Lesley Robertson l.a.robertson at stm.tudelft.nl
Wed Jan 24 07:36:24 EST 1996

```b4nd at aol.com (B4ND) wrote:
>I am a high school biology student in need of help.  We have been assigned
>a lab in which we are experimenting with the level of inhibition of
>tetracycline of cultures of bacteria from yogurt.  We are supposed to make
>solutions of tetracycline and distilled water.  We are given 250 mg
>Tetracycline with which to make a 10 ml solution.  Then we're supposed to
>pour 1ml of the solution into 10 different cups.  In these cups, we're
>supposed to vary the ratio of tetracycline to water and find the
>inhibition level.  What I don't understand is how to measure the ratio
>when water is added, or how to produce a greater ratio.  Any help would be
>GREATLY appreciated.  Thanks.  B4ND at aol.com

You have made a solution of 250 mg/10ml, or 25mg/ml. This is your highest
concentration. You change this by adding more (known amounts) water to
the other cups. I'm not sure what you mean by "ratio" in this context,
whether you mean tetracycline powder or the original solution. If you
mean the original solution then adding 1 ml of water to a cup will give
you a tetracycline:water ratio of 1:1, 2 mls would give a ration of 1:2,
etc.  Didn't your teacher explain all this?
Lesley Robertson

```