40 BACTERIOLOGY 



acid-producing bacteria, and, if the process is carried further, 

 the acetic acid itself may be broken up to form water and 

 carbonic acid gas (carbon dioxide). 



Butyric acid is similarly produced by a corresponding group 

 of bacteria which can attack almost any sugar or allied com- 

 pound, glycerine, and also in some cases proteins and fats. 

 Sour milk usually contains not only lactic but also butyric acid 

 so produced. 



These three forms of fermentation lactic, acetic, and butyric 

 very often occur in the stomach in cases of chronic indi- 

 gestion, and are due to the action of the specific bacteria 

 upon the retained food various gases such as carbon dioxide, 

 hydrogen, etc., being also produced, causing distension and 

 eructations. 



Putrefaction and decay of dead animal and vegetable matter 

 are highly complex processes which may here be shortly dis- 

 cussed. Living plants and animals have the faculty of resisting 

 more or less completely the attack of bacteria ; but after their 

 death, the complex compounds of which their tissues are built 

 are speedily attacked and broken down into simpler substances 

 in much the same way as food is digested, i.e. by the action of 

 ferments in this case produced by the bacteria. Organisms 

 which can live upon such dead organic matter are called sapro- 

 phytic (Greek : sapros, rotten, and phyton, a plant). Those 

 which actually cause the breaking up of the organic matter are 

 called saprogenic (Greek : sapros, rotten, and genes, producing) ; 

 and there are also numerous bacteria, moulds, etc., which can 

 live upon the products of putrefaction, and which are therefore 

 called saprophilic (Greek : sapros, rotten, philein, to love) ; 

 though in many cases both functions are combined. Putre- 

 factive processes go on most readily in absence of oxygen, i.e. 

 the majority of saprogenic bacteria are anaerobic, and" the pro- 

 ducts often have a very foul smell, especially those containing 

 the aromatic and sulphur constituent of the original protein. 

 Sulphuretted hydrogen, indole and skatole, are amongst the 

 substances so produced, and to them is due the characteristic 

 odour of animal excrements and other decomposing matter. 



Where abundant oxygen has access, the process is somewhat 

 different, and is sometimes known as decay, the products being 

 more freely oxidised and the odour much less marked. These 

 changes and the substances so produced from proteins, carbo- 

 hydrates, sugars, etc., can be studied in the culture tube, and 

 by their nature the differences in the action of the special 

 bacteria under investigation can be used as methods of identi- 

 fication of these bacteria. 



