454 BACTERIOLOGY. 
that under certain conditions it may become pathogenic 
to man. 
According to many authorities there are a great 
number of varieties of the colon bacillus, some main- 
taining even that it may become, under suitable con- 
ditions, identical with the typhoid bacillus; but there 
has been no proof whatever of this. 
Differential Diagnosis. By comparing what has been 
said of the bacillus coli and the bacillus typhosus it 
will be seen that while certain varieties of each simu- 
late each other in many respects, the characteristic 
varieties of each still possess individual characteristics 
by which they may be readily differentiated: 
1. The motility of the B. coli is, as a rule, much less 
conspicuous than that of the B. typhosus. It is also 
shorter, thicker, and has fewer flagella. 
2. In gelatin the colonies of the colon bacillus de- 
velop more rapidly and luxuriantly than those of the 
typhoid bacillus. 
3. On potato the growth of the colon bacillus is 
usually rapid, Iuxuriant, and visible, though not inva- 
riably so; while that of the typhoid bacillus is ordina- 
rily invisible. 
4. The colon bacillus coagulates milk in from thirty- 
six to forty-eight hours in the incubator, with acid 
reaction. The typhoid bacillus does not cause coagula- 
tion. 
5. The colon bacillus is conspicuous for its power of 
causing fermentation, with the production of gas in 
media containing glucose. The typhoid bacillus never 
does this. 
6. In nutrient agar or gelatin containing lactose and 
litmus tincture, and of a slightly alkaline reaction, the 
