1887] Parasitic Bacteria and their Relation to Saprophytes. 7 
juries and changes different from those which arise when they 
are loosely scattered. 
It is a curious fact that those bacteria which are strictly 
parasitic and which have not yet been cultivated in nutritive 
media, or only with considerable difficulty, cause diseases which 
are very slow in their progress, often lasting for years and fre- 
quently checked and cured. Tuberculosis, syphilis, and leprosy 
are illustrations of this fact. On the other hand, the diseases 
which are produced by bacteria that thrive in artificial media 
` are usually quite rapid in their course. The conflict in the latter 
case is much fiercer and more quickly decided. In other words, 
the bacteria are more virulent. The better adapted the parasite 
becomes, the more compatible will it be with the host and the 
less capable of carrying on an, independent existence. It is for 
the interest of the more strictly parasitic forms that their host 
live as long as possible. This is not necessarily so with those 
species whose life in nature may continue more or less inde- 
pendent of a parasitic existence. 
The more perfect parasitic bacteria, manifesting their presence 
in very slowly progressive maladies, usually reside within the 
protoplasm of the cells, where the feeble irritation leads to a 
hypertrophy and then to a gradual destruction of the cell itself. 
The bacteria are probably taken up in the same way in which the 
amceba takes in solid particles. The cell endeavors to destroy 
them in this way, but their persistence within the cell-protoplasm 
indicates that the struggle has resulted in the victory of the 
parasite, which even finds the battle-ground a convenient place of 
abode. There are one or two rapid diseases, such as mouse 
septiczemia, in which this intra-cellular habitat of the microbes 
is always observed. 
Another interesting feature which they share with entozoa is 
their limitation to certain species of animals. Some are peculiar 
to one species, others may thrive upon several. This suscepti- 
bility of certain animals to definite pathogenic germs is so con- 
stant a phenomenon that it has now become an indispensable 
means in the diagnosis and differentiation of bacteria, and in 
conducting investigations upon obscure points in the life-history 
which are of direct practical value. In other words, the smaller 
animals are to the pathologist what chemical reagents are to the 
chemist. 
