115 



193 



nitrogen in the nutritive substrate. That Witle peptone is inferior as a source of nitrogen 

 to casein peptone and this latter — in the case of the strongest acid-formers — again jioorer 

 than yeast extract, lies to some extent in the fact tliat the hulTer action of these sources 

 of nitrogen rises in the order given. 



21. That the difference between the mentioned sources of nitrogen is not exclusively 

 due to their difTerent buffer action, however, is distinctly evident from the relation of the 

 lactic acid bacteria to the sugars. Only a nitrogenous nourishment absolutely sufficient 

 in all respects will enable the lactic acid bacteria to produce the many heterogeneous 

 enzymes (invertase. maltase, lactase, inulinase, etc.) which are required to hydrolyse di- 

 and polysaccharides. It is therefore necessary to know which are the best sources of nitro- 

 gen for the difTerent bacteria before we can judge which sugars they are al)le to ferment 

 at all. 



22. Sugar fermentation should not, however, be tested only with a good source of 

 nitrogen, but also with a poorer one; this will give a more complete impression as to which 

 sugars are preferred. 



23. The lactic acid fermentation is impeded not only by the liy drogen ions, but also 

 by the lactate ions. The better the buffer action of the nutritive substrate, the more 

 will these latter make themselves felt. As, however, the lactate ions are not nearly so 

 dangerous to the life of the lactic acid bacteria as the hydrogen ions, nutritive substrates 

 with good bulTer action should be used for preservation cultures. In agar with only i/j % 

 dextrose and 14% nitrogen in the form of casein peptone, we have succeeded in preserving 

 lactic acid bacteria unweakened for over three years, and this, be it noted, without 

 re-inoculation. 



24. As the aerogenes bacteria will, with a good nitrogenous nourishment, turn all 

 the sugar into gas, and render the substrate alkaline, they are better preserved in sub- 

 strates with less abundant nitrogenous food. The same applies to the fluorescent bac- 

 teria, strong alkali formers, and the more aerobic micrococci. Many of these can be Well 

 preserved in water with 2 % soluble starch. 



25. Like most other bacteria, the lactic acid bacteria are also able to grow with hardly 

 perceptible quantities of inorganic salts. Potassium phosphate is the most important 

 nutritive salt, and furthers the development of the lactic acid bacteria in increasing quanti- 

 ties up to 2 %o or even more (this, however, with a single exception). The favourable effect 

 of this salt, however, depends like that of the nutritive substrate, partly upon buffer 

 action. This is very distinctly seen in the case of the aerogenes bacteria, which turn all the 

 sugar into gas, with great quantities of potassium phosphate. 



26. There is considerable difference in the amounts of common salt which the differetit 

 species of lactic acid bacteria can stand. As a rule, 2.5 % will produce no detrimental 

 effect; certain species are slightly impeded, others advanced in their development thereby. 

 With 5.5 %, all are impeded, and 10.5 % will in most cases stop the growth entirely. An 

 exception, however, is formed by the tetracocci, which can as a rule stand 15.5 ''o commom 

 salt. 



27. Important characters in a bacterium are the minimal, optimal and maximal tem- 

 peratures for its vital activity, as also the maximal temperature at which it can live at all. 



25* 



