88 MICROBES AND TOXINS 



bacterium which has produced it, but which can in its turn 

 become a food material if the conditions are altered, if the 

 bacterium takes on new properties, or if other microbes step 

 in " (Duclaux). Every living thing lives on the products of 

 others. 



As the richness of a culture increases, its growth slackens : 

 the medium becomes less and less favourable, the food material 

 becomes exhausted and the bacterium, by no means always 

 capable of living on its own residues, ends by being embarrassed 

 by the substances it has produced. Acid-producing bacteria 

 cease to grow when the acidity reaches the point where 

 vegetation is no longer possible. Alcohol acts like an 

 antiseptic towards the yeast that produced it and acetic 

 acid does the same for the ferment of vinegar. The bacterium 

 however can often fall back from the food of its real choice to 

 a sort of famine ration : when the acetic ferment has used up 

 all the alcohol it burns up the acetic acid. When a yeast has 

 no longer any sugar it consumes the glycerine which it has 

 produced at its expense. 



Products of excretion exist which stop the growth of cultures 

 by a sort of auto-intoxication. The foulest waters are those 

 which are least easy to infect, because, according to Miquel, they 

 contain substances of bacterial origin injurious to bacterial 

 growth. If such foul waters are concentrated at a low 

 temperature and the filtered result is added to pure water this 

 latter becomes incapable of supporting life. Boiling destroys 

 these inhibitory substances, which indicates perhaps that they 

 are of the nature of diastases. There is said to be in faecal 

 matter an inhibitory substance which checks the extraordinary 

 multiplication of bacteria in the intestine, and this also is to be 

 regarded as a diastase ; Conradi and Kurpjuweit compare its 

 energy to that of carbolic acid ; without having been able to 

 isolate it they were able by dialysis to make it act without the 

 bacteria themselves. They call it an " autotoxin." Others, how- 

 ever, question this, not having been able to find it either in 

 tube cultures or in the human intestine, and explain the 

 inhibition by the exhaustion of the medium, in the same way 



