Page 83 - My FlipBook

SEVENTH LECTURE.

My last lecture was regarding the forms presented by
micro-organisms, the physical forms. For a time I will
make a few plants before beginning my lectures, showing you
something of the methods of the cultivation of micro-organ-
isms. I have brought with me some culture media for that
purpose this morning. This (holding up tube) I cannot well
pass to the class, for turning the tube up wets the cotton
stopper and spoils it. It is simply a broth, made from lean
beef. We may use beef or mutton, or chicken or turkey, or
any of the meats for making this broth ; simply half a pound
of meat, bruised fine and placed in a quart of water and
brought to a boil and kept so for an hour. We usually
put it in a vessel within a vessel of water, so that it does not
actually boil. At least, it must be cooked until it is clear.
Then it is filtered and tested as to acidity ; it is generally
found to be slightly acid. We then neutralize it by adding bi-
carbonate of soda, bringing it just to neutral, or very slightly
alkaline. This is very exactly neutral. I did not make this
broth myself. I got it of Parke, Davis & Co. I am disap-
pointed in it because it isn't just a little over the line. Here
is some to which I added some litmus and you will see it is
an amber color instead of being a blue, as it would be if it
was but a little alkaline ; it is neither a red nor a blue. After
neutralizing we usually add some pepsin and place it in the
cold for twenty-four hours, or else place it in the incubator
for four or five hours, in order to digest the broth ; then it
will be of a white, milky appearance. Then we sterilize it by
placing it in a steam bath for half an hour or an hour, set it
away for a day, and again sterilize in a steam bath for an
hour ; then set it away for two days, and again place it in
the steam sterilizer for another half hour. The reason for
these repeated sterilizations is that we cannot destroy the
spores of the molds and some of the micro-organisms by an
hour's boiling, or two hours, even. We must put it away
under conditions for the beginning of growth, and allow the
spores to begin their development, and then we can destroy
them by boiling. We can destroy any of the active growing

71