42 Bacteria in Relation to Country Life 



is exerted by high pressure. It has been found that a 

 gradually increasing pressure up to 2,904 atmospheres 

 fails to destroy many of the bacterial species. When, 

 however, the pressure is raised and lowered successively 

 a number of times, the bacteria are considerably weak- 

 ened, as shown by less active motility, an impairment or 

 loss of the abihty to multiply, and impairment or loss 

 of abihty to produce typical reactions. Some of the 

 species prove to be more susceptible than others. 



Influence of the concentration of the culture medium. — 

 Large amounts of soluble salts in the culture solution 

 lead to the injury or destruction of the bacteria. Many 

 substances, when present in small proportion, may serve 

 as food to bacteria, or may be harmless to them, while 

 in greater concentration they may act as poisons. 

 Apart from such substances as alcohol, carbolic acid, 

 or salicylic acid, which are recognized as germicides, 

 there are many neutral salts that are commonly used as 

 nutrients in the preparation of culture media, and yet 

 they are poisonous to the bacteria when present in 

 greater concentration. 



Common salt and saltpeter (potassium nitrate), both 

 being used as preservatives in the pickling of meats and 

 of other organic materials, may serve as examples of 

 such substances. Because of greater resistance to the 

 injurious action of salt, some species are enabled to 

 develop in herring brine and similar substances, investi- 

 gations having demonstrated the presence of a rich 

 bacterial flora in such brines containing as much as 

 20 per cent of salt. 



Among other substances used for the preservation of 



