need help with lac- strain e.coli problem

Robert Randolph rand777 at ids.net
Wed Apr 10 05:05:45 EST 1996

On Wed, 03 Apr 1996 17:57:21 -0800, Matthew Chan <csv at calweb.com>

>I am sending this for a friend who is doing an individual experiment in 
>High School.  Message as follows:
>My name is Mike LeFevre and I am doing an experiment to try to test the
>mutation rate of E.Coli bacteria.  I am using the lac- strain and
>growing it on Endo Agar.  Supposedly, a coliform bacteria that cannot
>use lactose will grow on the agar, but one that can use lactose will
>react with the fischin in the agar and turn red.  My problem is that I
>cannot get either the lac- or the wild type to grow on Endo Agar.  I
>tried Serial Dilution, Streaking, and Direct Innoculation with a pour
>plate.  I have gotten absolutely no growth.  What am I doing wrong?

Endo agar is difficult to work with if it is not fresh.  The medium
should be a transparent very light pink, if not ...it is not fresh.
Endo is a very light sensitive and produces a toxic by-product when
exposed to sun light or fluorescent light.  Deterioration by light
(uv) is evidenced by the uninoculated media turning deep red in areas
or the complete plate.  A plate that is partially dehydrated due to
age or storage in a "frost free" refrigerator looks and acts the same
inhibitory way.  My suggestion, MacConkey or Hekteon (HE) agar are
eaiser to work.  If your Endo is fresh, then check your test organisms
for viability on a general growth medium ..TSA, BHIA, Blood Agar.


 Robert Randolph    Randolph Biomedical
 21 McElroy Street   West Warwick, RI 02893
     401-826-1407   rand777 at ids.net

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