Flashcards in L39 & 40. Using Antibiotics Deck (18):
What is the relationship between antibiotic usage and antibiotic resistance?
The higher the usage of antibiotics, the higher the incidence and development of antibiotic resistant infections
What considerations must be made when choosing antibiotic treatments? 
1. Making the right clinical diagnosis
2. Make the right microbiological diagnosis
3. Checking in vitro susceptibility
4. Host factors (eg. immunosuppression, pregnancy)
5. Properties of the Abx (eg. toxicity, kinetics, administration, distribution etc)
What is the MIC?
The Minimum Inhibitory Concentration: is the lowest concentration of the antibiotic required to inhibit bacterial growth in culture: gives an indication of susceptibility to the antibiotic
Describe the disc susceptibility test
A disc impregnated with antibiotics is plated onto a disc. The antibiotic is allowed to diffuse through the agar (concentration dependent). Susceptible bacteria grow on the plate with a Zone of Inhibition that is proportional to the MIC.
The Disk Diameter break points: determines the relationship between the disc diameter and the MIC
Describe the E-test strip
A modified method of the disc susceptibility test that uses a strip of exponentially graded antibiotic concentrations placed onto a plate that fives an MIC directly off it.
What specific considerations about the antimicrobial agent should be made? 
1. Spectrum of action
2. Clinical efficacy of the antibiotic (whether it works in vivo)
3. Route of administration (and excretion)
What are the three most common prescribing errors when it comes to antibiotics?
1. Prescribing them when they aren't needed (Eg. viral infections)
2. Prescribing the wrong antibiotic
3. Using the correct antibiotic but with the incorrect dose, course, route, patient
What is Empirical or Best Guess Antimicrobial therapy? Why do we use it?
- What considerations should be made when using it?
There are some infectious diseases that we don't know the causative and susceptibility of the organism and not treating leads to complications and symptomatic suffering.
Whether Abx is needed, safety and whether there is time to wait for a diagnosis, likely causative agents, susceptibility and whether evidence exists that treatment will help
Why do we use antibiotics in combination?
As a temporary measure (while waiting for more information)
To delay emergence of resistance
To treat mixed infections
Achieve synergistic effects
What is an example of using antibiotic combinations to decrease the emergence of resistance?
TB therapy: normally start with administering 4 frontline drugs because if one or two are resistant then another 2 exist to kill the bacteria before the resistant bacteria can spread.
Important to TB infections because a slow replication cycle means a higher likelihood for mutations (and resistance) to occur.
What are the three types of antibiotic combination interactions?
1. Indifference: an additive effect that is the same as using higher concentrations of either drug [A + B = A + A = B + B]
2. Antagonism: where they are worse than one alone
3. Synergy: where the combinations provide a supra-additive effect
What are the mechanisms of synergy?
1. Blockage of the sequential steps of a metabolic pathway (anything that gets through the first step is stopped by the next one)
2. Inhibition of enzymatic degradation (eg. co-amoxyclav)
3. Enhancing antimicrobial uptake by the cell wall
What is an example of synergistic antibiotics?
Penicillin G and Streptomycin
Amoxycillin and Covulanic acid
Folic acid synthesis is targeted by a combination of sulphonamides and trimethoprim
What are the mechanisms of antagonism?
1. Inhibiting bactericidal activity by a bacteriostatic agent (tetracycline and penicillin)
2. Inducing enzymatic degradation of one agent by another agent
3. Competition for binding to the same target
4. Inhibition of the target
What are examples of antagonism?
Ampicillin + piperacillin (ampicillin can be resistant and induces the B-lactamase too be produced and so the pipercillin which was susceptible is rendered useless because the beta lactamase degrades it)
Why is treatment of meningitis by penicillin + tetracycline not useful?
Tetracycline is bacteriostatic and stops the growth of all bacteria meaning that synthesis of the PG doesn't occur anymore. Thus the target for penicillin isn't available anymore and so it doesn't work at all. Such that the bacteria are now resistant and protected against penicillin
What are Jawetz's Laws?
A rough guide for combination interactions:
1. Bacteriostatic + Bacteriostatic = Additive or Indifferent
2. Bacteriostatic + Bactericidal = Antagonistic
3. Bactericidal + Bactericidal = Synergistic