METHODS OF STUDYING BACTERIA 39 



highly complex organic compounds are split up into simpler 

 substances by a process akin to, or in some cases identical with, 

 digestion. Proteins, sugars, starch, etc., may be attacked by 

 such fermenting organisms. Various gases are often given off 

 during the process, carbonic dioxide gas, nitrogen, hydrogen, 

 marsh-gas and sulphuretted hydrogen being amongst the 

 commonest of these (see Fig. 13, d). The bubbles rising to the 

 surface of stagnant water from the decaying matter at the 

 bottom, and the collection of gases in 

 the intestine, are familiar instances 

 of this process. In some forms of 

 gangrene, e.g. in Malignant Oedema, 

 the tissues of the living body are 

 attacked bygas-producing organisms ; 

 and gas-production is practically al- 

 ways present in the decay of the body 

 after death unless prevented artifici- 

 ally or by special climatic or other 

 conditions. It is by this means that 

 dead bodies immersed in water come 

 to the surface after a varying in- 

 terval. Sometimes, during the last 

 hours of life, when the tissues of the 

 body are devitalised by illness, putre- 

 factive bacteria from the intestine 

 and elsewhere pass into the blood- 

 stream and spread through the or- 

 gans and tissues. 



FIG. 13. Cultures, (a) Stab- 

 culture in nutrient gelatin. 

 The dark part represents 

 the bacterial growth in the 

 substance of the jelly. (6 

 and c) Surface-cultures on 

 sloped agar, (b) showing 

 scattered colonies, (c) a 

 line of growth, (d) Fluid 

 culture, showing fermen- 

 tation with -gas-production, 

 the gas being caught at 

 upper part of small in- 

 verted tube. 



Fermentation. Various alcohols 

 are produced by micro-organisms, 

 usually belonging to the yeasts, 

 though numerous bacteria also pro- 

 duce alcohol in small quantities. 

 Certain organic acids are important 

 by-products of fermentation, and are especially produced by 

 the splitting up of various sugars. Thus lactic acid is formed 

 by the splitting up of milk-sugar, about a hundred different 

 species of bacteria having this action being known. Other 

 by-products are usually produced at the same time, but gener- 

 ally in smaller amount than the lactic acid e.g., acetic and 

 formic acids, etc., as well as various gases and alcohols. 



Acetic acid, the most important constituent of vinegar, is a 

 common product of the fermentation of various sugars, the 

 formation of alcohol being an intermediate stage in the process. 

 A free supply of oxygen is necessary for. the action of acetic 



