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18 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

II. Action upon Lifeless Matter.
Bacteria are furthermore designated, with reference to their
action upon the substratum, as :
1. Zymogenic, or fermentation bacteria.
2. Chromogenic, or color-forming bacteria.
3. Aerogenic, or gas-forming bacteria.
4. Saprogenic, or putrefaction bacteria.

1. Fermentation Bacteria.
Ferment bacteria are such as bring about those changes in
the substratum which have been designated as fermentation.
According to Iloppe-Seyler,^^ these changes are intense decom-
positions of complicated organic compounds, through which sub-
stances arise " which have together less heat of combustion
than those bodies out of which they were formed." By others,
fermentation is described as a process of decomposition, which
is inaugurated and continued by a substance called a ferment,
without the ferment itself suffering any change ; this kind of
action is frequently designated as catalysis, -/.arahac^ (xara/jjio)
dissolution. During the process of fermentation the medium
gradually loses its nutritive power; it becomes used up. The
substances which are thereby produced are very numerous, and
of very different character. Many of them, the ptomaines, have
exceedingly poisonous properties. It is highly probable that all
bacteria, pathogenic as well as non-pathogenic, when brought
into a suitable substratum, possess the property of producing
changes which must be designated as fermentation. The fermen-
tation of nitrogenous, and more particularly of albuminous
substances, which is accompanied by the development of large
quantities of gaseous and stinking products, is called putrefactive
fermentation, or simply putrefaction, and the bacteria causing
this kind of fermentation are putrefactive or saprogenic bacteria.
Many years ago I^*^ called attention to the fact that the course of
the fermentation frequently depends more upon the substratum
than upon the micro-organism. Bacteria which grow upon white
of egg, producing an intensely offensive smell and a strong alka-
line reaction, when brought into carbohydrates exhibit entirely