30 
PRINCIPLES OF BACTERIOLOGY 
the fact that the morphological changes alone should 
not be regarded as definite evidence of mutation, but 
one should take into consideration the reaction of the 
given bacterium to its culture media, staining, patho¬ 
genic properties and the various serological reactions 
(agglutination, complement fixation, precipitation, etc.). 
We know that the various members of the typhoid-colon- 
dysentery group cannot be differentiated morphologically, 
yet their disease-producing properties and the fer¬ 
mentation of sugars remain distinct. Even when changes 
in bacteria have been brought about by artificial means, 
these secondary changes will disappear, as shown by the 
work of Yaughan, Eisenberg, and others, when normal 
conditions have been restored. The most startling work 
along these lines is that of Rosenow, who claims to have 
among other things, mutated hemolytic streptococci into 
pneumococci; so far these observations seem to be ac¬ 
cepted as interesting bacteriological phenomena, but 
how true and permanent these mutations are, time alone 
will show. 
So far we may accept the statement that bacteria are 
quite constant as to type and that all secondary changes 
can be banished when normal conditions of bacterial 
development have again prevailed. 
V. The Chemical and Physical Properties of the Bac¬ 
terial Cell 
The various chemical constituents of bacteria may be 
tabulated approximately as follows: 
Water.85% 
Proteins.10-12% 
Fats.1% 
Ash.1.5%-2% 
Residue.1-1.5% 
(Kappes, Nencki and Scheffer.) 
