534 BIOLOGY OF THE MICRO-ORGANISMS. 



very great differences in the requirements of the dif- 

 ferent species of bacteria. On the whole, the amount of 

 water in the nutrient material must be very great, and 

 the concentration slight. Fermentescible substances 

 can be protected against the invasion of bacteria by the 

 removal of a relatively small quantity of water, while 

 they still remain a favourable soil for the growth of the 

 yeast fungi, and more especially of the mould fungi. 

 Very little has been ascertained as to the limits of con- 

 centration, for this must vary according to the nature 

 of the nutrient materials ; quite as little is known as 

 regards the optimum of the amount of water. That the 

 latter can as a rule vary within wide limits is evident 

 from the fact that bacteria can be cultivated equally 

 well on semi-solid nutrient soils containing about 80 

 per cent, of water, in concentrated fluids containing 

 5 to 10 per cent, of solid constituents, and in very 

 dilute solutions containing scarcely more than traces of 

 nutrient substances. 



Eeaction. Excess of acid or alkali may be either injurious or 



favourable to the development of the bacteria ; the first, 

 however, interferes most readily with the growth. In 

 this respect there is an important difference between 

 many of the bacteria and the mould and yeast 

 fungi ; and hence in the acid reaction of the nutrient 

 medium we have an excellent means of protecting the 

 cultivations of the latter organisms against the entrance 

 of numerous species of bacteria. Many bacteria, for 

 example, bacillus subtilis, anthrax bacilli, &c., are very 

 sensitive to slight excess of acid ; but on the other 

 hand there are bacteria, such as bacillus butyricus or 

 the acetic bacterium, which can bear a very marked acid 

 reaction without injury ; indeed, many only grow when 

 there is a certain excess of acid in the nutrient medium 

 (for example the bacillus of blue milk, and the bacterium 

 of acetic acid which only grows when at least 2 per cent. 

 of acetic acid is present) . Hence an excess of alkali is 

 hurtful to these particular bacilli, while as a rule it has 

 by no means such an injurious effect on bacteria as the 

 free acids ; indeed, some organisms, for example the 



