66 APPLIED BACTERIOLOGY 



of the various nutrient media and processes too much 

 emphasis may appear to have been laid on the necessity for 

 the most absolute care to be taken in the sterilisation of 

 vessels, and so on, to prevent contamination, it must be 

 remembered that by neglecting to take the most trivial 

 precaution a great deal of labour may be rendered useless, 

 so that the work has to be done entirely afresh. 



In this connection it must also be remarked that bacteria 

 may display an extreme sensitiveness to differences in the 

 composition of nutrient media so minute that they have not 

 in all cases been identified chemically. Too much care 

 cannot be given to preparing such media with the utmost 

 accuracy, and, in particular, all precautions must be taken 

 to assure the identity of composition of media to be used in 

 experiments of which the results may subsequently have to 

 be compared with each other. The variation between 

 different races of an organism, or even between different 

 specimens of the same organism, and the possibility of 

 an effect of symbiosis or antagonism, has an important 

 practical bearing on experimental work. In comparative 

 experiments it is desirable to work with bacteria of the same 

 race which has been grown and subcultured, so far as 

 possible, under similar conditions, and to repeat the experi- 

 ments with specimens of another race. In plate cultures 

 of possibly mixed bacteria, the inoculated material should 

 be diluted, so as to avoid crowding of colonies. 



In the culture of the bacteria the store of nutrient 

 material necessary for their growth becomes gradually used 

 up by the vital activity of the organisms, and their gradual 

 development and reproduction comes to a standstill. Some 

 of the bacteria die from want of nourishment ; while 

 others, as has already been shown, develop permanent 

 forms, or ' spores,' which are able to remain quiescent for 

 long periods of time until favourable conditions of growth 



