1887] Parasitic Bacteria and their Relation to Saprophytes, 3 
of the higher mammals, to act as a true parasite and destroy life. 
This may explain the occasionally poisonous bites of animals. 
The sputum in pneumonia has been found equally fatal to rabbits. 
But here we are confronted with the important but still unsettled 
question whether the pathogenic microbe in the sputum is not 
the cause of the pneumonia. 
Whether we shall ever find bacteria within the organs, in the 
blood and lymph-channels of the animal body, as permanent 
parasites which do no appreciable injury, is very improbable. 
‘Many experiments which have been made lead to the conclusion 
that the animal organism in health is free from bacteria. This is 
an almost daily experience in the laboratory. Even the excre- 
tions, such as urine and milk, are free from bacterial life. More- 
over, if there were harmless parasitic forms present, why should 
we always obtain the same microbe alone from organs affected 
with the same disease? That bacteria do occasionally penetrate 
into the closed cavities from the mucous surfaces need not be 
disputed, but they are quickly destroyed. Large numbers in- 
jected directly into the blood have been found greatly reduced 
in a few hours, and entirely absent after twenty-four hours. To 
impress this fact more firmly we may picture to ourselves our 
skin‘ and the entire alimentary canal in contact with myriads of 
these organisms. A delicate mucous membrane is all that sepa- 
rates them from the vital organs. Yet not a single individual is 
capable of gaining a permanent foothold within this membrane. 
This applies only to non-parasitic species, however. 
In contrast with this lasting enmity between bacteria and 
the healthy tissues is the more friendly relation between animal 
parasites and the latter. Trichinz and tape-worm cysts enjoy an 
undisputed repose in the muscular tissue of their host. Some 
entozoa live in the connective tissue, others infest the blood; 
they have even been found within the blood-corpuscles of fishes 
and turtles of apparently normal vitality. 
A survey of the various biological properties of those bacteria 
which have been more carefully studied up to the present does 
not reveal to us two extreme classes,—those that are capable of 
a parasitic existence only on the one hand, and those that can 
only live upon dead organic matter. We actually find bacteria 
possessing the vicarious power of living, now a parasitic, now a 
saprophytic existence. The microbes which occasion such dis- 
