, ensuring continual nutrient supply
2. Man-LAM: inhibits phagosomal maturation
3. SapM: cleaves the late endosomal vesicular marker phosphatidylinositol 3-phosphate (PI3P) in the phagosome membrane, which in turn prevents fusion of the M. tuberculosis phagosome with lysosomes
Cathelicidin
- antimicrobial peptide
- require Vit D for expression in macrophages
- kills intracellular M. tuberculosis in macrophages
Nitric oxide
- produced by activated macrophages
- role in human infection with TB unclear.
Aims of tuberculosis control
- identify active infectious cases and treat (stop transmission)
- identify contacts of infectious cases (treat to prevent from progressing to disease)
- identify high risk people with latent infection (prevent progression)
- vaccination with BCG in children in highly prevalent areas
Sx of pulmonary TB
pulmonary TB:
cough
chest pain
hemoptysis
systemic sx:
fever, chills, night sweats, appetite loss, weight loss, and fatigue
Diagnosis of TB
Mantoux tuberculin skin test (primary method to test for infection, not used for diagnosis b/c 20% of pulm and 50% of disseminated TB cases will have negative result)
Interferon gamma release assays (IGRAs): measures release of IFN gamma in whole blood in response to stimulation by various antigens. Dx for latent TB infection
Direct exam of sputum (acid fast; fluorescent method): Cannot determine WHICH mycobacteria are present. Culture on specific media used for species ID and drug resistance.
First line antituberculous agents
“RIPE”
Rifampin: Inhibits RNA synthesis by binding RNA polymerase; turns body fluids red-orange; drug-induced hepatitis and hypersensitivity reactions
Isoniazid: Inhibits mycolic acid cell-wall synthesis; drug-induced hepatitis
Pyrazinamide: Derivative of nicotinic acid, target unknown, requires acidic pH for activity, and is highly specific for M. tuberculosis; can cause arthralgias and drug-induced hepatitis.
Ethambutol: Inhibits arabinogalactan cell wall synthesis and is specific for mycobacteria; bacteriostatic (others = bactericidal); can cause optic neuritis.
Give all 4 to start.
Pyrazinamide stopped after 2 months.
Ethambutol stopped as soon as an isolate is shown to be drug susceptible.
INH and rifampin for total of 6 mo.
Second line anti-TB agents
Oral: Cycloserine, p-Aminosalicylic acid, Ethionamide, or Levofloxacin, Moxifloxacin
Injectables: Streptomycin, Kanamycin, Amikacin, Capreomycin
Drug-resistant TB
-drug resistance does NOT occur by acquiring foreign DNA that encodes resistance
Multi drug-resistant TB (MDR TB): resistance to at least isoniazid and rifampin.
Extensively drug-resistant TB (XDR TB): resistance to isoniazid and rifampin, plus resistance to any fluoroquinolone and at least one of three injectable second-line anti-TB drugs.
LTBI Diagnosis
-targeted testing for higher risk individuals or populations:
high risk: HIV, close contacts, recently infected, abnormal CXR
intermediate risk: foreign born from TB prevalent countries, injection drug users, homeless, certain medical conditions (diabetes mellitus, end-stage renal disease, taking immunosuppressive drugs (TNFalpha inhibitors), silicosis, malnourished)
Definitions for positive PPD skin test
Low risk individuals: greater than 15 mm of induration
Intermediate risk individuals: greater than 10 mm
High risk individuals: greater than 5 mm
With positive PPD or IGRA: symptom review and chest XR to rule out active TB
Advantages of IGRAs (like Quantiferon-TB assay) compared to standard tuberculin skin test (Mantoux)
- less cross-reactivity with BCG and non-TB mycobacteria
- less susceptible to reader variability
- only one patient visit to obtain results
- read as positive or negative regardless of patient’s risk level
Treatment of latent TB infection
One of the following:
• INH alone for 9 months given daily or intermittently
• Rifampin for 4 months given daily
• INH plus rifapentine for 3 months given once weekly
BCG vaccination
- live vaccine from attenuated strain of M. bovis
- Not used in US due to low incidence of TB and low efficacy
- little or no protection against adult pulmonary TB, but good for preventing/minimizing the severity of meningeal and disseminated TB (esp in young children)
- Drawback: now see positive PPD skin test (BUT if BCG given in infancy, it is unlikely to lead to positive PPD response later in life)
AIDS and Mycobacterial infections
- with HAART, rate of mycobacterial infec in AIDS pts has declined a lot
- 26-31 times more likely to develop active TB than people without HIV
- Disease is more likely to be extrapulmonary
Clinical manifestations of Nontuberculosis Mycobacteria (NTM)
- lower respiratory disease similar to TB (M. kansasii, M. avium-intracellulare, M. abscessus)
- cervical lymphadenitis (M. avium-intracellulare, M. scrofulaceum)
- skin and soft tissue infections (M. ulcerans, M. marinum and rapid growers)
- disseminated disease in persons with HIV
Epidemiology of NTM
- not transmissible between humans, acquired from environmental sources
- no latent infection
- important opportunistic pathogens in immunocomp.
