Key Points - Microbiology Flashcards
What are the 2 purposes of antibacterial and antifungal agents?
- To kill microorganisms
- To be selectively toxic - it be less toxic to the host than to the microorganisms
What is the difference between empiric and targeted therapy?
Empiric therapy:
- initial selection of antimicrobial agents
- need to be of broad enough spectrum to cover the range of organisms likely to be causing the infection
Targeted therapy:
- use of narrowest possible spectrum agents based on the ID and sensitivities of the causative organism (s)
What are the 3 steps involved in choosing the correct empiric therapy?
Know the likely organisms:
- based on site of infection, patient’s immunological status and microbiological history
Antimicrobial spectrum:
- identify what antimicrobial/combination is likely to have a spectrum that covers the organisms identified
Site of infection:
- select an antimicrobial/combination that is appropriate for the patient and site of infection
- consider antibiotic allergy, availability at different body sites, drug interactions and adverse effects
What is the difference between antibiotics and antimicrobial agents?
Antibiotics:
- natural products made by microorganisms
Antimicrobial agents:
- can be antibacterial, antifungal, antiviral
- synthetic or semi-synthetic compounds (chemically modified antibiotics)
- similar effect to antibiotics
What is the difference between bacteriostatic and bactericidal?
A bacteriostatic compound will inhibit growth
A bactericidal compound will cause microbial killing
What is the definition of MIC?
Minimum inhibitory concentration
this is the lowest concentration of antimicrobial that will inhibit the growth of a microorganism
low MIC = sensitive
high MIC = resistant
What is the definition of synergy?
Two agents together have more activity that one or other of the agents would have had if used on its own
What is the definition of antagonism?
Two agents together have less activity that one or the other of the agents would have if used on its own
What are the main cell wall synthesis inhibitors and protein synthesis inhibitors (antibacterial agents)?
Cell wall synthesis inhibitors:
- beta lactams
- glycopeptides
protein synthesis inhibitors:
- aminoglycosides
- clarithromycin
- mupirocin
- oxazolidinones
What are examples of antibacterial agents that act as membrane-active agents and DNA replication inhibitors?
Membrane active agents:
- daptomycin
- colistin
DNA replication inhibitors:
- quinolones
What are examples of antibacterial agents that act as nucleic acid synthesis inhibitors?
- Sulphonamides
- trimethoprim
- rifampicin
What are the 4 mechanisms of action of antifungals?
What are examples of each?
Cell wall synthesis inhibitors:
- echinocandins
DNA / protein synthesis inhibitors:
- 5-fluorocytosine
Membrane synthesis inhibitors:
- azoles
- terbinafine
Membrane active agents:
- polyenes
How should the diagnosis of infection be considered?
The diagnosis of infection is rarely certain
it should be considered as a probability
the patient should be considered as having a low, medium or high probability of infection
What is a differential diagnosis?
How can this be channeled down to a single diagnosis?
A differential diagnosis involves a list of possible diagnoses
Tests help you to improve the probability that is assigned to a patient’s diagnosis
Coming to a single diagnosis is an iterative process of testing and re-evaluation
What is meant by a test being ‘indicated’ or ‘not-indicated’?
All tests should be considered indicated or not indicated
This determines whether or not a certain test should be performed in a particular situation
e.g. A urine culture test is not indicated in a patient with suspected cellulitis
What factors should be considered when deciding what tests to perform?
- How easy the test is to do
- how quick the test is to do
- how expensive the test is
- how practical the test is to do
What are the 4 stages involved in the step-wise process of antibiotic selection?
- Determine the infection diagnosis (or working / differential diagnosis)
- Identify which bacteria are causing the infection
- Identify which antibiotics are effective against these bacteria
- Of the antibiotics, which have the right characteristics
e. g. given orally, side effects, etc.
What is meant by the MIC?
How can this be used to predict the efficacy of antibiotics?
The MIC is the minimum concentration of antibiotic which inhibits bacterial growth
Efficacy of antibiotics can be predicted by knowing how blood concentrations of antibiotics relate to the MIC
How can the efficacy of most antibiotics be predicted?
By knowing the time (in hours) that the blood concentration of antibiotic is above the MIC value
For an antibiotic to be maximally effective, what should it reach?
It should reach a certain number of hours above the MIC
this is known as the pharmacodynamics target
What happens if the MIC value of an antibiotic vs. A bacterial species goes up?
The antibiotic dose may need to increase for the antibiotic to still be effective
Above a certain MIC, bacteria are resistant to an antibiotic
Below a certain MIC value, bacteria are sensitive to an antibiotic
What is the variation within bacteria and humans that influences MIC value?
There is variation in the MIC value within a single bacterial species
There is variation in blood concentrations for a single antibiotic when multiple patients are tested
How can you predict how effective an antibiotic will be, given the variations in MIC and blood concentration?
Statistical simulations predict the effectiveness of an antibiotic
for a single dose of antibiotic, some patients may have insufficient antibiotic concentration for it to be effective
some patients may have a concentration that will be toxic
most patients are within the therapeutic window - they get enough drug to be effective, without it being toxic
What should be done if a patient gets antibiotic toxicity?
Check the blood antibiotic concentrations and / or lower the dose (if another antibiotic cannot be used)
if a patient is not improving on a dose of antibiotic, consider increasing the dose
What is the difference in efficacy between oral and IV antibiotics?
When an antibiotic has reached systemic circulation, IV and PO antibiotics can be considered equal
What is the purpose of antibiotic duration recommendations?
What are the consequences of longer and shorter antibiotic durations?
Antibiotic duration recommendations try to maximise cure and limit resistance & toxicity
longer antibiotic durations:
- may improve cure (up to a point)
- increase antibiotic resistance and toxicity
shorter antibiotic durations:
- may reduce cure
- may reduce resistance and toxicity
When should antibiotics be started?
Antibiotics should be started when the benefits of starting are greater than the disadvantages
What is the purpose of therapeutic drug monitoring?
To ensure patients are within the therapeutic window
they are not sub-therapeutic or toxic
When should surgical antibiotic prophylaxis be given?
It should be given within the hour before surgery i.e. before incision
