TEXT-BOOK OF BACTERIOLOGY. 101 



must be remarked that the energy of the growth is very different 

 in different species while some grow very quickly and luxuriantly, 

 others require whole weeks to reach the same degree of develop- 

 ment. Occasionally, no doubt, this is because our food gelatin offers 

 better conditions for the development of some micro-organisms 

 than for others, which can only exist upon it with difficulty. Re- 

 member that for great numbers of bacteria the ordinary food 

 media offer no suitable nourishment, and we must, therefore, not 

 record the number of colonies that develop as an always reliable 

 index of the number of germs inoculated. 



There is also a second reason which prevents this being done. 

 If we have inoculated the gelatin with too many germs, they will 

 afterward disturb each other in their growth on the glass plates ; 

 many will be completely kept back by others and will thus be lost 

 for statistical calculation. 



Lastty, it may also happen that a colony, though looking just 

 like the others, may not have owed its origin to one single germ. 

 In the inoculated material small, firmly-united groups of bacteria 

 may have existed ; a series of cocci, a thread of bacilli, may have 

 passed into the nutritive liquid, and thence, undisturbed, on to the 

 glass plate, where they could only proceed conjointly to the forma- 

 tion of a colony 



With these exceptions, however, the colonies, as a rule, agree 

 in quantity and variety with the original number and kinds 

 of germs inoculated, and it is an extremely valuable quality of 

 the solid transparent food media that they are able to give us 

 such ready and exact information on this point. If we wish to 

 know the quality of bacteria present in any particular substance, 

 one only needs to bring a measured quantity of that substance into 

 gelatin, and after a few days can read off the result from the plate. 

 For the comparative examination of different fluids, etc., this proc- 

 ess is extremely valuable. The particulars of its applications will 

 be given hereafter. 



Much more important, however, is the certainty with which the 

 solid food media enable us to differentiate between the separate 

 species. 



As in every case the germs develop small pure cultures of 

 their own species only, all the peculiarities of the species are seen 

 in the colony in a more strongly-marked and striking manner; 

 features that, occurring separately, would scarcely be perceived by 

 the practised eye becoming clear and obvious when seen en masse. 

 This was noticed in the case of the potato, but it is seen still more 

 strikingly in the case of the transparent gelatin or agar. Forty- 



