Il8 MANUAL OF BACTERIOLOGY. 



permanent. For instance, it would be incorrect to imagine 

 that it is possible for the micrococci and spirilla to become 

 converted into species of bacilli, or for the bacilli of one 

 species to be transmuted into those of another. This does 

 not contradict the statement that we may frequently, through 

 erroneous and imperfect information, be in the habit of 

 including unlike species under one name, or of classifying 

 mere varieties of one species as entirely different species. At 

 present the simple division of bacteria into three great 

 generic groups is probably as good as any : micro cocci, 

 spherical forms; bacilli, rod-shaped forms, one diameter 

 being in excess of the others; spirilla, twisted like a cork- 

 screw, making long spirals or simply parts of spirals 

 ( comma-shaped forms ) . 



Recent investigations indicate that several species of bac- 

 teria often are closely related to one another, so as to form 



FIG. 39. 



Staphylococci. Streptococci. Diplococci. Tetrads. Sarcinae. 



a well-marked group. Such a group is constituted by the 

 bacillus of typhoid fever, bacillus coli communis and similar 

 forms. The spirillum of cholera and other comma-shaped 

 spirilla resembling it may be held to constitute another 

 group. Still another is that containing the tubercle 

 bacillus and other acid-proof bacilli. 



The micrococci are subdivided into Staphylococci, where 

 the spheres grow in clusters like a bunch of grapes; strep- 

 tococci, where they are arranged in long rows or chains, 

 like a string of beads ; diplococci, or pairs of micrococci ; 

 tetrads, where the individual spheres are grouped in fours; 

 sarcincr, where they are grouped in eights, making the out- 

 line of a cube, resembling a bale or package tied with rope. 



