i j ESSENTIALS OF MILK BACTERIOLOGY 2^ 



does at times exist surrounding the bacterial cell outside of the cell 

 wall, as the white of an egg surrounds the yolk.. Whether this is. 

 a secretion of the cell or a mere swelling up of the cell-wall, or 

 sometimes one, sometimes the other, is not known. 



Classification. — The bacteria have for convenience been divided 

 according to shape into families : the families have been divided, 

 partly according to shape, partly according to structure or methods, 

 of fission, into genera ; the genera have been divided, partly accord- 

 ing to shape and structure but chiefly according to methods- and 

 conditions of growth, into species. (See p. 281 for families and 

 genera. Chester's Manual of Determinative Bacteriology is the 

 most useful of the works in English for the recognition of species.) 

 But besides this formal classification, now becoming slowly ac- 

 cepted, a rougher classification has been in use for many years, and 

 is still largely employed. This informal classification furnishes, 

 many of the names in common use and follows here. 



LOWER BACTERIA. 



Coccus (from Greek, kokkus, a berry) — spheroidal bacteria (plural,, 

 cocci). 



Micrococcus — a synonym of. coccus. 



Diplococcus — a coccus, the individuals of which are generally at- 

 tached together in pairs. 



Staphylococcus — a coccus arranged chiefly in clusters. 



Streptococcus — a coccus arranged chiefly in chains. 



Tetracoccus — a coccus arranged chiefly in flat groups of four or 

 more. 



Sarcina — a coccus arranged chiefly in cubical packets of four or 

 more to each side. 



Bacillus — cylindroidal bacteria, straight or curved, but not spiral! 

 (plural, bacilli). 

 — diplobacillus — a bacillus arranged chiefly in pairs. 

 — streptobacillus — a bacillus arranged chiefly in chains. 



