The vaccines, with the exception of the tubercle vaccine, consist of
emulsions of heated cultures of the particular germ producing the
infection. Luxuriant cultures of the desired organism are grown upon
inclined agar; the growth is then removed with salt solution and a glass
rod, and thoroughly emulsified by shaking in a test-tube, in order to
get the germs well distributed and the clumps broken up. The number of
germs in a given quantity of the emulsion is then determined by
comparing the number of germs and red blood-corpuscles in a mixture of
one part emulsion, one part freshly drawn blood, and three parts of
normal salt solution. If there are ten times as many germs as
blood-corpuscles, the bacterial suspension contains approximately
45,000,000 germs per cubic centimeter. The number of germs present will
of course vary with the density of the emulsion. The bacterial
suspension is heated at the lowest temperature and for the shortest
possible time to kill it. The amount of sterilization will differ with
different organisms; 60° C. or less maintained for thirty minutes or
less is usually sufficient. Cultures from the vaccine are finally made
to insure that it is sterile and safe and some preservative added.
_Tubercle Vaccine._—This is the “New Tuberculin Koch,” an opalescent
fluid, containing the active principle of tuberle bacilli obtained
according to the method of Koch.