Disk diffusion method

[a_zed24]

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Hello there,
does anyone know why do we perform the diagnosis step during the disk diffusion method to test the bacterial susceptibility towards the used antibiotics?
By "diagnosis step" (as our prof. called it) I mean that step in which we examine the bacterial culture to determine what type of bacteria we have.
To put the question in another view, what happens if we don't perform diagnosis?
Thanks!

 

[WhiteCoatWonder]

not sure i understand your question but i might be able to help...

certain bugs have known patterns of resistance, “evidence based”, if you will. knowing which organism youre dealing with narrows what drugs have historically worked to kill it/slow it down, so you start with those. if you isolate and grow bacteria without identifying it, id roll the dice that you could kill it with vanc or pip tazo but for the sake of the patient, antimicrobial resistance and best practice, you should probably know what the bug is before you try to attack it specifically.

seems counterintuitive i guess when you consider the idea of treating initially w/ broad spectrum antibiotics that are later narrowed but again, not “performing the diagnosis” asap is less than optimal.

 

[a_zed24]

Yes, that was kind like what I answered my prof. when he asked that question in class, but he said there were 3 specific points, or 3 benefits in identifying the bacteria before the diffusion test.
I'll rewrite the question more clearly, sorry if I didn't do that in the first place:
why is it useful to diagnose or identify the type of bacteria in the culture before you try any antibiotic against them?

The first two points that we were able to find during class were:
1) If you know whether your bacteria are Gram +ve or Gram +ve, you can use narrow spectrum antibiotics instead of broad, un-specific ones.
2) If the bacteria are altered bacteria of the normal flora, it may be better to treat them with bacteriostatics instead of bacteriocides.

Now, the million-dollar question...what's the third point?

P.S.: May it be that by diagnosis you can identify the place of infection? I mean, the area of the body which the bacteria attack. Once you got the place of infection, you can select an antibiotic whose concentration is high in that place. Eg., for cystisis, you choose a drug that has a high level in urine...