Micro-organisms

Janet Elkins                    F.A. Douglass Middle Math & Science Academy
                                543 N. Waller
                                Chicago IL 60644
                                (312) 534-6176  VM 534-6299  x4110
 
Objectives:

     This lesson has been developed for 6th grade students, but is easily 
adaptable to other levels.
     Students will:  -follow written directions
                     -discover additives which will deter micro-organism growth

Materials Needed:

                 -250 ml beaker
                 -jar of bouillon (either beef or chicken will work)
                 -water
                 -3 test tubes
                 -labels "salt", "vinegar", "control"
                 -test tube rack

Strategy:

     One (1) bouillon cube is to be dissolved in 250 ml of very warm water.  As 
the cube is dissolving, add 1 t of salt to the test tube labeled "salt"; 1 t 
vinegar in the test tube labeled "vinegar".  Pour equal amounts of the bouillon 
into the test tubes.  The salt may NOT dissolve quickly, but leave it alone-if 
you try to stir it in, you may remove some of the salt, or "blurp" it, losing 
some of the liquid, or contaminate another container by using the stirrer more 
than once. 
     With the test tubes arranged in the rack, LEAVE ALONE, uncovered for 2-3 
days.  This can take place over a weekend.  Spores will drop into the tubes from 
the air.  Perhaps a duplication of Pasteur's experiment would be helpful at the 
same time:  put bouillon into 2 containers, one open to the air straight up, and 
one open to the air, but the opening is off to the side so that things could not 
"drop" into the container.  Compare these two containers to show that things 
"dropped" in from the air, and did not spontaneously grow in the containers. 
     Students are to examine and record their findings.
     Discussion.  Most will find the control tube to be very cloudy and the 
other two will be less so.  Both salt and vinegar inhibit the growth of micro-
organisms. 

Performance Assessment:

     The outcomes of each student should indicate that the directions were 
followed, therefore PASS.  Any other outcome will be a fail for the first 
objective, that of following written directions.  Teacher may use discretion in 
allowing a repeat of the experiment for half credit; 3/4 credit for a 2-3 
paragraph identifying the cause of the different outcomes.