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a_zed24

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Hello there,

does anyone know what are the basic steps for the disk diffusion method to test the bacterial susceptibility towards the used antibiotics?

Thanks!
 
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a_zed24

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Has nobody answered this? Did you find it elsewhere?

Well yes, I did. Sorry for the late reply.
First of all you should inoculate your Petri dish with the type of bacteria of interest. To ensure a homogenous growth, spread the bacteria in three directions all over the plate and around the rims.

Then, you use a forcep (or even better, a Disk dispenser) to place antibiotic disks at equal distance on the plate. You incubate the whole thing at 37°C for 24 hrs. and you look at the inhibition zones around the disks:
- no zone = bacteria are resistant to that specific antibiotic
- small zone = intermediate sensibility
- large zone = sensible, use that antibiotic in vivo to treat the infection

However, you should also be able to choose the right antibiotics to place on the disk. For this reason, you keep in mind what bacteria you're dealing with, after of course identifying them by various tests, the simplest of them is the Gram stain and visualization of their shape and arrangement under a microscope.
Doing this "diagnosis" step will save you a lot of time, because if you have for example, Gram negative bacteria, you won't choose a narrow spectrum antibiotic which works for G+ve ones, will you?
And if the bacteria you have taken are part of the normal flora of the human body, you may want to try with bacteriostatics rather than bactericides, because in vivo your goal will be not to kill those bacteria completely, but only to prevent excessive multiplication.
Unfortunately, i'm still looking for a third or even a fourth example of how beneficial this "diagnosis" step is when you're choosing the antibiotics, and i'd be glad if anyone who knows will tell me!
Cheers.
 

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