2 PRACTICAL BACTERIOLOGY 



A saprophyte may be defined as an organism obtaining its nutri- 

 tion from dead organic matter, whilst a parasite exists at the expense 

 of some other living organic creature, known as its host, and strictly 

 speaking cannot develop upon dead matter. 



There is, however, a group of so-called 'facultative'' saprophytes 

 and parasites which possess the power of accommodating themselves 

 to existing surroundings, at one time leading a parasitic, and at 

 another time a saprophytic form of existence. 



Decomposition, putrefaction, and fermentation result from the 

 activities of saprophytic bacteria, whilst the parasitic bacteria cause 

 changes in the tissues, resulting in disease processes, or causing the 

 death of their host. It has been found convenient in classifying 

 bacteria to describe their chief characteristics by the following 

 terms : 



Chromogenic, for pigment-producing bacteria. 

 Photogenic, for phosphorescent or light-producing bacteria. 

 Zymogenic, for bacteria concerned in the various fermentations. 

 Saprogenic, for bacteria producing putrefaction. 

 Pyogenic, for bacteria producing suppuration. 

 Thiogenic, for those converting sulphuretted hydrogen into higher 

 sulphur compounds. 



There is another very important saprophytic group which com- 

 prises the so-called ' nitrifying ' and ' denitrifying ' bacteria. The 

 4 nitrifying ' group oxidizing ammonia to nitrous and nitric acids ; the 

 4 denitrifying ' group reducing nitric acid to nitrous acid and ammonia. 

 Through their association (symbiosis) with the nitrifying bacteria and 

 the activity of the latter, as far as we know, leguminous plants are 

 enabled to make up their nitrogen deficit in part from the free 

 nitrogen of the air. 



This important discovery gave to free atmospheric nitrogen a 

 biological significance heretofore denied. 



Owing to the absence of chlorophyll, bacteria must have the carbon 

 and nitrogen necessary for their growth in the form of decomposable 

 organic substances, and the most favourable media for their develop- 

 ment are neutral or very slightly alkaline solutions of proteid 

 materials in one form or another. 



Bacteria growing, multiplying, and performing definite functions in 

 the absence of oxygen, and to the existence of which oxygen is 

 positively harmful, are known as 'Anaerobic 1 bacteria, in contra- 

 distinction to the Aerobic group, for the proper performance of whose 

 functions free oxygen is essential. 



