Chapter 15: Mycobacterium

Question 1

What are mycobacterium?

Answer 1

Rods with lipid-laden cell walls
High-lipid content makes them acid-fast staining
Obligate aerobe
Most commonly infects the lungs where oxygen is abundant
Grows very slowly - 6 weeks for visible growth

Question 2

What is acid-fast staining?

Answer 2

Smear of sputum covered in red stain carbolfuchsin and heated to aid dye penetration
Acid alcohol is poured over the smear
Coutner-stain methylene blue applied
Cell wall lipids of mycobacterium do not dissolve when the acid alcohol applied
Red stain does not wash off

Question 3

Mycosides

Answer 3

Mycolic acid: large fatty acid
Mycoside: mycolic acid bound to carb –>glycolipid
Cord factor: mycoside formed by 2 mycolic acids with disaccharide (found in mycobacterium tuberculosis), inhibits neutrophil migration, damages mitochondria, TNF,
Sulfatides: mycosides that resemble cord factor with sulfates, inhibit phagosome
Wax D: complicated mycoside, acts as adjuvant

Question 4

Describe the pathogenesis with facultative intracellular growth of Mycobacterium tuberculosis

Answer 4

First exposure usually by inhalation - no specific immunity
Local infiltration of neutrophils and Mo
Virulence factors cause phagocytosed bacteria to survive
Spread thru ymphatics and blood
Period short-lived bc of cell-mediated immunity

Question 5

Describe the pathogenesis of Mycobacterium tuberculosis with cell-mediated immunity

Answer 5

Sensitized T-cells enter circulation
Release lymphokines that attract Mo and activate them
Mo can destroy the bacteria
Mo attack results in local destruction and necrosis of llung tissue
Caseous necrosis: Mo, giant cells, fibroblasts, collagen deposits, calcifies

Question 6

What is the PPD skin test?

Answer 6

Checking if person has been infected with Mycobacterium tuberculosis
Latent tuberculosis
Exposure with cause Delayed-type hypersensitivity reaction
Positive test: treat and eradicate disease before significant damage to lungs

Question 7

What is considered a positive test for PPD? What does it indicate?

Answer 7

HIV pts: > 5mm
Common risk factors orHigh risk medical condition: >10 mm
In all others : > 15 mm

Does not mean the pts has active tuberculosis; indicates exposure and infection at some time in the past

Question 8

What causes a false positive PPD test?

Answer 8

BCG - bacillus Calmette-Guerin vaccine for tuberculosis

Effective in preventing sever forms of TB in children

Question 9

What causes a False negative PPD test?

Answer 9

Some pts do not react to PPD even if they have been infected
Pts anergic: lack normal immune response due to steroid use, malnutrition, AIDS, etc

Question 10

Primary Tuberculosis

Answer 10

first exposure via aerosolized droplet nuclei
Droplets land on middle and lower lung zones
Small area of pneumonitis with neutrophils and edema
Bacteria enter Mo, spread via lymph and blood

usually asymptomatic lung infection: cell-mediated defense kicks in, granulomas heal with scar formation, some bacteria remain viable, tiny tubercles too small on xray

Systematic: children, elderly, immunocompromised, symptoms occur, enlargement of mediastinal or hilar lymph nodes and lower middle lung infitrates –>lung necrosis –>holes

Question 11

Ghon focus:

Answer 11

calcified tubercle in the middle or lower lung zone

Accompanied by perihilar lymph node calcified granulomas called a Ghon, Ranke, complex

Question 12

Secondary or Reactivation Tuberculosis

Answer 12

Infection can occur in any organ system seeded during primary infection
Temporary weakening of immune system may precipitate reactivation (AIDS)

Question 13

What are the organ systems that can be involved in tuberculosis?

