50 Biology of Bacteria 



ticed, but if the bacterium be anaerobic and develop at 

 the lower part of a tube of culture media, a visible bubble 

 of gas is usually formed about the colonies. Such gas 

 bubbles are almost invariably present in cultures of the 

 bacilli of tetanus and malignant edema. 



To quantitatively determine the gas-production, some 

 form of the Smith fermentation-tube is most convenient. 

 The tube is filled with bouillon containing some sugar, 

 sterilized as usual, inoculated, and stood aside to grow. 

 As the gases form, the bubbles ascend and accumulate in 

 the closed arm. In estimating quantitatively, one must 

 be careful that the tube is not so constructed as to allow 

 the gas to escape as well as to ascend into the main reservoir. 



For the determination of the nature of the gases produced, 

 Theobald Smith has recommended the following method : 



"The bulb is completely filled with a 2 per cent, solution 

 of sodium hydroxid (NaOH) and tightly closed with the 

 thumb. The fluid is shaken thoroughly with the gas and 

 allowed to flow back and forth from the bulb to the closed 

 branch, and the reverse several times to insure intimate 

 contact of the CO 2 with the alkali. Lastly, before removing 

 the thumb all the gas is allowed to collect in the closed 

 branch so that none may escape when the thumb is removed. 

 If CO 2 be present, a partial vacuum in the closed branch 

 causes the fluid to rise suddenly when the thumb is removed. 

 After allowing the layer of foam to subside somewhat the 

 space occupied by gas is again measured, and the difference 

 between this amount and that measured before shaking 

 with the sodium hydroxid solution gives the proportion of 

 CO 2 absorbed. The explosive character of the residue is 

 determined as follows: The cotton plug is replaced and 

 the gas from the closed branch is allowed to flow into the 

 bulb and mix with the air there present. The plug is then 

 removed and a lighted match inserted into the mouth of 

 the bulb. The intensity of the explosion varies with the 

 amount of air present in the bulb. The relative propor- 

 tion of gases resulting from the fermentation is frequently 

 of importance for the differential diagnosis of related bac- 

 teria. Smith has designated this relation of ^ as the 'gas 

 formula.' The colon bacillus has a gas formula corre- 



TT 



spending to ^ f . Other aerogenic bacilli sometimes 

 show a formula - = = ." 



