Mycobacteria

Question 1

Mycobacteria species

Answer 1

• M. tuberculosis
• atypical mycobacteria
• M. leprae
• NOT mycoplasma

Question 2

M tuberculosis overview

Answer 2

• has existed as a human disease since at least 3000BCE
• the most common infectious cause of mortality worldwide
• > 1/3 of world population is infected
• was on the decline due to effective antiotic treatment (four drug regiments featuring isoniazid) until AIDS epidemic
• multidrug resistant (MDR) and extensively drug resistant (XDR) strains are becoming a public health emergency in US and abroad

Question 3

M. tuberculosis staining

Answer 3

• mycobacteria gram stain poorly, but stain with acid fast
  1) Cover smear with carbolfushsin. Steam over boiling water for 8 mins
  2) After slide has cooled colorize with acid-alcohol for 15-20secs
  3) Stop decolorization action of acid-rinsing briefly with water
  4) Counterstain with methylene blue for 30 secs
  5) Rinse briefly with water to remove excess methylene blue
  6) Blot dry with bibulous paper. Examine directly under oil immersion

Question 4

M. tuberculosis characteristics

Answer 4

• acid-fast
• grows in vitro but slowly and with special nutrients
• humans are natural host and reservoir
• can be intra- or extracellular
• mycobacteria produce no toxins
• drug resistance is chromosomal, no plasmids
• resistant to acid and alkali, environmentally hardy
• obligate aerobe
• pathogenic in guinea pigs- important for lab tests

Question 5

Important structural componenets of M. tuberculosis

Answer 5

• mycolic acids: acid fastness
• phosphatides: caseation necrosis
• Cord factor (trehalose dimycolate): virulence, microscopic serpentine appearance

Question 6

M. tuberculosis pathogenesis

Answer 6

• transmitted through inhalation of infected aerosols, rarely transdermal or GI infection
• extremely infectious: <10 organsims cause disease
• alveolar macrophages phagocytose the inhaled bacilli but are unable to kill the intracellular mycobacteria
• proliferates within mononuclear phagocytes traveling to extrapulmonary sites where it can establish latent or active infection in lymph nodes, kidney, bones, meninges
• swallowing infectious sputum infects GI

Question 7

Immunosuppression and M. tuberculosis

Answer 7

-immunocompetent hosts develop latent/dorment 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 8

Cell mediated immune response in M. tuberculosis

Answer 8

• a CMI response terminates the unimpeded growth of M. tuberculosis 2-3 weeks after initial infection
• 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 9

M. tuberculosis and active infections

Answer 9

• 85% of active TB includes lungs
• Bacilli proliferate locally and spread through the lymphatics to a hilar node, forming the Ghon complex, launch from there to the bloodstream
• exudative lesion plus draining lymph nodes are the Ghon complex

Question 10

Course of M. tuberculosis

Answer 10

• TB enters through inhalation
• TB lesion, replication
• Ghon complex (exudative lesion plus hilar node)
• infectious sputum, aerosal
• Milary TB
• TB meningitis
• TB granuloma
• Calcified TB granuloma
• reactive: Scrofula, Genitourinary, GI, Skeletal, Reactivating Pulmonary TB

Question 11

Risk factors for M. tuberculosis

Answer 11

• For infection:
• crowded at risk environments (prisions, hospitals, homeless shelters)
• HIV

For poor outcome is immunosuppression:

• Uncontrolled HIV (inadequate HAART)
• Steroids
• IFNgamma deficiency
• TNF-alpha antagonists (Remicade)
• Age <5 years

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

Question 12

Classive active pulmonary TB

Answer 12

• 75%
• presents with cough, weight loss (“consumption”), fever, night sweats, hemoptysis, and chest pain
• in CT cavity formation- indicates advanced infections, associated with a high bacterial load
• noncalcified round-infiltrates- may be confused with lung carcinoma

Question 13

TB scrofula

Answer 13

• reactivation in lymph node
• 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 by Mycobacterium tuberculosis
• Peds: trend is reversed: 92% of cases due to atypical mycobacterium

Question 14

Genitourinary TB

Answer 14

• most common site for extrapulmonary infection
• TB almost always reaches the kidneys during the primary infection but does not present clinically; may be 20 years of latency before symptoms
• genital tuberculosis is usually secondary to renal tuberculous infection

Question 15

CNS TB

Answer 15

• visualize by magnetic resonance imaging (MRI)

- CSF tests also useful

Question 16

Skeletal TB

Answer 16

• 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 17

GI TB

Answer 17

• CT scan show mesenteric lymphadenopathy with a hypoattenuating center suggestive of necrosis
• exploratory surgery
• can increase now with the raw milk movement

Question 18

Miliary TB

Answer 18

• 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

Question 19

TB meningitis

Answer 19

• develops in 5-10% of children younger than 2 years
• nuchal rigidity
• altered deep tendon reflexes
• lethargy
• cranial nerve palsies
• Brudzinski’s neck sign: because of inflammation when lift childs neck the knees pop up

