Il8 MANUAL OF BACTERIOLOGY. 



it must not be supposed that the species of bacteria are not 

 permanent. For instance, it would be incorrect to imagine 

 that the micrococci and spirilla become converted into species 

 of bacilli, or for the bacilli of one species to be transmuted 

 into those of another. This is not in conflict with the statement 

 that we may frequently, through erroneous and imperfect in- 

 formation, 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 it is sufficient for practical pur- 

 poses to divide bacteria into two great groups — the lower bacteria 

 and the higher bacteria; and to subdivide the lower bacteria 

 into: micrococci, spherical forms; bacilli, rod-shaped forms. 



vV, 



staphylococci. Streptococci. Diplococci. Tetrads. Sarcinffi. 

 Fig. 41. 



one diameter being in excess of the others; spirilla, twisted 

 like a corkscrew, making long spirals or simply parts of spirals 

 (comma-shaped forms).* 



Recent investigations indicate that several species of bacteria 

 often are closely related to one another, so as to form 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, smegma bacillus 

 and other acid-proof bacilli. 



The micrococci are subdivided into staphylococci, where 

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

 cocci, where they are arranged in long rows or chains, like a 



*Migula's system of classifying bacteria has found favor with many writers. 



