194 



116 



28. The optimal temperature for the acid formation, as also for the proteolytic action 

 often lies slightly below the optimal temperatures for growth. 



29. The optimal temperature is not affected by the conditions of nourishment, or 

 by the vitality of the bacteria. The minimal and maximal temperatures, on the other hand, 

 are to a certain degree affected, and weakened strains therefore exhibit far steeper tem- 

 perature curves than those whose vitality is unimpaired. 



30. Some few strains of Sc. cremoris and of the betacocci can grow already at 3°, 

 Whereas Sc. bovis and the thermobacteria do not as a rule develop until past 20"^. Sc. 

 fæcium and the thermobacteria thrive Well at 47i/i — 50°; some thermobacteria, indeed, 

 can even grow at over 50°. Sc. glycerinaceus, Sc. liquefaciens, and Sc. thennophilus , as well 

 as the coli and aerogenes bacteria, grow Well at 45°. The majority of the true lactic acid 

 bacteria on the other hand exhibit poor growth already at 37 — 40°, and their optimal 

 temperature lies at 30° or below this. The pathogenic streptococci and Bacterium bifidum 

 thrive best at 35 — 37°, and Sc. thennophilus, the thermobacteria and Betabaclerium Ion- 

 gum at 40° or over. 



31. In determining the maximal temperature for life (the death temperature), the 

 number of cells which can stand the temperatures used should be noted, as it is of far 

 greater practical importance to know how the majority of cells behave than what the few 

 specially resistant individuals can stand. It is only an extremely insignificant number of 

 cells in a bacteria species which can stand the so-called death temperature. 



32. The death temperature lies, for the pathogenic forms, below 60°. The betacocci 

 are killed, when not protected by slime, at 65°, and the most common lactic acid bacteria 

 of milk. Sc. laclis and Sc. cremoris at 70°, While most of the other lactic acid bacteria 

 can often stand 70 — -75°. The greatest power of resistance to heat is shown by Micro- 

 bacterium lacticum, which does not always perish even at 85°. The duration of the heating 

 in these experiments was a quarter of an hour. 



33. The true lactic acid bacteria — in contrast to most other bacteria — lack catalase 

 entirely. An exception is formed by the more aerobic forms; the tetracocci and most of 

 the microbacteria, but these two groups of bacteria cannot be reckoned entirely to 

 the true lactic acid bacteria. 



34. The true lactic acid bacteria are, in contrast to the pseudo lactic acid bacteria, 

 unable to reduce nitrate to nitrite. The tetracocci and microbacteria are here again excep- 

 tions. It is, however, by no means all the hydrogen-pcroxide-splitting lactic acid bacteria 

 which reduce nitrate to nitrite, in closely related strains, one may reduce, and the other not. 



35. The lactic acid bacteria have as a rule a great aversion to air. They form small 

 colonies on plates, and only an extremely thin veil in streak cultures, and they grow 

 evenly throughout the whole of the stab without any considerable surface growth. Some 

 few thermobacteria even grow better deeper down, and Bacterium bifidum is obligatorily 

 anaerobic. Exceptions are the microbacteria, Which with good nitrogenous nourishment 

 will as a rule exhibit some surface growth, and the tetracocci, which most frequently 

 have a strong surface groNvth. 



36. The true lactic acid bacteria arc as a rule not chromogenic. Some few pathogenic 

 streptococci can on casein peptone agar form a red colouring matter in the stab.and .S/rc/)- 

 tococcus maslilidis forms an'orange colour in casein peptone broth with soluble starch. Mi- 