- M. avium-intracellulare is the most common NTM causing lung disease
Diagnosis/Tx of NTM
- sputum examination (or other body tissues/fluids)
- drug susceptibility testing (many are drug resistant)
- surgical resection sometimes needed
- no vaccine
Mycobacterium avium Complex (MAC)
- M. avium, M. intracellulare, and M. chimaera
- Disseminated infections in immunocompromised
- Pulm disease in immunocompetent
- Children: cervical lymphadenitis
- likely environmentally acquired
- Tx: macrolide, ethambutol, rifamycin, and sometimes aminoglycoside
M. kansasii
- syndrome indistinguishable from pulm TB
- Tx: INH, rifampin, and ethambutol. The organism is inherently resistant to pyrazinamide
M. abscessus
- one of several “rapid growers” that can cause human disease.
- can produce pulmonary and cutaneous disease
- very difficult to treat because it is resistant to most antimicrobials including all of the standard TB drugs. Cure rates are usually below 50%.
Mycobacterium ulcerans
-Buruli ulcer
-linked to contaminated water
-produces mycolactone toxin– induces tissue necrosis leading to ulceration
Dx: by examination of the tissue for acid fast bacilli, mycobacterial culture at 32 degrees Celsius, histopathology or with PCR.
-Surgery is the treatment of choice for early disease.
-The WHO recommends treatment with rifampin and streptomycin for 8 weeks.
Leprosy
- Hansen’s disease
- M. leprae
- affects peripheral nerves, skin and mucosa leading to mutilation, social stigma, ostracism and denial of human rights.
- slow growing (can’t be cultured in vitro)
- propagated in armadillo and footpads of mice
- grows best at LOW temps, tends to affect skin and appendages
- primarily infects host cells of monocyte-macrophage lineage
- rare in US, important worldwide
- person to person spread (shed from nasal septa), but low rate of transmission
- 3-5 year incubation
- Two extremes: lepromatous vs tuberculoid
Lepromatous leprosy
- malignant form of disease
- strong Ab response but defect in cell mediated immune response and low/no delayed hypersensitivity to lepromin (due to active suppression by bacteria)
- loss of eyebrows, thickened and enlarged nares, ears and cheeks.
- Damage may be severe with loss of nasal bone and septa, and sometimes of digits.
- Both the skin and nerves are affected, with loss of local sensation.
- often leads to accidental injuries and secondary infections.
Tuberculoid leprosy
- better prognosis than lepromatous
- active cell-mediated immune response to lepromin
- blotchy red lesions on the face, trunk and extremities with loss of local sensation
- bacteria may grow in nerve sheath (more sensory nerve damage)
Diagnosis of leprosy
- usually clinical dx
(1) one or more hypopigmented, anaesthetic skin patches, (2) one or more thickened peripheral nerves, and (3) a positive skin smear. The most accurate way to diagnose leprosy is a tissue biopsy.
Tx of leprosy
dapsone
rifampin
clofazimine
Resistance is a problem
