39 



117 



IV. Attitude toward Different Temperatures. 



Important characteristic feature in a bacterium are tlie minimal, optimal and maximal 

 temperatures for its vital activity, and the death temperature. 



The 3Iiiiimal, Optimal and Maximal Temperatures for Vital Activity are determined 

 simultaneously, by sowing the bacterium in a series of tubes with a good nutritive 

 substrate, and placing them under observation at different temperatures. For the higher 

 temperatures, thermostats with water jacket were employed; these could, thanks to 

 the great heating capacity of the water, be regulated very accurately. For temperatures 

 lower than that of the room, we used a Panum's thermostat, heated with gas to 20° at one 

 end, and cooled with ice at the other. Unfortunately, the temperature in the dilîerent com- 

 partments varied from 1° — 2°, according to the quantity of ice in the receptacle. All our 

 strains were tested first at 5, 10, 15, 20, 25, 30, 35, 40, 45 and 50 degrees, and thereafter 

 at certain intermediate temperatures if required, in order to determine the minimal and 

 maximal temperatures more closely. Immediately after inoculation, the tubes must be 

 heated in a water bath to the testing temperature, if this is over 40°, otherwise, some 

 growth may take place before the air in the thermostat has communicated its temperature 

 to the content of the tubes. At temperatures over 20°, the tubes were left to stand as 

 usual for 14 days before titration; at 20° and under, on the other hand, they were left 

 to stand for a month, as experience had shown that the maximum of acid is generally 

 only reached after that time. When we are fortunate enough to strike the minimal or 

 maximal temperature exactly, it will be possible to observe growth without acid formation, 

 a state of things which is noted in Table XII by +• 



It should be pointed out that the optimal temperature for growth of lactic acid bac- 

 teria cannot be determined by acid titration alone but only by daily observation, and 

 where necessary by microscopic examination of the contents of the tubes. For the tem- 

 perature at which the greatest quantity of acid is formed can lie some- 

 what below the temperature at which liveliest growth takes place, owing 

 to the fact that the acid is more destructive in its effects at the higher than at the lower 

 temperature. This was very distinctly seen in the case of Sc. Ihermopliilus and the ther- 

 mobacteria, which quite indisputably showed most rapid growth at 40°, but formed most 

 acid at 30° and 35° respectively. And what is here said of the acid formation, applies 

 in an even higher degree to the proteolytic action. I have previously shown that Telracoc- 

 cus liquefaciens (= Micrococcus casei liquefaciensY) which grows most rapidly, and also 

 ferments most sugar, at 30°, exhibits the strongest proteolysis at 20°, and Barthel has 

 shown that streptococci^) whose optimal temperature is likewise about 30° can in the long 

 run split up the casein in the milk most powerfully at indoor temperature. This question, 

 however, is a more complicated one than that of the acid formation, since, as mentioned, 

 the proteolysis occasioned by the lactic acid bacteria in the surrounding substrate is 

 a pure enzyme action, which does not run parallel with the vital activity of the cells. 



It can even less be determined — as many writers have done — by the time required for 

 curdling milk, since milk will curdle with smaller quantities of acid as the temperature is increased. 

 1. c. 1904, p. 32. 



3) Meddelande Nr. 97 från Centralanstalten for försöksväsendet på jordbruksområdet 1914. 



