CHIEF CONDITIONS REGULATING GROWTH 71 



Now many solids which bacteria use as food, for example, proteids, 

 are not soluble in water, so, in the form in which they are presented, 

 they are useless. To overcome this difficulty, bacteria, like other 

 living organisms, have the power of secreting what are called ferments 

 or enzymes, the chief function of which is to change the insoluble into 

 soluble substances, which can then pass through the membrane. To 

 effect these transformations water must be taken up by the ferments, 

 otherwise they are not able to operate successfully, so that this is 

 another reason for the necessity of the presence of water in the culture 

 of bacteria. 



Next, with regard to oxygen, up to 1861 it was thought that without 

 oxygen no organic life was possible. In that year, however, Pasteur 

 discovered some bacteria which were able to thrive in an environment 

 devoid of oxygen, and since then several similar bacteria have been 

 discovered. With these exceptions, however, oxygen is essential to 

 the life and growth of bacteria. 



Finally, we have to consider the necessity of an adequate tem- 

 perature. As already explained, bacteria are not uniform in this 

 respect ; some, by gradual acclimatisation to hot springs, are rendered 

 capable of active multiplication at a temperature which would be 

 almost instantly fatal to others. Also, owing to extreme sensitive- 

 ness to change of environment, the mimimum, optimum, and maximum 

 temperature will vary with the species and with the different cultures 

 of the same species. High temperatures produce more disastrous and 

 immediate effects than low temperatures. At 0° C. there is no 

 multiplication : the optimum for most bacteria lies between 30° C. and 

 40° C, whilst, with a few exceptions, 45° C. is the higher limit. The 

 optimum temperature for almost the whole of the non-pathogenic 

 bacteria lies very near 32° C. 



The following table gives the minima, optima, and maxima of a few 

 of the pathogenic bacteria : 



MiN. Opt. Max. 



(Degrees — Centigrade). 



