Mycobacteria: TB

Question 1

Mycobacteria

Answer 1

mycolic acid exterior, arabinogalactan, peptidoglycan

Question 2

Steps for acid fast staining

Answer 2

1- carbolfuchsin, steam over boiling water for 8 minutes
2- decolorize
3- rinse with water
4- counterstain with methylene blue
5- rinse to remove excess methylene blue
6- dry

Question 3

M. tuberculosis bacteriology

What are important structural components?

Answer 3

almost uniquely acid-fast

M. tuberculosis grows in vitro, but very slowly, and requires special nutrients

Humans are natural host and reservoir

Can be intra- or extracellular

Mycobacteria produce no toxins

Drug resistance is chromosomal; no known plasmids

Resistant to acid and alkali, environmentally hardy

Obligate aerobe

Important structural components:
Mycolic acids: acid fastness
Wax D: adjuvant (used in Freund’s)
Phosphatides: caseation necrosis
Cord factor (trehalose dimycolate): virulence, microscopic serpentine appearance
Phtiocerol dimycocerosate: lung pathogenesis

Pathogenic in guinea pigs

Question 4

Pathogenesis of M. tuberculosis

How does it establish infection?

Answer 4

Transmitted primarily by inhalation of infected aerosols; rarely transdermal or GI infection

Aerosols are extremely infectious: <10 organisms can initiate infection

Alveolar macrophages phagocytose the inhaled bacilli.

Naïve macrophages are unable to kill the intracellular mycobacteria

M. tuberculosis proliferates within mononuclear phagocytes, traveling to extrapulmonary sites, where it can establish latent (immunocompetent) or active (peds, HIV+, immunosenescence) extrapulmonary infection:
Lymph nodes
Kidney
Bones
Meninges

Swallowing infectious sputum infects GI
Mycobacterium bovis – cow/milk

Immunocompetent hosts develop latent/dormant infection: only 5-10% lifetime risk of active TB

Current or later immunosuppression allows reactivation.

Non-TB infections may activate quiescent TB.
Measles
Varicella
Pertussis

Question 5

M. Tuberculosis pathogenesis

What terminates the unimpeded growth of the M. tuberculosis 2-3 weeks after initial infection

Answer 5

A cell-mediated immune (CMI) response terminates the unimpeded growth of theM. tuberculosis2-3 weeks after initial infection. 2 parts:

CD4 helper T cells activate some infected macrophages to kill intracellular bacteria

CD8 suppressor T cells lyse other infected macrophages → caseating granulomas (“tubercules”)

Mycobacteria cannot continue to grow within these granulomas, so the infectious process pauses (latency).

TNF plays an important role in maintaining latency: Patients receiving TNF-alpha antagonists (Remicade) may reactivate.

Question 6

M. Tb pathogenesis

Answer 6

85% of active TB includes lungs

The most common site of the primary lesion is within alveolar macrophages in subpleural regions of the lung.

Bacilli proliferate locally and spread through the lymphatics to a hilar node, forming the Ghon complex, launch from there to the bloodstream

The Ghon complex, typical of pulmonary tuberculosis, consists of a parenchymal focus and hilar lymph node lesions. The detailed section of the diagram shows typical features of tuberculous granuloma: central caseous necrosis surrounded by epithelioid cells, multinucleated giant cells, and lymphocytes. From Damjanov, 2000.

Question 7

M. Tb pathogenesis

Lesions that develop around mycobacterial foci can be either proliferative or exudative. Both types of lesions develop in the same host:

Answer 7

Proliferative lesions develop where the bacillary load is small and host cellular-immune responses dominate.

Exudative lesions predominate when large numbers of bacilli are present and host defenses are weak. These loose aggregates of immature macrophages, neutrophils, fibrin, and caseation necrosis are sites of mycobacterial growth.

Question 8

What are risk factors for infection and poor outcome?

Answer 8

Risk factors for infection:
Crowded at-risk environments (prisons, hospitals, homeless shelters)
HIV

Risk factor for poor outcome is immunosuppression:
Uncontrolled HIV (inadequate HAART)
Steroids
IFNɣ deficiency
TNF-alpha antagonists (Remicade) 
Age <5yrs

TB cases in the US are at an all-time low but XDR strains are disproportionately represented here.

Question 9

Classic active TB presentation

Answer 9

presents with cough, weight loss (“consumption”), fever, night sweats, hemoptysis, and chest pain

Chest radiograph:
Cavity formation - Indicates advanced infection, associated with a high bacterial load
Noncalcified round infiltrates - May be confused with lung carcinoma

Question 10

Pulmonary TB

Answer 10

Homogeneously calcified nodules (usually 5-20 mm) - Tuberculomas; represent old infection rather than active disease

HIV+ Xray may look normal despite symptoms and sputum+

Fiberoptic bronchoscopy is the most effective procedure for obtaining cultures (bronchoalveolar lavage)

Question 11

Tb extrapulmonary tuberculosis

Answer 11

Extrapulmonary involvement in 20% of patients

60% of these are sputum-negative w/ normal chest radiograph.

Nonpulmonary symptoms mimic a wide variety of diseases

Question 12

Rb scrofula (reactivation in lymph node)

Answer 12

Painless, enlarging, or persistent mass. Cervical lymph node affected in 2/3. Systemic symptoms include fever/chills, weight loss, or malaise.

~95% of mycobacterial cervical infections in adults are caused byMycobacterium tuberculosis

Peds: trend is reversed: 92% of cases due to atypical mycobacterium (acquired by putting contaminated objects in their mouths)

Useful tests are PPD and fine-needle aspiration for culture

Surgery should be considered only after antibiotic treatment is well underway

Question 13

Genitourinary TB

Most common site for extrapulmonary infection?

