Diagnostics - Bacteriology lab Flashcards
List 4 common diagnostic techniques
- Culture
- Serology
- Molecular Techniques
- Antimicrobial susceptibility testing
What are the pros and cons of bacterial cultures
Pros
- Can be used to determine the correct antibiotic
Cons
- It takes up to 24 hours to grow the bacteria and another 24 hours to do the susceptibility testing
Whats the difference between a sterile site and a non sterile site
A sterile site (e.g. CSF and blood) should not have any bacteria in them so anything that grows there is abnormal.
A non sterile site has loads of bacterial that could have colonised but may not be causing any problems.
What is serology
Used to look at the body’s response to infection
e.g. the presence of antibodies after the infection
Pros and cons of molecular testing
Pros
- Rapid
- Sensitive
Cons
- Resistance genes can interfere with results
However in some circumstances such as MRSA where the resistance gene is known (MecA) you can check for this gene to know if it is resistant
What is antimicrobial susceptibility testing
You impregnate agar with a microorganism and put antibiotic discs on it
Why is gram staining done in an infection
Gram negative and positive respond to different antibiotics and they tend to cause different types of infection
Gram positive = skin and soft tissue
Gram negative = abdomen and urinary tract
What are chocolate agar and macconkey agar
Chocolate agar is cooked blood - cooking it releases some of the nutrients allowing certain bacteria to grow (e.g. haemophilus influenza)
Macconkey agar is agar designed to grow gram negative organisms
When would you give antibiotics before sending cultures?
Meningitis or miningococcal septicaemia
Whats the difference between gram negative and gram positive bacteria
Gram positive have a thicker peptidoglycan cell wall which holds the gram stain and stains purple
Gram negative has as outer membrane outside the cell wall which stops them form taking up the stain and they instead take up the counter stain turning pink
What makes some antibiotics ineffective against gram negative bacteria
They may target the cell wall but be unable to pass through the outer membrane e.g. vancomycin
Describe clumping of gram negative cocci
Most bacteria you find are gram positive cocci
When staphylococci divide they clump looking like bunches of grapes
Streptococci divide end on end to form chains
Explain the staphylococci coagulase test
Staphylococci can also be coagulase positive or negative. Coagulase is a virulence factor which aids in causing infection. The most important coagulase positive staphylococcus is staphylococcus aureus. Coagulase negative staphylococci are common skin microbes which infect opportunistically
What are the two groups of haemolytic streptococci
Alpha haemolysis - incomplete haemolysis turns the agar green (e.g. streptococcus pneumoniae)
Beta haemolysis - complete haemolysis clears the agar (e.g. group A = streptococcus pyogenes, Group B = Streptococcus agalactiae)
Bacterial causes of diarrhoea
- Salmonella
- Shigella
- Campylobacter
- E. coli
- C. difficile
- Cholera
What three bacteria are stools commonly tested for
- Salmonella
- Shigella
- Campylobacter
An unusual history or travel history could mean you look for other things
Where did C. difficile get its name
It got its name because it is difficult to grow in labs so detection of their toxin is used as the marker for it
What agar specifically tests for salmonella
Xylose lysine deoxycholate (XLD) agar
- Most coliforms or enteric bacteria can ferment xylose turning it yellow but salmonella cant
- Salmonella also produces hydrogen sulphide turning the agar black
How do you test for campylobacter
Campylobacter takes 48 hours to grow and can survive at up to 42 degrees so the sample is incubated at 42 degrees to kill any other bacteria
What colour does vibrio cholerae turn TCBS agar
Green
What is positive predictive value
The positive predictive value is that the more likely a patient is to have a disease the more likely a positive test result is a true positive
What is minimum inhibitory concentration
The lowest amount of antibiotic required to inhibit the growth of bacteria in vitro. If the MIC is above the breakpoint then the strain should be reported as resistant
