Mycobacteria I and II

Question 1

Unique Properties of Mycobacteria

Answer 1

• 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

Question 2

Cell wall of mycobacteria

Answer 2

-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)

Question 3

What significance does the mycolic acid have?

Answer 3

-long chain lipids, make bacteria hardy and VERY DIFFICULT to STAIN
-once stained, resist destaining with acidified alcohol
= “acid fast”

Use Ziehl-Neelson stain

Question 4

Virulent M. tuberculosis contain

Answer 4

-cord factor (glycolipid trehalose dimycolate)

Question 5

What causes human TB?

Answer 5

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)

Question 6

Risk factors for TB infection

Answer 6

Those with co-morbidity (diabetes, HIV/AIDS, silicosis), malnourished, elderly, poor, smokers, and chronic alcoholics.

Question 7

Transmission of TB

Answer 7

• 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

Question 8

Forms of TB

Answer 8

• 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

Question 9

Initial TB infection/immunology

Answer 9

• 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.

Question 10

What happens to TB lesions of primary infection in most people?

Answer 10

-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.

Question 11

Reactivation of latent TB

Answer 11

• 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

Question 12

Where is the most common site of TB reactivation?

Answer 12

apex of lung (highly oxygenated)

Question 13

TB pathogenesis: What are the main elements of TB contributing to its survival?

Answer 13

• 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

Question 14

Cathelicidin

Answer 14

• antimicrobial peptide
• require Vit D for expression in macrophages
• kills intracellular M. tuberculosis in macrophages

Question 15

Nitric oxide

Answer 15

• produced by activated macrophages

- role in human infection with TB unclear.

Question 16

Aims of tuberculosis control

Answer 16

1. identify active infectious cases and treat (stop transmission)
2. identify contacts of infectious cases (treat to prevent from progressing to disease)
3. identify high risk people with latent infection (prevent progression)
4. vaccination with BCG in children in highly prevalent areas

Question 17

Sx of pulmonary TB

Answer 17

pulmonary TB:
cough
chest pain
hemoptysis

systemic sx:
fever, chills, night sweats, appetite loss, weight loss, and fatigue

Question 18

Diagnosis of TB

Answer 18

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.

Question 19

First line antituberculous agents

Answer 19

“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.

Question 20

Second line anti-TB agents

Answer 20

Oral: Cycloserine, p-Aminosalicylic acid, Ethionamide, or Levofloxacin, Moxifloxacin

Injectables: Streptomycin, Kanamycin, Amikacin, Capreomycin

Question 21

Drug-resistant TB

Answer 21

-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.

Question 22

LTBI Diagnosis

Answer 22

-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)

Question 23

Definitions for positive PPD skin test

Answer 23

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

Question 24

Advantages of IGRAs (like Quantiferon-TB assay) compared to standard tuberculin skin test (Mantoux)

Answer 24

1. less cross-reactivity with BCG and non-TB mycobacteria
2. less susceptible to reader variability
3. only one patient visit to obtain results
4. read as positive or negative regardless of patient’s risk level

Question 25

Treatment of latent TB infection

Answer 25

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

Question 26

BCG vaccination

Answer 26

• 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)

Question 27

AIDS and Mycobacterial infections

Answer 27

• 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

Question 28

Clinical manifestations of Nontuberculosis Mycobacteria (NTM)

Answer 28

• 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

Question 29

Epidemiology of NTM

Answer 29

• 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

Question 30

Diagnosis/Tx of NTM

Answer 30

• sputum examination (or other body tissues/fluids)
• drug susceptibility testing (many are drug resistant)
• surgical resection sometimes needed
• no vaccine

Question 31

Mycobacterium avium Complex (MAC)

Answer 31

• 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

Question 32

M. kansasii

Answer 32

• syndrome indistinguishable from pulm TB

- Tx: INH, rifampin, and ethambutol. The organism is inherently resistant to pyrazinamide

Question 33

M. abscessus

Answer 33

• 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%.

Question 34

Mycobacterium ulcerans

Answer 34

-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.

Question 35

Leprosy

Answer 35

• 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

Question 36

Lepromatous leprosy

Answer 36

• 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.

Question 37

Tuberculoid leprosy

Answer 37

• 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)

Question 38

Diagnosis of leprosy

Answer 38

• 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.

Question 39

Tx of leprosy

Answer 39

dapsone
rifampin
clofazimine

Resistance is a problem