Acid-Fast Bacteria (Mycobacterium) Flashcards
1
Q
What are the major features of Mycobacterium? What are the two major species and what diseases does each cause?
A
- these are rods with cell walls that are highly concentrated with lipids; this makes them acid fast on staining (they are also weakly Gram positive, but stain much better with acid fast)
- Mycobacterium tuberculosis causes TB
- Mycobacterium leprae causes leprosy
2
Q
What is acid-fast staining?
A
- the sample is dyed with red stain carbolfuchsin and heated (the heat assists the dye penetration)
- acid alcohol is then added, followed by the methylene blue counter stain
- samples that have lipid rich cell walls (like Mycobacterium) don’t dissolve in the acid and therefore remain RED
3
Q
What is tuberculosis? How many patients with Mycobacterium tuberculosis will develop the disease? Which population is at particularly high-risk?
A
- tuberculosis is a chronic disease that presents slowly with weight-loss, low-grade fever, and various presentations based on the organ involvement (this presentation resembles cancer!)
- occurs in 10% of patients infected with M. tuberculosis
- it has a primary (initial exposure) and secondary (reactivation) stage
- high-risk group: HIV patients; 1/3 of these patients harbor Mycobacterium tuberculosis and are prone to developing the disease (because of a weakened immune system)
4
Q
Mycobacterium tuberculosis
A
- this organism causes tuberculosis
- acid fast, obligate aerobe
- facultative intracellular organism
- grows very slowly (it takes up to 6 weeks before visible growth can be seen)
- contains specialized virulence factors called mycosides (these are large fatty acids bound to different carbohydrates)
5
Q
What happens with an initial exposure of Mycobacterium tuberculosis?
A
- (initial exposure = primary TB)
- no specific immunity, so neutrophils and macrophages infiltrate the area (usually the lungs, the organism is largely inhaled)
- the organism survives in the macrophages (a facultative intracellular organism) and spreads via the blood to other organs
- some macrophages are able to manage phagocytosis and present the antigen to helper T cells, these cells are then able to activate and “empower” the macrophages to finally destroy the organism
- this immune response causes the local destruction and necrosis of the lung tissue that characterizes TB
- primary TB is often a subclinical (asymptomatic) lung infection and it forms cell-mediated granulomas
6
Q
What type of necrosis occurs in tuberculosis? What is a granuloma?
A
- when the macrophages are able to destroy the bacteria, caseous necrosis results
- around the area of necrosis are macrophages, giant cells, fibroblasts, and collagen (this is called a granuloma)
- the granulomas wall off the organisms, but they are still viable!
- (granulomas often calcify; these are called ghon foci/complexes)
7
Q
What happens when the latent organisms (walled off by the granuloma) are reactivated? Which organs/systems can be involved?
A
- (reactivation = secondary TB)
- reactivation mainly occurs via a weakened immune system
- secondary TB makes up most cases of symptomatic TB
- can be pulmonary (most common), pleural or pericardial (infection of the fluid around the lungs and heart), lymph nodes (2nd most common), kidney (sterile pyuria), skeletal (Potts disease), joints (arthritis), CNS (subacute meningitis, brain granulomas), or miliary
8
Q
What is miliary tuberculosis?
A
- this type of TB is characterized by tiny granulomas that are disseminated throughout the entire body and organs
- it usually occurs in children and the elderly
9
Q
What is the PPD test? What is the basis behind this test?
A
- PPD: purified protein derivative (it is killed Mycobacterium tuberculosis)
- this test is used to test for infection with M. tuberculosis
- it is a re-exposure (secondary exposure) to the organism and is based on a localized delayed-type hypersensitivity reaction (type IV HS)
- patients that are infected (can be active, latent, or resolved) will develop a hard, red, raised lump at the injection site within 1-2 days (this is a positive rest)
10
Q
What can cause a false positive PPD test? A false negative?
A
- false positive: patients who have had the BCG vaccine (this vaccine is given in some countries and only temporarily prevents TB in kids)
- false negative: patients who are anergic (so those who have a weakened immune system won’t be able to develop the reaction)
11
Q
How do we diagnose tuberculosis?
A
- with a PPD test, chest x-ray, and sputum acid-fast stain and culture
12
Q
What is leprosy? What is the severity of the disease based on?
A
- leprosy is a disease caused by Mycobacterium leprae
- it is AKA Hansen’s disease
- severity of disease is based on the host’s cell-mediated immune response (there are 5 subdivisions)
- patients who lack a cell-mediated immune response develop the most severe form of the disease called lepromatous leprosy (LL)
- patients who have complete cell-mediated immunity develop a mild form called tuberculoid leprosy (TL)
- the spectrum of severity: TL
13
Q
Mycobacterium leprae
A
- causes leprosy (AKA Hansen’s disease)
- acid fast
- facultative intracellular organism
- it is impossible to grow M. leprae on artificial media!
- prefers cooler body temperatures and areas close to the skin surface (spares warm areas such as the armpit, groin, perineum)
14
Q
Lepromatous Leprosy (LL)
A
- the most severe form of leprosy, occurs in patients who fail to mount a cell-mediated immune response against the organism
- the organism is found everywhere, and the disease usually involves the skin, nerves, eyes, and testes
- patients present with lion-like facies, saddle nose deformity, sterility, blindness, glove-and-stocking sensation loss
- death will occur if untreated
15
Q
Tuberculoid Leprosy (LT)
A
- the least severe form of leprosy, occurs in patients who mount a full cell-mediated immune response against the organism
- the organism is hard to find in these patients
- the disease is self-limiting here with only 1-2 skin and nerve lesions
