122 



44 



liquid diverges from the neutral point. In the second place, there is really no sharply 

 marked limit at all which can be designated as the maximal temperature for the life of 

 a bacterium, but experience has shown that most of the cells are killed off at a far lower 

 temperature than is required to overcome the most resistant of the cells^). It is therefore 

 impossible to obtain a thorough insight into the question without investigating what 

 takes place at each of the temperatures tried. 



In order to obtain accurate results, it will be necessary first of all to dilute the starting 

 material before heating, to such a degree that its own concentration of hydrogen ions 

 cannot produce any effect; secondly, to note the number of the cells sown out Which survive 

 the different temperatures. Both can easily be done by sowing out a couple of drops of 

 the culture to be tested in agar tubes, and heating in the same. The surviving germs will 

 grow out into colonies in the usual way, and can then be counted. Wc used Bubri tubes 

 for this purpose, with a deep layer of casein peptone dexti-ose agar. It will of course be 

 necessary to see that the agar has, prior to sowing out, reached the temperature of the 

 water bath, and the germs must be distributed by rolling the tube to and fro in a vertical 

 position, so that they do not settle higher in the tubes than the level of the water outside. 

 When the heating is completed, the germs are thoroughly distributed by reversing the 

 tube once or twice; the tube is then cooled as rapidly as possible, in cold water. When deal- 

 ing with aerobic organisms — such as for instance the tetracocci — the agar is poured off 

 into a petri dish after heating. 



Table XIII a and b shows the results oMained with some of our strains after heating for 

 a quarter of an hour to 60°, 65°, 70°, 75°, 80° and 85° degrees. A temperature of 60° suffices 

 to kill off the pathogenic species such as Sc. mastitidis and Sc. pyogenes. Sc. pyogenes No. 

 10 is an exception; this bacterium is, however, as we shall see later on, not a true Sc. 

 pyogenes, but a pathogenic variety of Sc. fæcium. At 65°, the betacocci are killed, and 

 at 70°, the commonest lactic acid bacteria of milk. Sc. lactis and Sc. cremoris. Most of 

 the cells of these will, however, have perished already at lower temperatures. Taking for 

 instance Sc. lactis No. 2, we find from the table that only ^/loo % of its cells have been 

 able to endure heating to 60°. A greater power of resistance is exhibited by the remaining 

 lactic acid bacteria, of which some few cells can stand heating to 70° — 75°. The most re- 

 sistant species is Microbacterium lacticum, which is not always killed off entirely even at 

 85°. In milk pasteurised at low temperatures, therefore, we encounter chiefly this 

 and Sc. thennophilus^), besides certain tetracocci (micrococci); somewhat less frequently 

 Sc. fæcium and Sc. glycerinaceus. Even though these streptococci only exceptionally 

 survive heating to 75°, they have yet comparatively many cells which can stand 65°, 

 and in practice, it is of far greater importance how the majority of the 

 cells behave than what the most resistant individuals can stand, as these 

 few cells will in any case be unable to make their influence felt in the natural competition. 



') And even here we are not dealing with spore-formers, where the question is far more compli- 

 cated. 



''\ In true low-pasteurised milk (heated only to 63°) Mem. lacticum is, however, rare, at it is here 

 unahle to compete with the heat-resistiug cocci. In milk heated to a somewhat higher temperature, on 

 the other hand, we may find it almost as a pure culture. 



