56 PATHOGENIC BACTERIA. 



duce acids only, some alkalies only, others both acids 

 and alkalies. Like the acids, the alkalies, when in 

 excess, serve to check the further growth of the micro- 

 organisms. 



6. Production of Gases. — This seems, in reality, to be 

 a part of the process of decomposition and fermentation. 

 Among the gases due to bacterial action, C0 2 , H 2 S, NH 4 , 

 CH 4 , and others have been described. If the bacterium 

 be anaerobic and develop at the lower part of a tube of 

 gelatin, not infrequently a bubble of gas will be formed 

 about the colonies. This is almost constant in tetanus 

 and malignant edema. Ordinarily, the production or 

 liberation of gases passes undetected, the vapors escaping 

 from the surface of the culture-medium. 



To determine the gas production where it is suspected 

 but not apparent, the ordinary fermentation-tubes can be 

 employed. They are filled with glucose bouillon, steril- 

 ized as usual, inoculated and allowed to grow. If gases 

 are formed, the bubbles ascend and the 

 gas accumulates at the top of the tube. 

 In estimating quantitatively, one must 

 be careful that the tube is not so con- 

 structed as to allow the gas to escape as 

 well as to ascend in the main reservoir. 

 For the determination of the nature 

 of the gases ordinarily produced, some 



of which are inflammable and some not, 

 Fig. 5. — Smith's fer- ' 



mentation-tube. Theobald Smith has recommended the 



following methods: 

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

 lution 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 closed branch, and the reverse several times to insure 

 intimate contact of the C0 2 with the alkali. Lastly, 

 before removing the thumb all the gas is allowed to col- 

 lect in the closed branch so that none may escape when 

 the thumb is removed. If C0 2 be present, a partial 



