TO DETERMINE THE THERMAL DEATH-POINT.
SEVERAL methods may be employed for this purpose.
Roughly, it maybe done by keeping a bouillon-culture of
the micro-organism to be studied in a water-bath whose
temperature is gradually increased from that of the body
to 75° C.
Into a fresh bouillon-culture thus exposed to heat, the
experimenter cautiously, and at given intervals, intro-
duces a platinum loop or a capillary pipette, and with-
draws a drop of the culture which he inoculates into
fresh bouillon and stands aside to grow. It is economy
to make the transplantations rather infrequently at first
and frequently later on in the experiment, when the tem-
perature is ascending. In an ordinary determination it
would be well to make a transfer at 40° C., one at 45° C.,
another at 50°, still another at 55°, and then beginning at
60° make one for every additional degree up to 75° C.
or above. The day following the experiment it will be
observed that all the cultures grow except those heated
beyond a certain point, as 60° C. and upward, when it
can properly be concluded that 60° C. is the thermal
death-point. If all the transplantations grow, of course
the maximum temperature that the bacteria can endure
was not reached, and the experiment must be performed
again with higher temperatures.
When more accurate information is desired, and one
wishes to know how long the micro-organism can endure
some such temperature as 60° C. without losing its vital-
ity, a dozen or more bouillon-tubes may be inoculated
with the germ to be studied, and stood in the water-bath
at the temperature to be investigated. The first can be