30 BIOLOGY AND TECHNIQUE 



shown to occur in anaerobic cultures where the oxygen must have 

 come from some constituent of the medium. It is probable that 

 bacterial respiration is frequently of the anaerobic type in which, 

 for example, such compounds as nitrates are reduced to nitrites and 

 free nitrogen. Energy may be produced, however, by the oxidation 

 of other elements than carbon and hydrogen; thus, certain of the 

 iron bacteria seem to obtain energy from the oxidation of ferrous 

 to ferric compounds in their protoplasm, and the sulphur bacteria 

 oxidize H 2 S to free sulphur. The colored sulphur bacteria carry out 

 this change under the combined influence of sunlight and their pig- 

 ment in this way resembling the green plants, but the unpigmented 

 forms do not require radiant energy. 



While processes of oxidation are perhaps the most important 

 source of energy for certain bacteria, for other types and under 

 different conditions chemical reactions of another kind serve to pro- 

 duce energy. Processes of fermentation and perhaps of decay and 

 putrefaction liberate considerable quantities of energy. For ex- 

 ample, alcoholic fermentation in which neither oxidation nor reduc- 

 tion is involved produces about 33 calories for each gram molecule 

 of sugar fermented. Since complete oxidation of the same quantity 

 of sugar results in the liberation of about 680 calories it is apparent 

 that relatively large amounts of glucose must be fermented to supply 

 microorganisms with sufficient energy for their growth and, as a 

 matter of fact, bacteria will ferment many times their weight of 

 carbohydrates. Acid fermentation, such as the lactic fermentation 

 of milk and probably many of the processes of protein cleavage, are 

 exothermic and will supply bacteria with energy. Obviously these 

 reactions are particularly adapted to anaerobic growth since for 

 many of them oxygen is not required. 



While a profuse supply of oxygen absolutely inhibits the growth 

 of most anaerobes, a number of these may, nevertheless, develop 

 when only small quantities of oxygen are present. Minute quantities 

 of free oxygen in culture media have been shown by Beijerinck 9 

 and others not to inhibit the growth of Bacillus tetani and Theobald 

 Smith 10 has recently demonstrated that when suitable nutritive 

 material in the form of fresh liver tissue is added to bouillon, a num- 

 ber of anaerobic bacteria may be induced to grow in indifferently 



9 Beijerinck, Cent. f. Bakt., II, vi, 1900. 



10 Tli. Smith, Brown, and Walker, Jour. Mecl. Res., ix, 1906. 



