Key Points - Microbiology

Question 1

What are the 2 purposes of antibacterial and antifungal agents?

Answer 1

1. To kill microorganisms
2. To be selectively toxic - it be less toxic to the host than to the microorganisms

Question 2

What is the difference between empiric and targeted therapy?

Answer 2

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)

Question 3

What are the 3 steps involved in choosing the correct empiric therapy?

Answer 3

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

Question 4

What is the difference between antibiotics and antimicrobial agents?

Answer 4

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

Question 5

What is the difference between bacteriostatic and bactericidal?

Answer 5

A bacteriostatic compound will inhibit growth

A bactericidal compound will cause microbial killing

Question 6

What is the definition of MIC?

Answer 6

Minimum inhibitory concentration

this is the lowest concentration of antimicrobial that will inhibit the growth of a microorganism

low MIC = sensitive

high MIC = resistant

Question 7

What is the definition of synergy?

Answer 7

Two agents together have more activity that one or other of the agents would have had if used on its own

Question 8

What is the definition of antagonism?

Answer 8

Two agents together have less activity that one or the other of the agents would have if used on its own

Question 9

What are the main cell wall synthesis inhibitors and protein synthesis inhibitors (antibacterial agents)?

Answer 9

Cell wall synthesis inhibitors:

• beta lactams
• glycopeptides

protein synthesis inhibitors:

• aminoglycosides
• clarithromycin
• mupirocin
• oxazolidinones

Question 10

What are examples of antibacterial agents that act as membrane-active agents and DNA replication inhibitors?

Answer 10

Membrane active agents:

• daptomycin
• colistin

DNA replication inhibitors:

• quinolones

Question 11

What are examples of antibacterial agents that act as nucleic acid synthesis inhibitors?

Answer 11

• Sulphonamides
• trimethoprim
• rifampicin

Question 12

What are the 4 mechanisms of action of antifungals?

What are examples of each?

Answer 12

Cell wall synthesis inhibitors:

• echinocandins

DNA / protein synthesis inhibitors:

• 5-fluorocytosine

Membrane synthesis inhibitors:

• azoles
• terbinafine

Membrane active agents:

• polyenes

Question 13

How should the diagnosis of infection be considered?

Answer 13

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

Question 14

What is a differential diagnosis?

How can this be channeled down to a single diagnosis?

Answer 14

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

Question 15

What is meant by a test being ‘indicated’ or ‘not-indicated’?

Answer 15

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

Question 16

What factors should be considered when deciding what tests to perform?

Answer 16

• 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

Question 17

What are the 4 stages involved in the step-wise process of antibiotic selection?

Answer 17

1. Determine the infection diagnosis (or working / differential diagnosis)
2. Identify which bacteria are causing the infection
3. Identify which antibiotics are effective against these bacteria
4. Of the antibiotics, which have the right characteristics
   e. g. given orally, side effects, etc.

Question 18

What is meant by the MIC?

How can this be used to predict the efficacy of antibiotics?

Answer 18

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

Question 19

How can the efficacy of most antibiotics be predicted?

Answer 19

By knowing the time (in hours) that the blood concentration of antibiotic is above the MIC value

Question 20

For an antibiotic to be maximally effective, what should it reach?

Answer 20

It should reach a certain number of hours above the MIC

this is known as the pharmacodynamics target

Question 21

What happens if the MIC value of an antibiotic vs. A bacterial species goes up?

Answer 21

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

Question 22

What is the variation within bacteria and humans that influences MIC value?

Answer 22

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

Question 23

How can you predict how effective an antibiotic will be, given the variations in MIC and blood concentration?

Answer 23

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

Question 24

What should be done if a patient gets antibiotic toxicity?

Answer 24

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

Question 25

What is the difference in efficacy between oral and IV antibiotics?

Answer 25

When an antibiotic has reached systemic circulation, IV and PO antibiotics can be considered equal

Question 26

What is the purpose of antibiotic duration recommendations?

What are the consequences of longer and shorter antibiotic durations?

Answer 26

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

Question 27

When should antibiotics be started?

Answer 27

Antibiotics should be started when the benefits of starting are greater than the disadvantages

Question 28

What is the purpose of therapeutic drug monitoring?

Answer 28

To ensure patients are within the therapeutic window

they are not sub-therapeutic or toxic

Question 29

When should surgical antibiotic prophylaxis be given?

Answer 29

It should be given within the hour before surgery i.e. before incision