Microbiological diagnosis of Chest Infection Flashcards
What are the diagnostic techniques for chest infection?
Microscopy and culture of sputum and blood
Antigen detection methods
Nucleic acid amplification
Serology (measures antibodies)
What are the key aspects of microscopy and culture of sputum?
Gram stain
Culture for major respiratory pathogens
What are the significant major respiratory pathogens?
Strep pneumoniae
Haemophilus influenzae
Moraxella Catarhallis
How do you make sense of the sputum culture results?
Normal upper respiratory are not usually significant
Previous antibiotic therapy can be used to destroy E.Coli and Staph Aureus ruling them out from the cause of respiratory infection
Patient history can be used to distinguish between Community pneumonia vs aspiration pneumonia and ventilated patient
What stain is used for TB?
ZN or Auramine
TB is classed as an AAFB what does this mean?
Alcohol and acid fast bacilli - it cannot be decolourised by acids and alcohols
What length of time is required for TB culture?
Extended
When will M tuberculosis be able to be identified?
Only once it has grown
What is bronchiolar lavage?
A medical procedure in which a bronchoscope is passed through the mouth or nose into the lungs and fluid is squirted into a small part of the lung and then collected for examination.
In which part of the lung is airway sample collected?
Lower portions of the airway
What are the main advantages of a Bronchio-alveolar lavage?
Allows a quantitive culture in colony forming units per millilitre. (cfu/ml)
Less liable to contamination
Used for diagnosis of ventilator associated pneumonia
Who are blood cultures likely to be taken from?
Those with severe sepsis or bacteraemia.
Bacteraemia common in pneumonia patients
Is microscopy possible for blood samples?
No
When blood tubes are intubated how do they know if bacteria are present?
Build up of CO2 which will be picked up by a scanner - process usually takes less than 48 hours
What is done after bacteria in the blood is confirmed?
Overnight sub culture to assess identification and sensitivity
Day 2 confirms the bacteria and its sensitivity
Which organisms are not easily cultured (atypical causes)?
Legionella pneumophila
Mycoplasma pneumoniae
Chlamydia psittaci
Coxiella burnetti
What is a non-cultural method of demonstrating presence of an organism?
Detect specific antigen immunologically
PCR
Which antigens can be detected in urine and naso-pharyngeal secretions?
Legionella and pneumococcal antigen
What is latex agglutination?
Monoclonal antibodies are added to latex particles
This is then mixed with the clinical specimen
Agglutination is then possible with the naked eye if the organism is present.
What is a EIA?
Enzyme immunoassay
What does ELISA stand for?
Enzyme linked immunosorbent assay
How does an EIA work?
Specific antibody detects antigen
Linked enzyme on 2nd antibody generates colour change when the substrate is added
How does PCR detect DNA?
Specific short primers are added
Multiple copies of the target sequence are produced and detected
How can false positive results come about when using DNA detection?
Contamination
What is real time PCR?
Quantitive PCR - allows the product of the reaction to be measured as the reaction progresses
How is respiratory diagnosis achieved by PCR?
Throat swabs or other respiratory samples are used
What are the common respiratory viruses?
Influenza A and B Adenovirus RSV Parainfluenza viruses Metapneumovirus Coronavirus Rhinovirus
What is meant by serological diagnosis?
Antibody detection - It is a measure of the hosts antibody response to an organism, usually used when the culture or detection of the organism is difficult.
Previously used for viral infections infections and atypical causes of pneumonia
What does IgM represent in serology?
Current infection
What does IgG represent in serology?
Previous infection
How should the level of antibody change during the course of an infection?
Increase, although it takes some time for the antibody levels to rise (rising titre)
What other practical use does antibody measurement have?
Can be used to determine immunity (vaccine response)
What bacteria are difficult to diagnose?
Legionella pneumophila (urinary antigen and sputum culture) Chlamydia psittaci and C. pneumoniae Coxiella burnetii (Q fever