Answer 13

Pulmonary tuberculosis: most common site, Apical regions of lungs bc oxygen tension highest. lowgrade fever, night seats, weight loss, cough with blood
Pleural and pericardial infection: infected fluid collections
Lymph node infection: most common extrapulmonary manifestation of tuberculosis, cervical lymph nodes, SCROFULA
Kidney: Red and white blood cells in urine, but no bacteria bc it is slow to grow. Sterile Pyuria
Skeletal: spine - destroys intervertebral discs and adjacent vertebral bodies (Pott’s disease)
Joints: chronic arthritis of 1 joint
CNS: subacute meningitis, granulomas in the brain
Miliary tuberculosis: tiny tubercles (granulomas) all over the body (elderly and children)

Question 14

How do you diagnose tuberculosis?

Answer 14

PPD skin test
Chest X-ray to possible pick up Ghon focus, scarring, or granuloma
Sputum acid-fast stain and culture
Rapid molecular detection of MTB
Blood assays IGRAS

Question 15

What is MDR/XDR tuberculosis?

Answer 15

Multi-drug resistant tuberculosis and extremely drug resistant tuberculosis
Pts need extended care - 2 years!!

Question 16

What causes Hansen’s disease?

Answer 16

also known as leprosy
Mycobacterium leprae

impossible to grow on artificial media
India, Brazil, Burma, Indonesia, Madagascar and Nepal

Question 17

Clinical manifestations of leprosy are dependent on what 2 phenomena?

Answer 17

1. bacteria appear to grow better in cooler body temperatures closer to the skin surface
2. Severity of the disease is dependent on the host’s cell-mediated immune response to the bacilli (facultative intracellular)

Question 18

What does Mycobacterium leprae destroy?

Answer 18

skin (sparing warm areas such as armpit, groin, and perineum) superficial nerves, eyes, nose, and testes

Question 19

Leprosy is broken up into five subdivisions based on the level of cell-mediated immunity, which modulates the severity of the disease: name them

Answer 19

Lepromatous Leprosy
Tuberculoid Leprosy 
Boderline Lepromatous
Borderline
Borderline tuberculoid

Question 20

Lepromatous leprosy

Answer 20

Severest form
canNOT mount cell-mediated immune response
Defective T-suppressor cells (T-8 cells) block T-helper cell’s response

Face looks lionlike (leonine facies) 
Saddlenose deformity 
Infertility 
blindness
loss of sensation in extremities: glove and stocking neuropathy 
Leads to death if untreated
No reaction to skin test

Question 21

Tuberculoid leprosy

Answer 21

Can mount cell-mediated resistance, delayed hypersensitivity reaction intact,
Localized superficial, unilateral skin and nerve involvement
Only 1 or 2 skin lesions: well-defined hypopigmented, elevated blotches, hairless, diminished sensation
Enlarged nerves close to skin: greater auricular, ulnar, posterior tibial, peroneal

Bacilli hard to find in blood and lesions: pts noninfectious and spontaneously recover

Strong positive test to skin test

Question 22

Borderline leprosy (3)

Answer 22

Several skin lesions
Slightly decreased hair growth on skin lesions
Moderate lost of sensation in lesions of the extremities
Several acid fast bacilli in skin scrapings
Skin test shows no reaction

Question 23

What is used to test for leprosy?

Answer 23

lepromin skin test: similar to PPD
Measures ability of the host to mount DTH reaction
More prognostic than diagnostic

Question 24

What are NTM

Answer 24

Nontuberculous mycobacteria: expansive group of organisms that are ubiquitous in the soil and water
AIDS pts with disseminated MAC disease
Unexplained fevers, weight loss, diarrhea, general malaise, elevation of alkaline phosphatase

Question 25

What is the most common cause of NTM lung disease?

Answer 25

MAC
presents either as
1. upper lung cavitary disease predom male smokers
2. lower and middle lung involvement with bronchiectasis and nodular infiltrates din middle ages non-smoking women

Question 26

What can NTM cause?

Answer 26

pulmonary disease, lymphadentitis, skin lesions, bone and joint infections etc