THE CHEMISTRY OF BACTERIA. 81 



contact with the cells that produced them. The accumula- 

 tion of waste-products eventually stops the growth of the 

 organism, and as pointed out on p. 50, in many species it 

 will induce spore formation. Prolonged contact with these 

 chemical products is injurious and may result in the death 

 of the organism. If the bacteria are grown in a tube, the- 

 walls of which are permeable, as in the case of a collodium 

 sac, the waste-products will diffuse through the wall into 

 the surrounding liquid, and, as a result, growth will continue 

 as long as nutrient substances are present. A similar diffu- 

 sion, or rather dilution, enables these organisms to grow, 

 more or less continuously, in various waters. 



The accumulated waste-products under ordinary experi- 

 mental conditions soon stop the growth and multiplication 

 of the germs present. Inasmuch as the chemical products 

 of one organism are more or less different from those of an- 

 other, it will be evident that in some instances the inhibition 

 will be more marked than in others. Thus, in lactic acid 

 fermentation the process stops when about 0.8 per cent, of 

 free lactic acid has formed. On the other hand, in acetic 

 acid fermentation the growth may not be inhibited by 10 

 per cent, of the free acid. Similarly, the yeast-plant may 

 continue to grow until about 15 per cent, of alcohol has ac- 

 cumulated. Certain bacteria give rise to phenol or carbolic 

 acid which is a well marked antiseptic even when consider- 

 ably diluted. 



Although bacteria are small in size and simple in struc- 

 ture they, nevertheless, elaborate a great variety of chem- 

 ical compounds. The changes which these minute organ- 

 isms induce are exceedingly complex. In the first place it 

 must be borne in mind that the cell builds up its protoplasm 

 and other cell constituents out of the food supplied. Such 

 a chemical change is obviously carried on within the cell. 

 It is intracellular and synthetic. The bacterial proteins, 

 toxins and ferments are undoubtedly formed in this way. 



G 



