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THE MICRO-ORGANISMS OF THE HUMAN MOUTH.
,16
No sharp line of distinction can, however, be drawn between
pathogenic and non-pathogenic bacteria, for every bacterium
brought into the animal organism in sufficiently large masses will
be able to maintain itself for a certain length of time and to
excite more or less inflammatory reaction.
Pathogenic in a high degree are those bacteria which, like the
anthrax bacillus, produce dangerous disturbances, even though
singly inoculated into the organism ; very slightly pathogenic
are those which can cause disturbances only when introduced
in large quantities. i\"o»-pathogenic are those, which, brought
into the organism in large numbers, soon disappear without
having produced any apparent disturbance.
Pathogenic bacteria exert very different and diverse influences
upon different species of animals. An animal of one species
may manifest symptoms of disorder soon after inoculation,
while members of another species may remain altogether unaf-
fected by it. Even different varieties of one and the same species
do not always manifest the same degree of susceptibility when
e.g. house mice inoculated
inoculated with the same bacterium ;
with the bacillus of mouse septicaemia in a skin pocket suc-
cumb within forty to sixty hours, whereas field mice suffer no
inconvenience from the inoculation. Koch ^° explains this phe-
nomenon by the difference of the l)lood of these two nearly
related varieties.
A species or variety is consequently said to be either suscep-
tible (disposed) or unsusceptible (immune, refractory). "With
every animal the degree of susceptibility varies at different times,
according to the condition of the body at the time being (tempo-
rary susceptibility). Furthermore, since in all epidemics the dis-
ease does not rage everywhere with equal severity, but attains its
greatest intensity at certain circumscribed points or localities, it is
customary to speak also of a local disposition or susceptibility.
Bacteria are further divided into parasitic : such as live in oi
upon living organisms, and saprophgtle : such as are confined to
dead substances. Alost parasitic bacteria, however, arc also
able to live upon dead matter, and may consequently be culti-
vated in artificial media, while on the other hand pure sapro-
phytes occasionally take ou the habitus of parasites.