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PA THOGENIC BA CTERIA.
media, and cease to be pathogenic after a few days. In his
experiments with antipnetimococcic serum Washbourn
found, however, that a pneumococcus isolated from pneu-
monia sputum and passed through one mouse and nine
rabbits developed a permanent virulence when kept on
agar-agar made carefully, so that it was not heated beyond
100° C., and alkalinized 4 c.cm. of normal caustic soda
solution beyond the neutral point determined with rosalic
acid, to each liter. The agar-agar is first streaked with
sterile rabbit's blood, then inoculated. The cultures are
FIG. 99.—Diplococcus pneumonise: colony twenty-four hours old upon gelatin;
x 100 (Frankel and Pfeiffer).
kept at 37.5° C. Not only is this true, but ordinarily
they seem to be unable to accommodate themselves to a
purely saprophytic life, and unless continually trans-
planted to new media die in a week or two, sometimes
Kinyoun recommended to the writer that virulence
could be retained for a considerable time by keeping
blood from an infected rabbit, in a hermetically sealed
glass tube, on ice. This plan seems to work admirably
if the blood is not kept too long.