Introduction to Mycobacteriology Flashcards
Describe the cell wall of Mycobacteria
- High lipid content
- Mycolic Acids
- Lipoarabinomannan
- Waxy coat
**Peptidoglycan layer is linked to arabinogalactan which is then linked to high-molecular weight mycolic acids. The arabinogalactan/mycolic acid layer is overlaid with a layer of polypeptides and mycolic acids consisting of free lipids, glycolipids, and peptidoglycolipids.
**It looks like lipoarabinomannan runs the width of the entire cell wall
How do you stain Mycobacteria for Identification?
- Acid Fast Staining
- They are acid fast bacilli which means that they bind to carbol fuchsin dye ina way that is not removed by acid alcohol (Resistance to decoklorization by acid)
Draw out the important pathogens table on SLIDE 4
Write them all out.
What causes the majority of TB in the US?
Mycobacterium Tuberculosis
**M. Bovis can also cause TB disease in people
Describe transmission of Mycobactria TB Via the respiratory route
- Particles can remain suspended in air for several hours. Large droplets are carried:
- > 6mm away by exhaled air at 50 m/s (sneezing)
- > 2mm away at 10m/s from coughing
- <1mm away at 1 m/s from breathing
What is is a granuloma?
- a mass of granulation tissue, typically produced in response to infection, inflammation, or the presence of a foreign substance.
- Granulomas form when the immune system attempts to wall off substances it perceives as foreign but is unable to eliminate.
- May include Epitheliod macrophages, Langhans giant cells, lymphocytes, plasma cells, and some PMN’S
What is Caseous Necrosis?
Morphological changes indicative of cell death caused by progresssive enzymatic degradation.
**The cellular outline is lost and the tissue appears crumbly and cheese-like
What is the Ghon Complex
-Calcified focus of infection and associate lymph node
- Tan-yellow subpleural granuloma that is the characteristic gross appearance with primary TB.
- Over time the granulomas decrease in size and can calcify as a reaction to tissue damage
Describe Latent TB infection. How prevalent is it among persons that have TB?
- Persons infected with TB but not sick, have no symptoms, and cannot spread TB
- This is about 90-95% of persons with TB
What is secondary TB? How is Progressive disease characterized?
- Reactivation of a previous latent infection
2. Progressive disease characterized by weight loss, toxicity of tumor-necrosis factor, cavitation, and fibrosis
What are the 2 categories for persons at risk for developing TB active disease?
- Those who have an increased likelihood of exposure to persons with TB disease
- Close Contacts
- Healthcare workers
- Congregate settings (shelters, prisons, nursing homes)
- Intravenous drug abusers
- Recent immigrants
- Those with clinical conditions that increase their risk of progressing fro LTBI to TB disease
- HIV-1 infection/AIDS
- Transplanation
- Age < 2 years
What is the Mantoux tuberculin skin test (TST)?
- Also called PPD for purified protein derivative
- It is the standard method of determining whether a person is infected with M. TB
How is TST Administered?
By injecting 0.1ml of tuberculid purified protein derivative into the inner surface of the forearm with tuberculin syringe and bevel of the needle facing upwards
How is TST read?
Read within 48 and 72 hours after administration by measuring millimeters of the induration
How is TST interpreted?
- Measurement in millimeters of the induration
2. Person’s risk of being infected with TB and of progression to disease if infected
What is the strategy behind doing a 2-step TB test?
To expose any thing that may be considered a “boosted reaction” due to a TB infection that occurred a long time ago. This would make the first test negative but then the 2nd test could come back positived
What is Type IV Hypersensititivy?
Delayed Type hypersensitivity.
The reaction takes 2-3 days to develop
What are Interferon gamma release assays? Name 3 types approved in the US.
- They are whole blood tests that can aid in diagnosing M. TB infection
The 3 approved in the US are:
- Quantiferon-TB Gold test
- Quantiferon-TB Gold in-tube test
- T-SPOT
How do IGRA’s work?
