SYSTEMATIC CLASSIFICATION 19 



indicator to our sugar media and immerse a small inverted 

 tube full of media in the culture-tube, then we shall find 

 that the fermentation of a given sugar by one organism gives 

 rise to acid as shown by a change in the colour of the indicator, 

 while fermentation of the same sugar by another organism 

 gives rise to acid and also gas as shown by the accumulation 

 of bubbles inside the inverted tube. If we extend our range 

 of fermentation substrates to include such as xylose, mannitol, 

 dulcitol, glycerol, salicin, etc., we shall find that we have 

 already a method for the separation of certain individuals 

 from others by the range and nature of their fermentations. 

 With these tests we can combine others which involve simple 

 manipulation and observation, such as those for the formation 

 of indole from protein or amino-acid media (see p. 172) or 

 for the formation of acetylmethylcarbinol during the fermen- 

 tation of glucose (see p. 136). By using a series of such tests 

 we find that it is possible to differentiate many common 

 organisms, as can be seen from the selection of organisms and 

 their reactions in Table II (see p. 22). 



The following are some of the common species mentioned 

 in this book: 



Family I 



NiTROSOMONAS species all utilise ammonia as source of nitrogen and 



energy. These organisms are very difficult to isolate in pure 



culture and no methods of separating species have yet been 



described. 

 NiTROBACTER specics utilise nitrite as nitrogen source and cannot 



utilise ammonia. 

 Thiobacillus thio-oxidaxs is found in sulphur- containing soils and 



waters, and obtains energy by the oxidation of sulphur to sulphuric 



acid, to which it is very resistant. 



Family II 



PsEUDOMONAS AERUGINOSA (pyocyanea) is a common soil organism 

 which produces a distinctive blue-green pigment, pyocyanine, 

 which is excreted into the medium. It is non- pathogenic but 

 proves to be extremely diflScult to eradicate from wounds. 



AcETOBACTER spccies are found in the vinegar industry and as con- 

 taminants in brewing vats. Acetobacter xylinum produces a form of 

 capside made of cellulose. 



