Chapter 15: Mycobacterium Flashcards Preview

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Flashcards in Chapter 15: Mycobacterium Deck (26):

What are mycobacterium?

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


What is acid-fast staining?

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



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


Describe the pathogenesis with facultative intracellular growth of Mycobacterium tuberculosis

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


Describe the pathogenesis of Mycobacterium tuberculosis with cell-mediated immunity

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


What is the PPD skin test?

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


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

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


What causes a false positive PPD test?

BCG - bacillus Calmette-Guerin vaccine for tuberculosis
Effective in preventing sever forms of TB in children


What causes a False negative PPD test?

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


Primary Tuberculosis

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


Ghon focus:

calcified tubercle in the middle or lower lung zone
Accompanied by perihilar lymph node calcified granulomas called a Ghon, Ranke, complex


Secondary or Reactivation Tuberculosis

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


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

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)


How do you diagnose tuberculosis?

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


What is MDR/XDR tuberculosis?

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


What causes Hansen's disease?

also known as leprosy
Mycobacterium leprae

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


Clinical manifestations of leprosy are dependent on what 2 phenomena?

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)


What does Mycobacterium leprae destroy?

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


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

Lepromatous Leprosy
Tuberculoid Leprosy
Boderline Lepromatous
Borderline tuberculoid


Lepromatous leprosy

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
loss of sensation in extremities: glove and stocking neuropathy
Leads to death if untreated
No reaction to skin test


Tuberculoid leprosy

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


Borderline leprosy (3)

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


What is used to test for leprosy?

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


What are NTM

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


What is the most common cause of NTM lung disease?

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


What can NTM cause?

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