“(2) It is probable that there are present many varieties of opsonins in the blood plasma, each having to do with combating a particular kind of microbic invasion.
“(3) Opsonins have been shown to be distinct from other bacteriotropic substances, such as the bacteriolysins, the agglutinins, and the antitoxins.”
The leucocytes of healthy or diseased persons seem to be equally active when brought in contact with the same serum, hence the amount of opsonins present in the blood of an individual determines, according to the opsonic theory, his susceptibility to bacterial invasion.
TECHNIQUE.
To measure the resistance of the patient to such invasion, or to find out his opsonic index, special technique has been developed, which may be briefly described as follows:
I. EMULSIONS OF BACTERIA.
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 BaSO4, put up in sealed tubes, which correspond to solutions containing from 1 to 10 per cent. of BaCl2, which serve as standards. The turbidity of the emulsion is compared in similar layer with the standard tubes of BaSO4. They found that the tube containing “5 per cent. of BaCl2 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.