Question on a MacFarland Standards
ricoco at worldonline.nl
Wed Mar 3 02:42:07 EST 1999
It's been a long time since I have been working with the MacFarland standard
but in answer on the question:
One of the properties of a standard is that it should be stable for a long
time. This meens that there must be no question about in- or decreasement of
As BaCl2 is a very stable compount that is nat attact bij bacteria or others
and wil not change for a long period of time, it surely is the mest
If one wants to use mo for such a standard the problem comes with the
If the suspension is sterilised, the properties of the mo will change
due to the letal process.
If the suspention is not sterilised the properties will change because
of the growth of the mo.
A possible alternative could be a suspension of B. cereus spores. Thes are
quit stable but also in this case you should ask yourself how the standard
wil look like after 5 years.
With BaCl2 there is no question. A bactieria suspension according to the
macfarland-scale 2 is after 5 years the same als the first.
With any standard one should ask himself how can I relate it to a higher
standard just like the metric metre in Paris.
Quality assurance in Food
Van Rossem & Partners
"William J. Mason" heeft geschreven in bericht ...
>I am currently teaching an honors microbiology lab and was posed with a
>question that I could not fully answer.
>The MacFarland standards are solutions of BaCl2 and H2SO4 that are added
>to a spec cuvette to create a turbid solution of measurable OD. This tube
>is then correlated to the OD of a growing bacterial sample. Once the ODs
>are the same an alloquat of the bacterial culture is removed, diluted, and
>CFU/ml are determined. By increasing the concentration of the BaCl2, one
>can establish a set of ODs that correspond to a specific number of
>bacteria/ml (obviously for a specific bacteria). The question was Why is
>it necessry to use the chemicals to create the standard. I tried to
>explain that it is better to have a non-biological solution to create the
>standards, but this did not satisfy the student. She left with an
>understanding of how to do it, but not a good and detailed reason as to
>WHY? Does anyone have any practical but good advice on how to explain
>this? Your help would be greatly appreciated.
>University of Arkansas, Biological Sciences/Microbiology
>wmason at comp.uark.edu
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