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. After a 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 