White blood cells from most persons that have been infected the M. TB will release interferon-gamma when mixed with antigens drived from M. TB
Compare advantages and disadvantages of using TST or IGRA
TST ADVANTAGES:
- No special equipment
- Lots of experience
- Predictive value
- Effectiveness of treatment for LTBI
TST DISADVANTAGES:
- Requires follow-up
- False positives
- False negatives
- Rare adverse skin reactions
IGRA ADVANTAGES:
- No follow-up visit
- No Interference from BCG vaccine and most other NTM
IGRA DISADVANTAGES:
- Requires a blood draw and equipment
- Cost
- False positives and negatives
*The CDC recommends using one or the other but not both. And neither one can tell you if the patient has Active TB
Describe the clinical features of active TB
- Cough is one of the earliest signs with production of sputum as tissue necrosis progresses
- Dyspnea a later symptom indicating extensive involvement of pulmonary parenchyma
- Fever and Weight loss reflecting systemic actions of IL-1 and TNF-a secreted by activated macrophages
What are some ways to collect specimen to test for active infection?
- Respiratory samples
- Ideally 3 early morning sputa
- Young children–gastric aspirate
- Ideally 3 early morning sputa
- Sterile Tissue/body fluid
- CSF, bone, pleural fluid, tissue but NOT blood culture
- Urine if signs of urinary tract disease
- early morning urines
What is needed to detect mycobacteria TB in a sputum sample?
An excess of 10,000 organisms per ml of sputum are needed to visualize the bacilli with a 100X microsope objective
Describe Kinyoun staining
- Uses carbol fuchsin as a primary stain, followed by decolorization with an acid-alcohol solution and methylene blue as a counterstain
- Has a greater concentration of phenol and basic fuchsin and does not require heating in order to stain properly as compared to other methods (Ziehl-Neelsen)
- Shows acid fast as red and non acid fast as blue
What does NAAT stand for? Give examples and describe it
- Stands for: Nucleic Acid Amplification Tests
- Examples: PCR, Gene Xpert MTB
- Can detect as little as 10 bacilli per sample. Results within 24-48 hrs. CDC recommends NAAT on at least 1 respiratory specimen from each patient
Describe the Runyon classification system and what each class is known as
Divided into 4 groups based on rate of growth, production of yellow pigment, and whether the pigment is produced in the dark or only after exposure to light
Runyon I : Photochromogens
Runyon II: Scotochromogens
Runyon III: Nonchromogenic
Runyon IV: Rapid Growers
Describe and give examples of each runyon class
Runyon I:
- Slow growth, yellow-orange pigment only when exposed to light
- Examples: M. Kansasii, M. Marinum
Runyon II:
- Slow growth, yellow-orange pigment in the light or dark
- Examples: M. Scrofulaceum
Runyon III:
- Slow growth, no pigment
- M. Avium Intracellulare Complex
Runyon IV:
- Colonies in 5 days, no pigment
- M. Fortuitum, M. Chelonae-abscessus
What type of infections would be associated with Rapid growers?
- Skin and soft tissue as well as lung infections
- Cytstic fibrosis, Lung transplant
What is a lymphocutaneous infection? Give examples of organisms that could cause them
- characterised by the appearance of subcutaneous nodules that progress along dermal and lymphatic vessels
- M. Marinum, Sporothrix Schenckii, Nocardia spp.
Describe M. Marinum.
- Photochromagen
- grows best at 30-32 degress C
What could be a result of traumatized skin coming into contact with water containing M. Marinum?
- Tensynovitis
- Arthritis
- Busitis
- Osteomyelitis
- Lymphatic spread
What different diagnoses would be relevant for acute/subacute or unilateral/bilateral cervcial lymphadenitis?
- Acute/Bilateral
- Viral upper respiratory tract infection or Streptococcal pharyngitis
- Acute Unilateral
- Streptococcal or staphylococcal infection
- Subacute or Chronic unilateral
- Bartonella (cat scratch)
- Mycobacteria-MTB, MAC, M. Scrofulacium
What is Scrofula?
Cervical Lymphadentitis
Name and describe some additional slow growers
- MAC
- NonChromogen
- M. Scofulacium
- Scotochromogen
- Slowgrower in lab (4-6 wks) all temps
- Environmental organism (Swamp Water)
