AGf~ICULTURAL RESEARCH, PUSA.

76 PRACTIC'AL BACTERIOLOGY
protcins and carbohydrates. At the same time simple
organic substances-e.g. amino-acids and other products
of protcin disintegration-arc utilised in the
synthetic mctabolism of thcr-;e organisms.
The food requirements of bacteria vary with the
natural environment and the particular r6le they have
in nature, but generally speaking the pathogenic
organisms require a medium approaching in composition
and reaction to the body tissues and fluids.
Some organisms arc ablc to grow under a ·wide range
of conditions, whercas other more bighly rarasitic
bactcria, such as the gonococcus, are restricted ill
their requirements, with regard not oIlly to food but
also to temperature alld other factors. It is usually
impossible to rcproduce exactly the natural conditions
under ·which pathogenic micro-organisIns live.
On the othcr hand it must be realised that a c(Jllsiderablc
degree of adaptability exists among them, and for
the great majority of pathogenic bacteria suitable
l:tl'tificialmedia have been found.
All bacteria require nitrogen, and in the case of the
pathogenic organisms it is generally obtained from
protein. In the usual culture media nitrogen is conveniently
supplied in the form of "peptone" which
is a commercial product obtained by peptic digestion.
Peptone is a crude product and consists of a mixture
of proteose, pcptone, polypcptides and amino-acids.
This simpler digested form of' protein is used because
it is soluble and does not coagulate on heating. Moreover
it is better adapted to synthetic metabolism than
native protein, especially if it contains a sufBciency
of amino - acids. Alternatively, instead of' adding
the digested protein in the form of commercial peptone,
complex native proteins arc split up during
the preparation of the medium by the action of pepsin
01' trypsin to form simpler uneoagulable nitrogenous
compounds. This is the principle of the so-called
" digest media," of which Hartley'S broth (vide p. 83)
is an example. Other forms in which protein is provided
are serum protcin--e.g. serum-agar and I,cifflcr's