62 



Biology of Micro-organisms 



/^-\ 



the meat. Kaensche* has carefully investigated the subject, and 

 given a synoptic table containing all the described bacteria of this 

 class. His researches show that there are at least three different 

 baciUi whose growth causes the meat to become poisonous. 



With the increase of knowledge upon the toxic character of the 

 bacteria themselves, the importance of the toxic ptomains has 

 diminished, until at present we have come to regard them as very 

 rare causes of disease. 



Production of Gases. — Various gases are given off during decom- 

 position and fermentation, among them being CO2, H2S, NH4, 

 H, CH4. Gases produced by aerobic bacteria 

 usually fly off from the surface of the 

 culture unnoticed, but if the bacterium 

 be anaerobic and develop in the lower part of 

 a tube of soUd culture media, a visible bubble 

 of gas is usually formed about the colonies. 

 Such gas bubbles are almost invariably pres- 

 ent in cultures of the bacilh of tetanus, and 

 malignant edema. 



To quantitatively determine the gas-produc- 

 tion, 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: 



Fig. 18.— Smith's fer- 

 mentation tube. 



"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 CO2 

 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 CO2 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 differ- 

 ence between this amount and that measured before shaking with the sodium 

 hydroxid solution gives the proportion of CO2 absorbed. The explosive charac- 

 ter 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 proportion, of gases resulting from the 

 fermentation is frequently of importance for the differential diagnosis of related 



* "Zeitschrift ftir Hygiene," etc., June 25, 1896, Bd. xxn, Heft i. 



