CHAPTEE III. 



§1. VARIOUS GEOWTH-FORMS OF BACTERIA. 



As described in Chapter II. multiplication usually takes place in 

 bacterial cells by division, followed by separation of the daughter- 

 cells, which in turn repeat the process of division and separation, until 

 a vast number of individuals has been produced. If, however, no 

 separation takes place after division, chains of cells are formed in the 

 case of the rod-bacteria, and in the case of the cocci-species we have 

 seen that chains (Streptococcus), plates (Micro- and Planococcus), 

 packets (Sarcina and Planosarcina), or irregular groups (Staphylo- 

 coccus) may result. These modes of combination are not invariably 

 fixed for any particular species, except in the case of the Streptococci 

 species, which produce chains, and chains only. On the other hand, a 

 Sarcina species can form packets, irregular groups, plates, and some- 

 times the culture may consist of groups of from one to four only in 

 each group. Growth-forms similar in nature to the above are absent 

 from the Spirillum species. 



An important growth-form is the pellicle or skin which forms on the 

 surface of the liquid in which bacteria are growing. This pellicle 

 consists of millions of bacteria held together by their membranes, the 

 whole forming a comparatively thick white, or grayish-white skin of 

 firm consistency on the surface. This formation takes place only in 

 some species of bacteria, e.g. Bac. subtilis, and Bac. fluorescens-lique- 

 faciens, and is an important aid in the diagnosis of those species which 

 form the pellicle. 



In some species this banding together of individuals takes place 

 under the surface of the nutrient liquid, and in irregular masses. This 

 is called a Zoogloea. In this condition the bacteria are better able to 

 resist unfavourable circumstances. This formation is of common 

 occurrence among other low forms of vegetable life. The zoogloea 



