78 PHYSIOLOGY OF BACTERIA 



not be discussed in this part of the book. However, the 

 source of energy is one of the most important factors in 

 growth, and so the two might well be combined and 

 considered here. 



It might seem that we could define strict aerobes 

 as bacteria which must have oxygen for hydrogen accep- 

 tors. One may object that this definition will fail as 

 soon as new hydrogen acceptors are found which permit 

 growth without oxygen gas. Since vinegar bacteria 

 might obtain their oxygen for fermentation from methy- 

 lene blue, and since oxidation of a medium with per- 

 manganate makes aerobic bacteria grow farther down 

 below the surface (see p. 71), we are warned against 

 making our statements too positive. 



Another factor enters in, namely, the growth require- 

 ments. Sohngen (1909) has shown that urea fermenta- 

 tion (which needs no hydrogen acceptors) is entirely 

 sufficient for the energy requirements of some urea 

 bacteria, and that very small amounts of organic acids 

 suffice for the carbon of the cell construction; no growth 

 will take place, however, in the absence of oxygen. In 

 the construction of cell compounds from organic acids, 

 dehydrogenation must become necessary for certain 

 parts of the building material, and oxygen is the only 

 acceptor for this hydrogen. Urea cannot possibly act 

 as hydrogen acceptor. 



When it comes to the differentiation between faculta- 

 tive and strict anaerobes, another factor enters, namely, 

 the toxic effect of oxygen. Really, the only definition 

 that holds with all anaerobes is that oxygen of the 

 concentration found in air is toxic, and prevents growth. 

 Anaerobiosis has no direct connection with the source 

 of energy, and some Beggiatoa species have been claimed 

 to grow only with reduced atmospheric oxygen pressures, 



