AGf~ICULTURAL RESEARCH, PUSA.

20 PRACTICAL BACTERIOLOGY
virtue of which disease processes result from bacterial
infection. They have not bcen isolated as chemically
pure substances, but arc generally regarded as being of
protein nature. They are classified as :
1. Extracellular toxins, or exotoxins, which diffuse
readily from the bacteria into the surrounding
medium.
2. Intracellular toxins, or endotoxins, which are :..'ebined
within the cells until the bacteria die and
disintegrate.
The majority of the pathogens produce endotoxin only,
bnt certain develop powerful cxotoxills-e.g. B. diphtheriac,
B. dysenteriae (Shiga), B. ietani, B. botulinus,
certain types of staphylococci and streptococci.
Exotoxins are generally unstable substances, their
toxic effect being annulled readily by chemicals, free
oxygen, and heat-e.g., in the case of the diphtheria
toxin, at 65° C. They tend to bc specific in their
action on particular tissues. Preparations of these
toxins call be obtained by growing the bacteria in
fluid culture which is then filtered through an earthenware
or other bacterial filter-the filtrate contains
the toxin (viele pp. 73, 284). After introductioll into
the body thel'e is usually a short incubation period
(e.g. some hours) before symptoms of poisoning are
manifest. By immunisillg animals with such preparations
a specific neutralil>ing antibody (anti-toxin) is
developed which can be demonstrated in the blood
serum of the immune animal (vide p. 27).
Most pathogenic organisms do not produce specific
diffusible toxins, and their cultUl'e-filtrates are nOlltoxic;
on the othel' hand, their dead bodies are toxic,
and it is assumed that theil' poisons are bound up in
the bact.erial cell-hence the designation endotoxin.
These toxins al'e less specific in their action and
produce a more general poisoning effect. They arc more
stable than exotoxins and can withstand a temperature
of lOOoe. They can be demonstrated experimentally
by injecting dead cultures or hacterial extl'acts into
animals. 'There is no incubation period following