RELATION OF BACTERIA TO DISEASE. 87 
Let us now cultivate these same bacilli in bouillon, 
which is a little too acid or a little too alkaline for their 
maximum development, and we shall find that, while 
all of them will grow, only the one which produced 
the most toxin under favorable conditions will continue 
to develop it, while the others will fail to produce 
any specific poison. This shows that growth of bac- 
teria may occur in the body and yet no specific poison 
be produced, and that of the same species of bacteria 
some varieties are capable of producing toxin under 
less favorable circumstances than others. 
Slight variations in the culture media are, moreover, 
of great importance in aiding or inhibiting the growth 
of bacteria. Thus the. diphtheria bacillus grown in 
neutral bouillon containing a little glucose will at first 
thrive luxuriantly; but as the result of its growth fer- 
mentation of the glucose takes place and acid is pro- 
duced, which then inhibits the further development of 
the bacillus. Aftera while, however, the glucose having 
been entirely destroyed, acid formation ceases while 
alkaline products continue to be formed, and thus render 
the medium neutral or slightly alkaline again, and now 
a vigorous growth again starts up. 
The cultivation of the tetanus bacillus also furnishes 
some interesting facts which illustrate the complicated 
ways in which bacterial growth is hindered or assisted. 
The tetanus bacillus, when placed in suitable media, 
will not grow except in the absence of oxygen; but 
place it under the same conditions, together with a 
bacillus which actively assimilates oxygen, and the two 
in association will grow in the presence of air. 
The tubercle bacillus, when taken direct from an 
animal, will only grow in a few selected media, such as 
