Twenty-four-hour or younger growths of the rapid-growing bacteria, as
Staphylococci, Streptococci, Pneumococci, Gonococci and Colon bacilli,
upon inclined agar are washed off with normal saline solution. After the
mixture has sedimented, the upper, whitish layer composed of fluid and
bacteria is removed with a pipette, and the finer clumps of bacteria
precipitated by placing the fluid in a rapidly rotated centrifuge for a
few minutes. The supernatant layer, which is still opalescent and is
called a bacterial emulsion, should if suitable for work contain the
germs in a well-separated condition.
Cultures of tubercle germs are heated, and ground in a mortar with salt
solution until the mass is well broken up, and then centrifugated. In
case glycerin cultures are used, such as are left in the manufacture of
Koch’s old tuberculin, the glycerin must be removed by repeated washing
with water and finally with 1·5 per cent. salt solution. The washed
culture is worked up in a mortar and centrifugated until the clumps are
practically all thrown down, and the cloudy layer or emulsion is
removed.
The emulsions must be of uniform density. Wright computed the number of
germs in a given volume by counting, but McFarland and L’Engle devised
an apparatus which is called a nephelometer, consisting essentially of
mixtures of BaSO_{4}, put up in sealed tubes, which correspond to
solutions containing from 1 to 10 per cent. of BaCl_{2}, which serve as
standards. The turbidity of the emulsion is compared in similar layer
with the standard tubes of BaSO_{4}. They found that the tube containing
“5 per cent. of BaCl_{2} corresponds to the most useful bacterial
suspension.” The permanency of the emulsions varies a good deal.
Suspensions of the gonococci should be used at once, staphylococci
within two days, etc., while the emulsion of tubercle germs may be
employed indefinitely.