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24 THE MICRO-ORGANISMS OF THE HUMAN MOUTH. ;

attain to a diaraoter of 2.6/7.. (Fig. 6, a,b.) Tlie plasma of the
cells is thereby condensed and becomes highly refractive ; the
membrane also appears considerably thickened. This change
of form is soon followed by the formation of the spores. The
Bacillus butyricus is typically anaerobic ; access of air impedes
or entirely prevents its development and fermentative action.
In solutions of carbohydrates (starch, sugar, dextrine), and of
lactates, this micro-organism produces butyric acid under evolu-
tion of carbonic acid and hydrogen. The fermentation is most
intense when air is excluded ; optimum temperature, 35° to 40° C.
The butyric acid fungus shows a peculiar reaction in regard to
iodine, which is also incidental to several other bacteria. " Under
certain conditions it gives rise to a compound which is colored
blue to dark violet by iodine, and which consequently is to be
regarded as an analogon of starch." This reaction occurs when
the micro-organism is cultivated in solutions of starch, cellulose,
glycerine, and lactate of lime, rarely when solations containing
dextrine and sugar are made use of. The young rods are colored
blue, the older ones dark violet some only in scattered transverse
;
zones, others in continuo. The reaction depends also upon the
intensity of the fermentation.
The conditions obtaining in some parts of the human mouth
are not adverse to the development of the butyric acid bacterium
hitherto, however, no proof has been adduced for the statement,
regularly found in hand-books of dentistry, that butyric acid is
formed in the oral cavity. In the first place, no micro-organism
has as yet been discovered in the mouth which gives rise to this
fermentation, nor has any trace of butyric acid ever been detected
In regard to this point, however, experiments of a suifi-
in it.
ciently exhaustive nature are still wanting.
That butyric acid may be formed in the oral cavity as a
hii-prodacf in lactic acid fermentation, is at least highly probable.
The fermentation of a large quantity of saliva and starch yielded
about 2.0 c.cm. of lactic acid, and a few drops of a liquid faintly
smelling of butyric acid, but the quantity was too small to allow
of a more accurate determination.