Answer 13

Most common site for extrapulmonary infection

TB almost always reaches the kidneys during the primary infection but does not present clinically; may be 20yrs of latency before symptoms

Genital tuberculosis is usually secondary to renal tuberculous infection.

Females with genital tuberculosis may present with infertility, menstrual disorders, and pain, usually infected fallopian tube.

Pregnancy is unusual in the presence of genital tuberculosis. When pregnancy occurs, spontaneous abortion or ectopic pregnancy usually results.

Intravenous urography best option for identification of renal, ureteric, and bladder tuberculosis

Also detectable by ultrasonography, CT scanning, or MRI

CT scanning useful for assessment of renal function and severity of the disease, may also detect the involvement of other abdominal organs

All findings may be normal – small areas of calcification are difficult to detect. “Sterile pyuria” is suggestive

Surgical care: both infection and healing can block tubes

Question 14

CNS TB

Answer 14

Visualize by magnetic resonance imaging (MRI) with gadolinium enhancement

MRI is the most sensitive test for detecting the extent of leptomeningeal disease and is superior to computed tomography (CT) scanning in detecting parenchymal abnormalities, such as tuberculomas, abscesses, and infarctions.

Cerebrospinal fluid (CSF) analysis is usually used to detect a decreased glucose level, elevated protein levels, and a slight pleocytosis. Results of CSF polymerase chain reaction (PCR) assays may be diagnostic.

Question 15

Skeletal TB

Answer 15

Two main manifestations:
Arthritis of one joint

Pott disease (spinal infection): back pain, stiffness, paralysis of lower extremities

If suspect Pott, CT/MRI but do not delay treatment: paralysis can become permanent

Question 16

GI Tb

Answer 16

Rare
Abdominal pain
Weight loss
Anemia
Fever with night sweats 
Obstruction
Palpable mass
Radiograph for calcified granulomas
CT scans show mesenteric                                               lymphadenopathy with a                                                          hypoattenuating                                                  center suggestive of necrosis.
Exploratory surgery

Question 17

Miliary Tb

Answer 17

~1.5% of TB cases

Hematogenous spread of TB throughout body, many tiny noncalcified foci of infection appear “like millet seeds” in lung on chest Xray

Miliary form more likely to develop right after primary infection, less likely as a reactivation

Highest risk in very young and old (65y)

Fatal if untreated

History of cough and respiratory distress
Lymphadenopathy
Hepatosplenomegaly
Tachypnea
Cyanosis
Subtle: papular, necrotic, or purpuric lesions on the skin or choroidal tubercles in the retina.
Tiny nodules best visualized by chest Xray w/ bright spotlight, lateral Xray, chest CT
Begin treatment on first suspicion: one week of delay increases mortality

Question 18

Tb meningitis

Answer 18

Develops in 5-10% of children younger than 2 years

Nuchal rigidity

Altered deep tendon reflexes

Lethargy

Cranial nerve palsies

Question 19

Special pediatric considerations

Answer 19

Special Pediatric Considerations
TB in a child indicates recent transmission: track the contacts and index case
Unusual sites:
Middle ear
Skin
Ocular structures

Rule out TB if presents w/ pneumonia, pleural effusion, or a cavitary or mass lesion in the lung, failure to thrive, significant weight loss, or unexplained lymphadenopathy

Gastric aspirates are used in lieu of sputum in children younger than 6 years. (Don’t cough forcefully enough to bring up sputum)

Begin treatment as soon as samples have been taken for culture. Pediatric TB can be lethal before TST turns positive.

Question 20

M. Tb Diagnosis

look at slide 50 for sizes

Answer 20

Tuberculin skin test by purified protein derivative (PPD, TST)

Positivity develops 2-10wks post-infection.
Alternative: IFNɣ release assay using TB peptides (IGRA) blood test doesn’t require 2nd visit and is specific for TB, not vaccine
Either PPD or IRA may be false-negative if patient is badly immuno- suppressed or late in the course of TB
HIV+ patients must be regularly screened for TB and vice versa.

Sputum smears and culture are very specific but not very sensitive – repeat often

Urinalysis and urine culture can be obtained for patients with genitourinary complaints.

Cultures are needed for antibiotic resistance testing, but molecular (rRNA probe and PCR) tests are available for faster +/-

Traditional antibiotic resistance tests take 3-4wks because of TB’s slow doubling time; check for local alternatives – DNA sequencing or microscopic-observation methods are faster, can be as good depending on the reagents&personnel

TB bacteremia can be detected from blood cultures, particularly in immunosuppressed patients, but special media are required – alert lab

Question 21

M. tuberculosis Prevention

Answer 21

Keep at hospital for first two weeks of treatment if patient lives:
with immunocompromised persons
with children younger than 5 years
in a communal residence type of facility (eg, homeless shelter, senior citizen facility, jail, prison)

Alternate: home rest w/ TB mask
Control HIV (HAART), diabetes (insulin), underweight (better diet)
Good housing and nutrition are disproportionately helpful – healthy CMI drives TB latent (no new infections!). Population load of TB can be significantly decreased by improved standard of living even without access to antibiotics.

Question 22

M. Tb prevention

Answer 22

BCG vaccine:
Live attenuated M. bovis
prevents up to 70% of symptomatic infections -> forces into latency, doesn’t cure
seldom used in US
Watch for 3-6mm PPD+ if vaccinated abroad or in military: can differentiate w/ IGRA
NOT for immunocompromised