Bacteria and disinfectants

About microscopic forms of life, including Bacteria, Archea, protozoans, algae and fungi. Topics relating to viruses, viroids and prions also belong here.

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Bacteria and disinfectants

Post by falconfurball7 » Sat Jun 14, 2008 7:47 pm

Hi everyone
im doing a biology project for my final assignment in sophomore year

i had an original experiment but my teacher modified it, its based upon the effectiveness of household disinfectants on bacteria

anyways... my teacher told me to grow the bacteria on a agar plate and then put filter discs saturated in varius disinfectants on the bacteria

the bacteria cant grow around the area of the disinfectant if it kills it, so there is a clean zone of inhibition around each disc. im supposed to measure the diameter of the zone of inhibition for each bacteria. However, when other people did this experiment online, they put the disinfectant discs on the agar plate right after they swabbed it with a bacteria sample. I was told to let the bacteria grow first, and once it did over two days, i put the disinfectant plates on the agar and its been 4 days and nothing has changed... it all looks the same. will i ever see a zone of inhibition now that the bacteria is developped?

sorry for the choppiness of my words, im in a hurry

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King Cobra
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Re: Bacteria and disinfectants

Post by biohazard » Mon Jun 16, 2008 6:00 am

It is possible that your disinfectants are bacteriostatic rather than bacteriocidic, that is, they only inhibit bacterial growth. In this case you may not see (any visible) effect after the bacteria have already grown. It may also take a while for the dead bacterial colonies to disappear from your plate - you could check their viability for example by making a replicate plate from your test plate, or by inoculating some of the bacteria close to the disc to another agar plate, and see if they grow.

In case your disinfectants do not kill bacteria but are bacteriostatic instead, you can find some comfort from the fact that the people you said did this online might've used bacteriostatic substances as well, and mistakenly think they were killing the bacteria, whilst they actually only prevented their growth - because the effect looks the same when you put the discs on the agar right after inoculation ;)

After all, as far as I know most of the common household disinfectants are fairly weak, and many bacteria tolerate them, or easily develop resistance. What sort of bacteria and disinfectants you are using?

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Inland Taipan
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Post by canalon » Mon Jun 16, 2008 1:44 pm

What you read on the web is the correct way to test for inhibition of growth, and what you did was wrong, in spite of what your teacher told you.
Bacteriostatic or bactericidic do not change the result: once the colonies have grown in a lawn (to confluence), you won't see a difference if they die (they usually do not melt away, the walls are still there) or if they simply stop growing... or if they are perfectly healthy. In fact the disc diffusion method do not alllow to make the difference between bacteriostatic and bactericidic substances. The only thing it allows you to do in such setting is to compare the relative efficiency of the same antibiotic relatively to different bacteria. In fact results between different species of bacteria should be handled with precautions, but that is another story. So if I were you, I would simply restart the experiment if possible, and put the disc on freshly inoculated plates.

Science has proof without any certainty. Creationists have certainty without
any proof. (Ashley Montague)

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