CORE PRACTICAL 15: Investigate the effect of different antibiotics on bacteria.

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cherryhitchkins
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Why do we need to control concentration of antibiotic? for valid results?

Why is incubation time needed to be controlled?

Why does size of paper disc need to be controlled?
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macpatgh-Sheldon
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Hi there! I presume you are still sitting on a cake [sorry! - pls keep laughing during this era when Homo sapiens appears to be heading towards extinction!]

Why do we need to control concentration of antibiotic? for valid results?

We need to work out the MIC of the antibiotic - let me explain: MIC is the lowest conc-n of a particular antibiotic that will kill the bacteria in question - in a pathology lab, the technician will need to determine this for the clinicians [doctors], so that the patient can be given the correct dose: not too much to induce toxicity yet enough to treat the infection, yeah?

Why is incubation time needed to be controlled?

The effective antibiotic [one that kills bacteria] will create a ring of inhibition around the area of bacterial growth i.e. where the antibiotic kills the bacteria - if we keep the culture too long, the bacteria will grow exponentially and the antibiotic will not be able to contain them [a bacterium becomes 2 bacteria in about 20 minutes, then 4 in 20 more, then 8, then 16 etc. How long will it take for 1 bacterium to become a million? Work it out NOT that long! [It takes minimum 18 months for 2 humans to become 4, agreed? Ok not if twins are born!

Why does size of paper disc need to be controlled?

Not sure, sorry!

Have a great day!
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HarisMalik98
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The antibiotic diffuses through the agar from the edge of the disc. Therefore, I think that if you used a larger and smaller disc, impregnated with the same antibiotic, then the zone of inhibition would likely be larger when using the larger disc. So, keeping the same disc diameter standardises the measurements of the zone of inhibition.

I'm pretty sure there's very strict guidelines in place for susceptibility testing by the CLSI and EUCAST, and one is that the discs should be ~6mm; using different sized discs (such as 8mm) will invalidate any results you produce. So I guess that's a good enough incentive!

I did a microbiology degree, doing this procedure many times, and not once did we ever question (or ever told) why disc size should be the same - so this is just me hypothesising!
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macpatgh-Sheldon
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Thanks for your input Mzee Haris - sounds logical!
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