Mycobacterium

Question 1

List at least 5 general characteristics of mycobacterium.

Answer 1

1. Thin, rod shape
2. Obligate aerobe
3. Cell wall with N-glucolylmuramic acid (high LIPID content)
4. Resistant to antibiotics (b/c of cell wall)
5. Slow-growing (4-8 weeks for colonies to be seen)

Question 2

List the components that can cause TB.

Answer 2

Mtb complex can cause tuberculosis (TB). These are non-pigmented colonies when seen ion Lowernstein-Jensen agar.

Question 3

What is the mode by which M. Tuberculosis (Mtb) is reanimated and causes human disease?

Answer 3

Transmitted from person to person via droplets in the air (sneeze, cough)

Question 4

Name at least 5 factors that affect the probability of Mtb transmission.

Answer 4

1. Susceptibility of exposed person
2. Infectiousness of TB patient
3. Environment in which the exposure occurred
4. Exposure length, proximity and duration
5. Virulence (strength) of tubercle bacilli

Question 5

What is the histopathological feature of Late TB Infection (LTBI)?

Answer 5

Granulomas as low power and Langerhan’s giant cell in faster

Question 6

_________ is less transmissible than M. Tuberculosis.

Answer 6

M. Bovine BCG

Question 7

Describe the characteristic of mycobacteria that can be obtained from a lung biopsy.

Answer 7

Slender acid-fast positive, slightly curved and beaded bacilli

Question 8

Describe the Tuberculin Skin test.

Answer 8

aka the Mantoux test involves a tuberculin (Mtb extract) that can recruit specific T-cells that make IFN-gamma to activate macrophages > inflammation.

Question 9

Contrast latent Mtb infection from active Mtb infection.

Answer 9

Latent Mtb infection can remain latent due to delayed type hypersensitivity seen by (+) IFN-gamma release of TST.
Active Mtb infection presents as reactivation, reinfection or progressive primary infection.

Question 10

How is the TST read and measured?

Answer 10

The Ring of induration is measured at 48-72 hours (DTH).
> 5 mm: (+) for HIGH-risk populations (organ transplants, HIV)
> 10 mm: (+) for moderate-risk populations (recent immigrants of high prevalent countries)
>15 mm: (+) for any persons

Question 11

List the advantages of Interferon-gamma release assays (IGRAs) over TST.

Answer 11

Whole blood plus Mtb peptides measure response to RECOMBINANT SPECIFIC Mtb antigens

Question 12

Under what conditions would both TST and IGRAs be performed?

Answer 12

1. Likely to be infected, HIGH-risk progression in children < 5 y.o.a. (TST > 5mM)
2. Likely to be infected at low to moderate risk progression (TST > 10 mM)
3. Unlikely to be infected (TST > 15 mM)

Question 13

Why would a physician perform a 2nd TST with an initial negative result? What of after an initial positive result?

Answer 13

1. Doc would perform a 2nd test when the initial is negative to increase sensitivity (and pick up a possible missed positive result)
2. She may perform another test after an initial positive as a confirmatory test for people unlikely to be infected with MTB

Question 14

List some high risk factors for HIGH risk of TB progression.

Answer 14

HIV infection, immunosuppressive therapy, silicosis and present in children <5 y.o.a.

Question 15

Compare LTBI (latent) from Active TB Disease.

Answer 15

LTBI = no symptoms, cannot spread to others, needs treatment for LTBI to prevent active form for up to a year
Active TB = symptoms of persistent, productive cough (>3 weeks), chest pain, fever, sweating at night; may spread to others, abnormal chest X-ray, needs multi drug therapy as treatment

Question 16

List some advantages of using molecular tools to detect mycobacteria over direct microscopic culture.

Answer 16

1. Fluorocarbons stain is more sensitive than Acid-fast stain. Positives must be confirmed by acid-fast stain
   2.

Question 17

What is an important virulent factor for Mtb that is dangerous for phagocytes?

Answer 17

Cord factor (trehalose dimycolate);
*Anti-cord antibodies are protective.*

Question 18

Describe the purpose of using nuclei acid amplification test on respiratory specimen of patients susceptible to TB.

Answer 18

A positive NAAT can be used as a presumptive piece of evidence of pulmonary TB disease in susceptible patients.

Question 19

What are non-tuberculosis mycobacteria?

Answer 19

These are ubiquitous env organisms that can be caused by trauma, inhalation or ingestion of them. They are Runyon classified into 4 groups

Question 20

What Group I NTM causes chronic pulmonary infection of the upper lung lobes and can be ID by DNA probe?

Answer 20

M. Kansasii is a photochromogen that has a major reservoir in tap water and grows slowly

Question 21

What Group I NTM causes “swimming pool granuloma” and can be identified by biochemical or molecular methods?

Answer 21

M. Marinum causes a cutaneous infection associated with exposure to salt/ freshwater after trauma

Question 22

Describe at least 5 features of M. Leprae.

Answer 22

1. LEPROSY aka Hansen’s disease
2. Spread from person-person via nasal secretions
3. Causes single, multiple or widespread anaesthetic skin lesions
4. Bacteria grow very slowly (years to decades incubation period)
5. Tuberculosis

Question 23

What feature of M. tuberculosis leads to effective transmission between hosts?

Answer 23

Impermeable lipid-rich envelope