Description of Species, and Systematisnn. 



In discussing the separate lactic acid bacteria, iL would be most natural to commence, 

 as 1 have done in my '-Dairy Bacteriology", with the rod forms, which arc as a rule the 

 strongest acid formers, proceeding then by way of the streptococci, which are all typical 

 lactic acid bacteria, to the micrococci, where we find all possible transition forms between 

 acid-forming and non-acid-forming liacteria. For practical reasons, however, I prefer in 

 the present work to commence will) tlie streptococci, as it seems easier here, in most cases, 

 to define the separate species than is the case with the two other groups. 



Streptococci. 



By streptococci we understand, as is generally known, spherical bacteria dividing 

 as a rule in one direction only. Distinction is made between the proper streptoccocus type, 

 and the diplococcus type, according as the cells after division are inclined to remain 

 hanging together in long chains, or to fall apart rapidly. As a rule, the streptococci stretch 

 before division, so that the cells are then oval. After division has taken place, the daughter 

 cells are often egg-shaped, the pointed ends turning outwards. Where the growth is lively, 

 several of the long-chained forms will not have Lime to stretch before division, but form 

 disc-like segments. Again, the double hemisphere form, with flat surface at the break, so 

 typical among the micrococci, is one which we have encountered in some species (Slrep- 

 iococcus fæcium, Streptococcus liquefaciens and Betncoccus bovis). In broth, the long-chained 

 strains form flakes which easily settle, so that the liquor above them rapidly clears, 

 whereas the short-chained strains remain suspended for a long time, so that the liquid 

 takes longer to clear. 



These morhological difl"erences have proved constant for all our strains of streptococci 

 throughout the years during which we have had them under observation, and we cannot 

 therefore refrain from considering them of some value as species characters, though 

 it must be admitted that most strains in a weakened state form shorter and in particular 

 far thinner chains than they did when at their full vitality. Here as with all other qualities 

 in bacteria, it will be necessary to take into consideration the effect of temperature and of 

 the nutritive substrate. Broth, for instance, increases the tendency to chain formation, 

 whereas milk produces a reverse efltect, and we have therefore only reckoned strains 

 which also in milk grow in long chains as typical chain forms. In agar streak, and on 

 gelatin plates, both types can produce rod forms, or other divergent forms. As regards 



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