Page 174 - My FlipBook

In gelatine the growth is very slow and small in amount.
The colonies are microscopic in size for the first two or three
days, but in four or five days are readily seen by the naked
eye as small, round, yellowish colonies with smooth borders.
There is no liquefaction of the gelatine. Much difference in
the growth is observed in different specimens of the gelatine.
In some which do well for the ordinary culture of pathogenic
micro-organisms, the caries fungus will scarcely grow at all,,
while in others it will make a fairly good growth. However,,
the growth upon gelatine is never very abundant.
In Nutrient Agar Agar the growth is better and otherwise
has the same appearance. Colonies will often grow to an
eighth of an inch or more in diameter. In prick cultures it
grows about as well in the depths of the media as upon the
surface, and appears to be indiflerent to the presence of air.
In any of the media it grows best if the reaction is neutral, or
slightly alkaline. It will grow, however, if the media is
slightly acid, but not so abundantly. If planted in slightly al-
kaline broth, containing sugar or glucose, to which enough
litmus has been added to color it blue, acid will be developed
and the litmus will become red in from eight to twelve hours
and the acidity will continue to increase from three to five
days. The acid formed, as was determined by Dr. D. W.
Miller in i'884, is lactic acid. When sweetened Agar Agar, to
which litmus has been added, is planted with the fungus, acid
will be formed and the blue color will first be changed to red
about the growing colonies, and this color change will spread
gradually to the whole mass.
If planted in litmus broth, to which no sugar or glucose
has been added, no change in the color will occur, for the or-
ganisms do not form an acid without the presence of some
form of sugar or starch ; but otherwise the growth will appear
the same.
It stains fairly well with the ordinary analine dyes, when
dried upon a cover glass, but the color is readily removed by
washing with alcohol or with acidulated water.
The formation of a gelatinous material is one of the pe-
culiar features of the organism, peculiar because of its ap-
parent fickleness in forming and refusing to form this gela-
tine. Often I have made two specimens of broth from the
same formula, and in the one the gelatine would form abun-
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