Question 20

TB skin test

Answer 20

• PPD, TST
• purified protein derivative
• positivity develops 2-10 weeks post infection
• alternative: IFNgamma 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 immunosuppressed or late in the course of TB
• HIV+ patients must be regularly screened for TB and vice versa
• 15mm: positive, 10mm: positive if have risk factors, 5mm: positive if has deficient CMI

Question 21

Lab identification of TB

Answer 21

• acid fast staining of sputum- positive but some smear negative are still infectious
• culture: liquid media, takes 2 weeks
• begin susceptibility testing, but results take weeks

Question 22

TB treatment

Answer 22

• isolate infectious patients, negative pressure, high-efficiency masks
• 4 drug regiment featuring isoniazid
• for MDR-TB need to use 4-6 drugs
• take 3-6 months, (9-12 if CNS involvement)
• Directly Observed Therapy is recommended- nurses watch them take it
• pregnant women should have monthly ALT monitoring

Question 23

TB prevention

Answer 23

• good housing and nutrition

- semieffective vaccine availible

Question 24

BCG vaccine

Answer 24

• M. tuberculosis prevention
• live attenuated M. bovis
• prevents up to 70% of symptomatic infectious (wide batch variation)
• seldom used in the US
• watch for 3-6 mm PPD+ if vaccinated abroad or in military, can differentiate with IGRA
• not for immunocompromised

Question 25

Atypical Mycobacteria overview

Answer 25

- cause neither TB nor leprosy - environmentally acquired - PPD, TST usually negative - less aggressive infections, not lethal in guinea pigs - systemic disease very rare without predisposing condition - cuteneous infection most likely in immunocompetent adults, scrofula in children

Question 26

Group 1: Photochromogens

Answer 26

- produce pigment when grown in light - M. kansasii is environmental in Midwest, Texas, England produces pulmonary/systemic disease most closely resembling TB, killed by same antibiotics - M. marinum found in fresh and salt water, forms "fish tank" granulomatous, ulcerating lesions on abrasions exposed swimming water or aquariums, treat with tetracycline - the most common atypical mycobacterial infection

Question 27

Group 2: Scotochromogens

Answer 27

- produce pigment when grown in dark or light - M. scrofulaceum produces scrofula - reservoir is in water, can be harmess in respiratory tract - fix by surgically removing affected nodes

Question 28

Group 3: Nonchromogens

Answer 28

- no pigment 1) M. avium/ M. intracellulare are very difficult to distinguish, jointly called MAI, MAC - cause pulmonary disease indistinguishable from TB in severely immunocompromised pateitns - environmentally widespread, found in soil and water - high drug resistant, use claritheomycin in combinatin with ethambutal, rifabutin, or cipro

Question 29

Group 4: Rapidly Growing Mycobacteria

Answer 29

- no pigment - culturable in <1 week - M fortuitum/ M clelonei: - found in soil and water - cause problems in immunocompromised, prosthetic hips, indwelling catheters, puncture wounds - treat with amikacin+doxyclin plus surgical excision of site M. abscesssus: - environmental - chronic lung infections, skin, bone joints - highly antibiotic resistant M. smegmatis: normal flora under foreskin

Question 30

M. leprae Bacteriology

Answer 30

- Not culturable - reservoirs are humans (major) and armadillos (minor) - 14 day doubling time; slowest growing human pathogen - prefers 30C for growth, sticks to periphery of humans - genetically, appears to be a stripped-down version of M. tuberculosis

Question 31

M. leprae Pathogenesis

Answer 31

- causes leprosy, aka Hansen Disease - symptoms from both infection and immune respnonse - worldwide incidence is at historic lows, but Leprosy is still deemed a public health problem in 9 countries; they account for 85% of reported cases - ~150 cases/yr in the US - transmission is unclear- requires prolonged contact with infectious case, rare zoonosis from armadillos - extremely long incubation: months to 50 years - most commn sequel of exposure is asymptomatic seroconversion: only 5-10% of population is immunologically susceptible to symptoms

Question 32

Paucibacillary leprosy= tuberculoid form

Answer 32

- vigorous CMI contains disease (CD4+, Th1) but causes immunogenic problems - <5 dry skin lesions containing few bacteria - asymmetric immunogenic peripheral nerve damage - lepromatin PPD (+)

Question 33

Multibacillary leprosy= lepromatous form

Answer 33

- inadequate CMI response (useless Th2, nonprotective antibodies) - extensive skin involvement: >6 lesions, infiltrated nodules & plaques, bacilli may be visible on smears from lesion fluid - symmetric peripheral nerve damage from bacterial growth in Schwann cells - Lepromatin PPD-

Question 34

Lepromin skin test

Answer 34

-not diagnostic of exposure; used to determine patient's ability to raise immune response

Question 35

M. leprae treatment

Answer 35

- Tuberculoid: dapsone+ rifampin, 2 yrs - Lepromatous: dapson + rifampin +clofazimine, 2 years + (until lesions are free of organism) Peds: prophylaxis with dapsone if exposed