For those taking or have had Micro...this is stumping me

[Shay08]

I have Micro this semester and am working through a lab exercise on Kirby Bauer testing for a bunch of different antibiotics. Here is the question I can't seem to get my head around...

Two antibiotics are tested for their efficacy against a single bacterial species. If antibiotic A and antibiotic B produce zones of inhibition with the same diameter, how can the bacterium be considered resistant to antibiotic A but sensitive to antibiotic B?

So I was thinking - could it be that each antibiotic may have varying diffusibility into the medium? Or does that sound way off base? Could it be simply that one measurement of the zones means resistance to one type of antibiotic, but sensitivity to the other?

Any insight or help would be appreciated, thanks!

[coolpeach]

Are you sure you didn't contaminate antibiotic A's zone with Anitbiotic B? Did you flame between applications? Its been about a year since micro, but if I remember right if they both have the same zones then the efficacy would be the same.

[obicurn]

Im gonna see if I can explain this to where it makes any sense at all.....

When we did Antimicrobial Testing, we used Mueller-Hinton.

After measuring the zones of inhibition, we compared our results with a chart that showed whether or not the bacteria was sensitive or resistant, and whether or not the drug was broad or narrow spectrum.

For instance.....our bacteria samples were E-coli and S. aureus.

There were too many drugs to list them all, but here is one example.

Where staph aureus is concerned, with Piperacillin, the zone of inhibition was 15mm. S.aureus is resistant to that drug if the zone is 17mm or less. At the same time, Streptoycin gave a zone of 15mm. S.aureus is sensitive to that drug because it is considered resistant with a zone of 11mm or less.

So basically, it all depends on which bacteria and which drug is being used.

[Shay08]

Thanks for your responses. I think it does make sense now, and we also used a similar chart with measurements that correspond to sensitivity. So essentially, what I think you're saying is that you can have 1 drug with a zone of inhibition of 15 mm which = sensitivity toward that drug, and another drug with the same 15 mm zone but for that drug it = resistance.

As far as technique, our instructor had a special method of punching the drugs onto the plate for us, all of them at the same time. There was no flaming involved.

Thanks!!

[obicurn]

Correct.

Our instructor put our drugs in for us also. They were on tiny discs and she just put then onto out agar plates.

[obicurn]

I can't spell at all today. Wow!!! You all speak fluent typo, right? lol