METHODS OF EXAMINATION OF BACTERIA 



51 



(6) Presence or absence of a pellicle extending from the agar 

 surface over the surface of the condensed water, e.g. Bac. 

 subtilis, the Hay-bacillus, invariably shows a thick pellicle. 



6. Growth in Various Liquid Nutrient Media. The bacteria require 

 to be supplied with their nitrogen and carbon in different ways. The 

 usual medium employed is a mixture made up of Lemco, common salt, 

 peptone and water, which mixture suits the needs of 



almost all bacteria. To obtain a complete knowledge 

 of the organism, it should be grown in different liquid 

 media, in which the nitrogen and carbon supplies 

 are different. The table on next page, taken from 

 Fischer's Torhsungen ilber Bakterien, illustrates the 

 aid given by these cultures to the diagnosis of the 

 species. 



The results of this set of experiments are quite 

 sufficient to show that this method of cultivation is 

 valuable as an aid to the diagnosis of species. In 

 this connection it must be noticed that the effect 

 of the excretion products is sometimes of an acid 

 and sometimes of an alkaline nature. The same 

 organism may induce acidity in some culture-liquids 

 and alkalinity in others. Bac, hirtus is an example 

 of this kind. 



7. Treatment with Stains. It is always advisable 

 to treat with stains in order to ascertain whether any 

 cell-contents are perceptible, and if so, to determine 

 their nature. Thus Spirillum giganteum, Bac. tumes- 

 cens, and others have fat globules, Bac. asterosporus 

 has glycogen, and Spirillum giganteum and Bac. 

 alvei, volutin. These can be determined by the use 

 of appropriate stains. 



8. Cultivation on such Substances as Sterilised 



Potatoes, Carrots, etc. These are good as subsidiary cultivations. 

 After a species has been cultivated by all the methods detailed above, 

 we have gained a large amount of information concerning it. But it 

 must be remembered that we are dealing with organisms very sensitive 

 to changes of environment and therefore liable to show different kinds 

 of growths. So, for successful work, one must not rely on any one or 

 two modes of cultivation, but must do as many as possible, and thus 

 lessen the likelihood of confusing one organism with another. 



FIG. 63. Agar slope- 

 culture. 



