66 GENERAL CONDiriONS FOR GROWTH 



which produce colors may grow in the Ught, since the pig- 

 ments protect them. Some few kinds, like the sulphur bac- 

 teria, which contain a purplish-red pigment that serves 

 them to break up HjS, need light for their growth. Since 

 disease-producing bacteria are all injuriously affected by 

 light, the advantage of well-lighted habitations both for 

 men and animals is obvious. 



OXYGEN SUPPLY. 



Oxygen is one of the constituents of protoplasm and is 

 therefore necessary for all organisms. This does not mean 

 that all organisms must obtain their supply from free oxygen, 

 however, as animals and plants generally do. This fact 

 is well illustrated by the differences among bacteria in this 

 respect. Some bacteria require free oxygen for their growth 

 and are therefore called aerobic bacteria or aerobes (sometimes 

 strict aerobes, though the adjective is unnecessary). Others 

 cannot grow in the presence of free oxygen and are therefore 

 named anaerobic bacteria or anaerobes (strict is unnecessary). 

 There are still other kinds which may grow either in the 

 presence of free oxygen or in its absence, hence the term 

 facultative anaerobes (usually) is applied to them. The dis- 

 tinction between facultative aerobe and famltative anaerobe 

 might be made. The former means those which grow best 

 in the absence of free oxygen, though capable of growing in 

 its presence, while the latter term means those which grow 

 best in the presence of free oxygen, but are capable of grow- 

 ing in its absence. The amount of oxygen in the atmos- 

 phere in which an organism grows may be conveniently 

 expresssed in terms of the oxygen pressure, i. e., in milli- 

 meters of mercury. It is evident that the maximum, mini- 

 mum and optimum oxygen pressures for anaerobic bacteria 

 are the same, namely, mm. Hg. This is true only for nat- 

 ural conditions, since a number of anaerobic organisms have 

 been gradually accustomed to increasing amounts of 0, so 

 that by this process of training they finally grew in ordinary 

 air, that is, at an oxygen pressure of about 150 mm. Hg. 

 (Normal air pressure is 760 mm. Hg. and oxygen makes up 



