Help: A Level assessment

Vitor Pinheiro vbbp2 at
Tue Mar 23 01:50:09 EST 1999

Hello Eric,

I have done an experiment such as the one you describe using E.coli in
standard agar growth media during my first year at Uni. It was just a
qualitative experiment, so it will probably fit the bill.

The experiment consisted of diluting the desinfectant in orders of 10
and spotting the growth media. Using the control spot (water) as the
negative control we simply compared how translucid the media was in the
various spots. Very crude, yes... But it was a side experiment to what
we were really dealing with, antibiotic resistance.

The dilutions are very simple. Because I'm aware you're not given
Gilkison pippetes for a-level experiments, try using ml pippetes. 1 ml
os the concentrated desinfectant into 9 ml of distilled water for the
first dilution and then 1ml of the fisrt dilution into 9 ml of water for
the second and so on.

You seem to be using a liquid growth media... What you can do that might
be available at A-level schools is to reduce the glucose left in the
medium after your period of incubation (24hs at 30ºC) so that you have a
coloured solution if there is glucose in it. (E.g.: a yellow solution
that goes green as it oxidises such as dichromate -> chromate, although
I'm pretty sure you're gonna oxidise too many other things with it)

Consequently, the more effective your desinfectant is the less bacteria
will grow, hence less sugar used up, hence the greener (as suggested in
the example) your solution. This way you can also quantify your
experiment if you standardise the times for the assays ans so on...

Hope I have been of any help, 


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