Bacteriology Flashcards
How is MIC tested?
least amount of antibiotics required to inhibit growth
Gradient antibiotic strips on agar - look for crossover of bacteria on the strip
Discs impregnated with different conc of antibiotic. Measure clerance zone guidelines have zone sizes on how this translated to MIC
What kind of investigations may be performed?
culture e.g. agar or a broth
Serology
Molecular techniques
What are the complications of starting patients on antibiotics before sending off samples?
The antibiotics can interfere with the test results - potential for false negatives as the dose administered may not have a clinical effect but it might have an effect at the cellular level on the agar
In meningitis or sepsis you administer anyway
What kind of samples might be used?
stool samples, mouth & vaginal swabs, taking blood
Describe susceptibility testing
Still use agar plates and MICs, takes about 18 hours
Describe how blood cultures are run
Different bottles for collection
pH indicator on bottom of bottle changes colour as pH lowered from bacteria producing CO2
takes about 18 hours
What kind of microbiological tests might be carried out?
Gram staining - double membrane are negative, single membrane are positive (peptidoglycan retains purple dye)
Microscopy - look at shape and organisation (clumping or chains)
Coagulase e.g. staf aureus is positive but other stafs are negative
What are the limitaions of microbiological investiagtions
slow - some e.g. TB can take 6 weeks to grow
MIC tests difficult to interpret
Explain the results of MIC
3 types of result resistant intermediate sensitive
guideline values for MIC enables the lab to say whether its sensitive or resistant – resistant because not enough of the drug will be able to reach the organisms. Guidelines have cross over values and zone sizes
What are the limitations of PCR?
- need to know what looking for to design primers
- potentially hundreds of primers needed as many genes confer resistance
- PCR less sensitive than culture
