43 



121 



ting the great majority of the streptobactcria to form appreciable quantities of acid at 

 10°, despite the fact that the growth at this temperature is as a rule not perceptible until 

 after the lapse of 14 days. The maximal and minimal temperatures of the bac- 

 teria are affected far more by the vitality of the bacteria themselves 

 than by conditions of nourishment, and 'weakened strains therefore exhi- 

 bit much steeper temperature curves than those whose vitality is unim- 

 paired. 



Even though the attitude of the bacteria towards different temperatures may not 

 be altogether constant, there is nevertheless hardly any other quality which better charac- 

 terises the various species. It will be seen from Table XII a, b and c that Sc. fæciuni and 

 the thermobacteria thrive well at 471/9 — 50°; some thermobacleria, indeed, at over .50°; 

 that Sc. glycerinaceus, Sc.liquefacicns, and Sc. thermophilus as well as the coli and aerogencs 

 bacteria grow well at 45°, that Sc. bovis and the thermobacteria as a rule do not grow at 

 ordinary indoor temperature, and that a few strains of Sc. cremoris and of the beta- 

 cocci can grow already at 3°. 



As regards the optimal temperature, this lies, as already mentioned, in the 

 case of Sc. thermophilus and the thermobacteria, at40°or even a little higher ; the same applies 

 to Belabacterium longum (not shown in Table XII); for most pathogenic streptococci it 

 is 35° — 37°, whereas for all other lactic acid bacteria it is 30° or even lower. 

 This last point cannot be too much emphasised, as many bacteriologists erroneously 

 believe the optimal temperature of lactic acid bacteria to lie generally somewhere about 

 blood heat, a temperature which on the contrary is detrimental to most of them. And in the 

 fermentation test which is so important for the cheese maker, milk is placed just at this very 

 critical temperature in order to determine whether good forms (i. e. true lactic acid bac- 

 teria) or bad (i. e. the pseudo lactic acid bacteria) predominate therein. For at low tempe- 

 ratures, the good bacteria, and at high temperatures the bad ones will too easily be able 

 to get the upper hand (We have seen that the coli and aerogenes bacteria thrive well 

 right up to 45°), even though they may by no means have been in the majority to begin 

 with. The employment of different temperatures leads altogether to very active enrich- 

 ment methods. Thus if milk be placed to stand at 45°— 50°, then at first. Sc. fæcium and 

 Sc. thermophilus, more rarely Sc. glycerinaceus will gain the mastery, to be subjugated later 

 by the thermobacteria, which are far more powerful acid formers. 



The Death Temperature. In order to determine the highest temperature to which 

 a bacterium can be subjected without perishing, the capillary tube method is generally 

 employed. It consists in drawing up a little broth culture of the particular bacterium into 

 a series of capillary tubes, which are then fused up at the ends, and heated for a longer or 

 shorter time in a water bath to different temperatures. Where the contents of the tubes 

 exhibit no growth when sown out in a good nutritive substrate, the bacteria will have been 

 killed. 



This method is subject to two sources of error. In the first place, we work 

 with highly varying hydrogen ion concentration, according as more or less powerful acid 

 formers — or possibly alkali formers — are being treated. And it is Well known that 

 the effect of heating upon the bacteria is greatly augmented the more the surrounding 



16* 



