This week in lecture we discussed antimicrobial susceptibility tests. Antimicrobial agents are separated into classes based on their mechanism of destroying the bacteria. Beta lactams (penicillins and cephalosporins) and glycopeptides (vancomyecin) act on the cell wall. Fluoroquinolones inhibit protein synthesis. Other drugs such as tetracyclines, aminoglycosides (gentamicin) and macrolides (erythromycin) interfere with protein synthesis. Susceptibility testing is performed on pathogens to determine if bacteria express resistance to agents that are potential choices to be used in treatment.
There are several different methods to detect antimicrobial resistance. A broth or microdilution method can be done in which two-fold dilutions of antimicrobics testes against standardized concentration of an organism. The minimal inhibitory concentration (MIC) or the lowest concentration of the drug that will inhibit the growth of the organism can be determined by looking for visible growth of the microorganism.
An agar dilution consists of agar plates each with a doubling dilution of antimicrobial agent incorporated into the plate. One or more bacterial isolates can be inoculated onto the plates by spotting the plate with a suspension. After incubation the pates are examined for growth. The MIC is the lowest concentration where growth is inhibited.
Agar disk diffusion or the Kirby Bauer method is where filter paper disks containing antibiotics are placed on a plate inoculated with standardized suspension of bacteria. As antibiotic diffuses through tha agar, its concentration is reduced until bacterial growth is no longer inhibited and a zone of inhibition is formed. The zone size is measured and can be used to determine susceptibility or resistance to a drug.
An Epsilometer Tests or E-test can be performed by placing a plastic strip which contains an exponential gradient of dried antibiotic on one side and a calibrated MIC reading scale on the other side onto an inoculated agar plate. After incubation bacterial growth becomes visible and production of a symmetrical inhibition ellipse is produced. MIC values are read from the scale where the ellipse edge intersects the strip.
I like the overview you've written on this lecture. It really breaks things down to a basic level. There was a lot of information in this lecture and I wasn't sure I retained all the important stuff but this helped. I also liked the link to the case study. Thanks!
ReplyDeleteLaura, That is a very interesting case study. As I know you have heard me say a million times, CF hits home with me because of my cousins. They go anually to the doctor to have plates inocculated with their sputum because they are testing for all of the bugs that can be harmful to CF patients. The one they are always talking about is Pseudomonas. I guess this is because it can be harder to treat sometimes. I am just glad there is a way to test the antibiotic to make sure it is going to work so that people with CF are not having to take a crazy amount of drugs that are not working.
ReplyDeleteThis case study is an interesting one. I really enjoyed reading this post, it also helped me to understand what I didn't understand from lecture. thanks!!:)
ReplyDeleteInteresting article on the 2 strains of Pseudomonas and what is recommended as far as testing!
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