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226                 PA THOGENIC BACTERIA.

are injected into the subcutaneous tissues of rabbits
small local abscesses develop in the course of a couple
of weeks, showing that the tubercle bacilli are chemotac-
tically potent.

While it is extremely interesting to observe that this
chemotactic property exists, it seems to be by some other
irritant that most of the lesions of tuberculosis are caused.
When the dead tubercle bacilli, instead of being injected
en masse into the areolar tissue, are so introduced into
the body—as by intravenous injection—as to disseminate
themselves or remain in small groups, the result is quite
different, and much-more closely resembles that of the
action of the living organism.

Baumgarten, whose researches were made upon minute
tubercles of the iris, has shown that the first manifesta-
tion of the irritation caused by the bacillus is not the
attraction, of leucocytes, but the stimulation of the fixed
connective-tissue cells of the part affected. These cells
increase in number by karyokinesis, and form about the
irritating bacterium a minute focus which is the primitive

The leucocytes are of secondary advent, and are no
doubt attracted both by the substance shown by Prudden
and Hodenphyl to exist in the bodies of the dead bacilli
and by the necrotic changes which already affect the
primary cells. For reasons not understood, the amount
of chemotaxis varies greatly in different cases. Some-
times the tubercles will be sufficiently purulent in type
almost to justify the name "tubercular abscess;" some-
times there will be a marked absence of cellular ele-
ments derived from the blood.

The important toxic substance produced by the bacillus
is evidently not associated with chemotaxis, for when the
leucocytes are absent the necrosis which is so characteris-
tic persists.

The groups of cells constituting the primitive tubercle
have scarcely reached microscopic proportions before a
distinct coagulation-necrosis is observable. The proto-