Chapter 15: Mycobacterium Flashcards
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
Mycosides
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?
- bacteria appear to grow better in cooler body temperatures closer to the skin surface
- 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 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 Infertility blindness 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
