Structure of the Bacteria. 7 



respiratory purposes from oxygen in combination, and on 

 account of their ability to thrive in an atmosphere devoid 

 of free oxygen are called anaerobic, while those requiring 

 air are called aerobic. Some species grow strictly under 

 one condition or the other, and hence are obligate aerobes 

 or anaerobes; others known as facultative or optional forms 

 possess the ability of growing under either condition. The 

 majority of milk bacteria are either obligate or facultative 

 aerobes. 



Rate Of Growth. Growth takes place very slowly at or 

 near the minimum temperatures, but as the temperature 

 is increased the rate of growth becomes more rapid, until 

 at the optimum point, a single bacterial cell, in an active 

 vegetative condition may divide into two cells in twenty 

 minutes. If the temperature is raised to the maximum the 

 rate of development is very rapidly lessened, until finally 

 cell-multiplication ceases altogether. Even under opti- 

 mum conditions this initially rapid rate of development 

 cannot be maintained indefinitely, for growth is soon 

 limited by the accumulation of by-products of cell activity. 

 Thus, the sour milk germ grows rapidly at ordinary tem- 

 peratures until the accumulation of lactic acid checks its 

 own growth. If this is removed by neutralizing it with 

 chalk, the lactic bacteria renew their growth. 



Detrimental effect of external conditions. Enviromental 

 influences of a detrimental character are constantly at work 

 on bacteria, tending to repress their development or destroy 

 them. These act much more readily on the vegetating cell 

 than on the more resistant spore. A thorough knowl- 

 edge of the effect of these antagonistic forces is essential, 

 for it is often by their means that undesirable bacteria may 

 be killed out. 



