INFLUENZA BACILLUS. 321 
Morphology. Very small, moderately thick bacilli, 
usually occurring singly or united in pairs, but threads 
or chains of three, four, or more elements, are occa- 
sionally found. 
The bacillus stains with difficulty with the ordinary 
aniline colors—best with dilute Ziehl’s solution or 
Léffler’s methylene-blue solution, with heat. When 
faintly stained the two ends of the bacilli are some- 
times more deeply stained than the middle portion. 
Those we have examined, all obtained from cases in 
New York, were not stained by Gram’s method, but 
some report instances in which they were. 
Biological Characters. An aérobic, non-motile bacil- 
lus; does not form spores; no growth occurs below 
26° C., or above 43° C., or in the entire absence of oxy- 
gen. This bacillus is best cultivated at 37° C., and on. 
the surface of the ordinary nutrient culture media con- 
taining hemoglobin or purulent material. Plain or 
glycerin-agar, or blood-serum streaked with sputum, 
pus, or blood, make a good soil for their growth. At 
the end of eighteen to twenty-four hours in the incu- 
bator very small, drop-like colonies are developed, 
which, under a low magnification (100 diameters), ap- 
pear as shining, transparent, homogeneous masses, and 
even under a No.7 lens scarcely show at all the indi- 
vidual organisms. Older colonies are sometimes col- 
ored yellowish-brown in the centre. A characteristic 
feature of the influenza bacillus is that the colonies tend 
to remain separate from each other, although when they 
are thickly sown in a film of moist blood upon nutri- 
ent agar they may become confluent. Transplantation 
of the original culture to ordinary agar or serum can- 
not, as a rule, be successfully performed, owing to the 
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