39° PA THOGENIC BA CTERIA.

The bacillus is rather variable culturally, and is some-
what polymorphic. Probably both size and form depend
to a certain extent upon the culture-medium on which
it grows. On the average, it measures 1-3 X 0.4-0.7^.

It usually occurs in the form of short rods, but very

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short coccus-like elements and quite elongate forms are
often found in the same culture. The individual bacilli
are frequently isolated or in pairs. Chains are the ex-
ception. They are provided with flagella, which are
very variable in number, generally from four to a dozen,
though there may be more. It forms no spores.

The bacillus stains well with the ordinary aqueous
solutions of the anilin dyes, but does not retain the stain
after immersion in Grain's solution.

The bacillus is motile, though in this particular it is
subject to irregularity, the organisms from some cultures

FIG. no.—Bacillus coli communis: superficial colony two days old upon a
gelatin plate; x 21 (Heim).

always swimming actively, even when the culture is
some days old, others being exceedingly sluggish even
when young and actively growing, and a few cultures
seem to consist of bacilli that do not move at all. Fresh
cultures which, when grown at incubation temperature,
consist of entirely non-motile bacteria are probably Bacil-
lus coli immobilis, not Bacillus coli communis.

The bacillus is readily cultivated upon the ordinary