Antibiotics 4 Flashcards
What must be considered when choosing an antibiotic?
Clinical diagnosis, microbiological diagnosis, the in vitro susceptibility, the condition of the patin (allergies, weak immune system) and the properties of the antibiotic (whether it can reach the target tissue)
Why is susceptibility testing done?
Because many bacteria have acquired resistance
What are the two methods of susceptibility testing?
Dilution method and diffusion method
What is the dilution method?
A series of test tubes contain different concentrations of antibiotics in two fold dilution series - bacteria is added to the tube to look for growth - the tube with the lowest concentration which inhibits growth is called the minimum inhibitory concentration (MIC)
Does the MIC indicate whether an antibiotic is bacteristatic or bactericidal?
bacteristatic
What is the diffusion method?
Place discs of antibiotic on a growth plate and allow the antibiotics to diffuse away from the disc - the point where the bacteria grow is the MIC
Is the size of the circle an indicator of the strength of the antibiotic?
No - different antibiotics have different diffusion rates so you can’t compare different circles
What is the E-test strip?
A diffusion test with a piece of filter paper with a gradient of antibiotic on it - where growth intersects the paper can read MIC directly
What are the specific considerations when choosing an antibiotic?
spectrum, clinical efficacy, route of administration, route of excretion, pharmacokinetics, pharmacodynamics, availability, cost
Why are antibiotic combinations sometimes used?
If someone is very sick, to delay the emergence of resistance (TB), to treat mixed infections, to reduce toxicity or to achieve a synergistic effect?
What are the ways that antibiotics can achieve a synergistic effect?
By blocking sequential steps in a metabolic pathway (ensures that if something gets through the first step it won’t get through the second), by inhibiting enzyme degradation or by enhancing antimicrobial uptake
What is a combination antibiotic which blocks sequential steps in a metabolic pathway?co-trimoxazole - interferes with two steps in the synthesis of folic acid
co-trimoxazole - interferes with two steps in the synthesis of folic acid
What is a combination antibiotic which inhibits enzyme degradation?
co-amoxyclav - inhibits beta lactamase so allows the beta lactam to work
What is a combination antibiotic which enhances antimicrobial uptake?
beta lactams and aminoglycosides - beta lactams degrade peptidoglycan wall to allow entry of aminoglycosides
What are the ways that antibiotics can achieve an antagonistic effect?
Inhibition of bactericidal activity by a bacteriostatic agent, induction of enzymatic degradation competition for binding the same target, inhibition of the target
