Genus: Streptobacterium (Abbr. Sbm.). 



These rod forms we have called streptobacteria, from their tendency to chain for- 

 mation. As they grow, even at their optimal temperature, more slowly than the lactic 

 acid cocci, they do not make themselves apparent in- spontaneously souring milk until 

 after the milk has curdled, but they will then, on the other hand, gradually supersede 

 the lactic acid cocci, being able to stand, and capable of forming, up to twice as much 

 acid as the latter. They therefore abound in butter and cheese after more or less time 

 has elapsed. They are likewise always found where vegetable matter is left to sour. There 

 are probably a great number of species, which are difficult to distingusih one from another, 

 owing to the many gradual transitions between. I will here content myself with making 

 distinction between the two extreme groups, the typical cheese bacterium, Streptobac- 

 terium casei, represented by the Bacterium casei a which I have formerly described, and 

 the typical vegetable bacterium, Streptobacterium plantarum. The former we have never 

 met with in vegetable matter, but the latter is of frequent occurrence in milk and dairy 

 products; it cannot escape being introduced into the same from the food and bedding 

 (grass or straw) of the cows. 



The streptobacteria are always killed by heating to 75°, many strains already at 

 70°, and some few even at 65°. The optimal temperature is 30°, and the maximal as a 

 rule from 37i/2° — 40°. Only a few strains can grow more or less well at 45° (Sbm. 

 casei Nos. 4, 26, 31, 32, 33 and 34, and Sbm. plantarum Nos. 28, 29, 30, 32 and 34). The 

 minimal temperature lies probably in most cases at about 10°, but the growth here is, even 

 with the most favourable source of nitrogen (Y), so slow that it will often be impossible 

 to discern anything at all until after 14 days. The growth on gelatin plates, also, at 

 ordinary temperature, is very slow, and the colonies are still considerably smaller than 

 those of the cocci. They grow better, of course, on SG than on AG. 



Streptobacterium casei (Table XXIX) forms either pure dextro-lactic acid or more 

 or less considerable quantities of lævo-lactic acid in addition, so that we also find inactive 

 lactic acid in the cultures. The power of forming dextro-lactic acid is, however, by far the 

 most constant, and many strains which at first formed almost exclusively inactive lactic 

 acid have yet in the course of years ended by forming pure dextro-lactic acid^). As men- 



') We have conse([uently been led to investigate considerably more strains than we otherwise 

 should have done, for of course we could not know that two bactei'ia, otherwise alike — even though 

 isolated from the same place — were identical, despite the fact of their forming different lactic acids. 

 These parallel investigations of identical strains have, however, further increased our knowledge of the 

 same, and it is interesting to see that they often exhibit exactly the same mutations at the same time. 

 They maj", however, also be found to differ suddenly in their relation to one or other of the sugars. 



