288

PATHOGENIC BACTERIA.

from the throat does not necessarily prove the patient to
be diseased. Virulent bacilli can often be discovered in
the throats of healthy persons who have knowingly or
unknowingly come in contact with the disease. The
bacteriologic examination is only an adjunct to the
clinical diagnosis, and must never be taken as positive
in itself.

The bacillus grows similarly upon blood-serum and
Loffler's mixture. Upon glycerin agar-agar and agar-agar
the colonies are much larger, more translucent, always

a & c

FIG. 78.—Diphtheria bacilli (from photographs taken by Prof. E. K. Dun-
ham, Carnegie Laboratory, New York): #, pseudo-bacillus; b, true bacillus;
c, pseudo-bacillus.