THE BACILLUS OF TYPHOID FEVER. 447 
bacillus as obtained, from a single colony, by special modes of culti- 
vation. From a consideration of these facts certain authors have 
been led to the conclusion that Bacillus typhi abdominalis and Bacillus 
coli communis are simply varieties of the same species. This view, 
however, is not generally acespted, and the characters which serve to 
differentiate the two bacilli are sufficiently well defined when typical 
cultures are compared. These characters, briefly stated, are: The 
invisible growth of the typhoid bacillus on potato; its failure to give 
the indol reaction; its failure to coagulate milk, or to produce a 
change of color in litmus milk; its failure to produce gas in culture 
media containing glucose or lactose; its failure to grow in formalin 
bouillon (1:7,000); and its active motility. Whether the closely re- 
lated bacilli which present some of the characters above indicated, 
without corresponding in all particulars with typical cultures of the 
typhoid bacillus, are varieties of this bacillus, which under favorable 
circumstances could give rise to typhoid infection, has not been defi- 
nitely determined, but appears to be quite probable. It may be that 
such varieties are developed when the typhoid bacillus in feeces finds 
its way into surface waters, under conditions which are favorable for 
its continued development as a saprophyte. On the other hand, it 
may be that one or more of the saprophytic bacilli, which are found 
in water and which closely resemble the typhoid bacillus, may give rise 
to the infectious disease which we know as typhoid fever when in- 
troduced into the alimentary canal of a particularly susceptible indi- 
vidual, and that the special conditions attending its development as 
a parasite give rise to certain modifications in its biological charac- 
ters of a more or less permanent kind. 
Frankland (1895), as aresult of extended experiments, has arrived 
at the conclusion that when the typhoid bacillus is cultivated for a 
long time in media which are more and more largely diluted with 
water, it acquires an increased ability to survive in river water. 
A predisposition to typhoid infection is established by various 
depressing agencies, such as inanition, overwork, mental worry, in- 
sanitary surroundings, etc. And there is considerable evidence in 
support of the supposition that exposure to the offensive gases 
given off from ill-ventilated sewers constitutes a predisposition to 
the disease. 
Experiments made by Alessi (1894), in the Hygienic Institute 
of the University of Rome, give support to this view. The ex- 
periments were made upon rats, guinea-pigs, and rabbits. The 
rats were confined in a close cage with perforated bottom, which was 
placed over the opening of a privy; the guinea-pigs and rabbits in 
similar cages having a receptacle below in which their own excreta 
was allowed to accumulate. The animals which breathed an atmo- 
