2 Parasitic Bacteria and their Relation to Saprophytes. [Jan. 
greatly preponderating over other species, although months 
elapsed between consecutive examinations. 
Such bacteria cannot be considered strictly parasitic. It is 
true that they have adapted themselves to conditions which are 
now necessary to the continued existence of many of them, yet, 
if we draw the line at which saprophytic phenomena end and 
parasitic begin, they are not true parasites. For they do not in- 
vade the living tissues to meet the resistance which the living 
cells interpose, but live upon dead organic matter present upon 
the skin, in the mouth, and the digestive tract in general. 
This adaptation to certain media is common to many micro- 
orggnisms. The juice of the grape becomes the habitat of a 
saccharomyces (Cerevis@) which converts the glucose into alcohol 
and carbonic dioxide. When this fermentation has ceased the 
bacterium aceti oxidizes the alcohol into acetic acid. When the 
medium is too acid the bacterium aceti cannot exercise its fer- 
menting power, and another saccharomyces (Mycoderma) first 
reduces the acidity of the liquid by oxidation. Examples may 
be multiplied in illustration of this fact that bacteria as well as 
fungi select certain media as most favorable to their growth. 
It now and then occurs that bacteria not strictly parasitic may 
prove pathogenic in setting up fermentations and decompositions 
in the alimentary canal. The substances thereby produced are 
absorbed, and act as chemical poisons. It seems very probable 
that our information of digestive derangements will be made 
more precise and better methods of relief applied when more 
attention has been bestowed upon the bacteriology of the di- 
gestive tract. Under certain conditions the Leptothrix buccalis, 
the most common microbe in the mouth, may become in a sense 
parasitic. When the enamel of the teeth has been removed by 
acids formed in the mouth during the fermentation of food, this 
microbe causes the slow disintegration known as caries by in- 
vading the dentinal tubules and the pulp-cavity. Now and then 
bacteria which carry on a harmless existence in one place may 
become very virulent in others. A few years ago Dr. Sternberg 
found that rabbits died within a few days after the injection be- 
neath the skin of some of his saliva. This virulence may last 
for years. For it is extremely difficult to dislodge a microbe 
` from a place which it finds conducive to its vital activity. Harm- 
less in the human mouth, it is able to multiply in the body of one 
