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Bacteria , their Nature and I unction. 
Pathogenic Bacteria. 
The different pathogenic bacteria connected with and causing 
the different infectious diseases have, then, the power of growing and 
multiplying within the infected individual and, through the different 
poisonous substances — toxins— which they therein produce, of 
causing the changes which characterise the particular disease. 
Photographs of a variety of such pathogenic bacteria serve to 
show that both as regards the manner of distribution of these bac- 
teria in the tissues of the infected individuals, as also in their mor- 
phological and biological characters in artificial cultures, most of 
them are sufficiently distinguished from one another and from 
other non-pa'thogenic bacteria. In considering the general action 
of pathogenic bacteria we find that they may be arranged in two 
groups : — 
a. Such as are entirely, so far as our knowledge at present goes, depen- 
dent on the living body of man or animals. These are endogenic bacteria 
or true parasites, for they do not appear to lead an existence independent 
of the living body. "When, therefore, infection by them takes place, it is 
effected by direct transference from an infected individual to a new one ; 
this is so in small-pox, in vaccinia, and in hydrophobia. 
b. A second group comprises those which are capable, besides a para- 
sitic life, i.e. growing and multiplying within the animal body, of leading 
also an existence independent of the animal body : that is to say, they, like 
many other non- pathogenic bacteria, are capable of thriving in suitable 
materials in the outside world ; such are anthrax and fowl cholera, Asiatic 
cholera and typhoid fever, tetanus and diphtheria, and others. But also 
amongst these some can lead such an ectogenic life comparatively easily, 
while others do so only in a restricted sense. While, for instance, anthrax, 
tetanus, typhoid fever can lead such ectogenic life easily, i.e. growing and 
multiplying outside the animal body, others, like tubercle and glanders, do 
so only to a very small extent. The former are obviously the more danger- 
ous to man and animals, on account of their more ready distribution, than 
the latter, of which the ectogenic existence is considerably restricted by 
various conditions, e.g. they require higher temperatures to grow at, and 
they require a much more specialised nutritive medium than is generally 
attainable by them. 
Many and wonderful are the results which have been obtained 
within a comparatively short recent period by a large number of 
workers as regards the identification of many of the pathogenic 
bacteria, their habits of life, their mode of spread and infection ; 
the way in which their action can be attenuated, their effects 
weakened, and such weakened cultures used for protective inocu- 
lations ; — the brilliant results achieved by Pasteur and many others 
in these protective and curative inoculations against anthrax, 
against fowl cholera, against tubercle, against hydrophobia, against 
tetanus, and other diseases. 
E. Klein, 
St. Bartholomew’s Hospital, London. 
