AGf~ICULTURAL RESEARCH, PUSA.

BIOLOGY OF MICRO-ORGANIS,J.Y[S 19
tion of complex organic suhstanccs--e.g. proteins and
earhohydl'ates---by means or t.heir enzymes. Among
the pathogens, fermentatin: proptrties are important
features in the identification of ecrtain spceiC's. Thus,
various bacteria decompose particular carbohydrates
(e.g. sugars, hexahydric alcohols, polysa.ccharides)
with the formation of acids (e.g. formic, aeetic and
lactic aeids) and, in many cases, gases (e.g. eurbon
dioxide and hydrogen). Certain species are also
capable of breaking down salts or the organic adds.
Some bacteria possess marked proteolytic properties
and digest complex protein substances, such as
gelatin, coagulated serum, etc., liberating the various
products of protein disintegration-e.g. amino-acids,
ammonia compounds, indole, skatole, hydrogen
sulphide, etc. Certain species, while unable to digest
complex proteins, net. on peptone with the procluetion
of indole, etc.
Some bacteria, including certain pathogens, are
CHROMOGENIC, und produce characterist.ic pigments,
which in some cases diffuse readily from the haeterial
cclIi'> into the surrounding mediunt.
GUOWTU PIIAsEs.-'iVhen bacteria grow in it Huia medium,
their development proceeds as follows: there is rust a perio!l
of" hlg " (e.g. two hours) during which there is no illme:.I.Re in
number or only slow multiplication; this is followed by a
phase of rapid llluitiplic:ttion (e.g. during sevell to eight honrs
at optimum temperature); the increase in number is in geometric
progression and the logarithmic curve of growth is a
straight line; this is called the "logarithmic" phase; then
follows :t " stationary" phase, mllltiplieatiou gradually ceasing;
ill this phase (lasting from a few hours to several days) the
number of' new individuals tends to be equalled by those dying;
finally there is a phase of deeliue or diminution of vinhle
individuals, the organisms progressively dying until (after
seventl days to months) no Jiying bacterin remain. It shonlrl
be noted that the initial "lag" phase may be absent whell
fresh medium is illoeulatrd with organisms 'which nrc in the
phase of multiplication.
BACTERIAL TOXINs.-Theseare defined as the products
of bacteria which are injurious to the tissues and in