6 AIDS TO BACTERIOLOGY 



(2) Crenothrix. Threads thicker at apex than at base. 

 Each thread is tubular, and inside each a linear series of 

 cells is arranged, each cell possessing a membrane of its 

 own. The cells are thrown out at the top, and elongate 

 to form new threads like those of the parent plant. 



(3) Leptothrix. Threads with or without sheaths, 

 showing no branching. 



(4) Cladothrix. The threads possess pseudo-branches. 

 Strings of rod-like cells are enclosed in a sheath. 



V. BEaaiATOACE^. Thread forms containing sulphur 

 granules. 



(1) Beggiatoa. Long motile free-swimming threads of 

 colourless cells containing strongly refracting granules of 

 sulphur. 



(2) Thiothrix. Threads that differ from beggiatoa by 

 having the filaments attached at one end. 



Organisms other than the true bacteria are dealt with 

 later, and descriptions of them are left to the special mono- 

 graphs and chapters. 



Growth of the Bacteria. Few bacteria can derive their 

 nourishment from inorganic sources, and an absence of 

 chlorophyll prevents all species from utilising atmospheric 

 carbon dioxide. The large majority require complex 

 organic compounds such as proteins and carbohydrates as 

 well as mineral salts for food. Suitable mixtures are pre- 

 pared for laboratory purposes (culture media v. Chap- 

 ter III.). While some simple media allow growth of most 

 species, by the introduction of other constituents certain 

 bacteria are favoured and nourish, perhaps with specific 

 indications of their presence, while other species are killed 

 or suppressed (selective media). By studies of the pref- 

 erences and conditions of vigorous growth in this and other 

 respects, the extraction of a required organism from ad- 

 mixture with others is facilitated. 



Bacteria derive their oxygen either from the air (aerobes) 

 or from compounds containing oxygen (anaerobes). The 

 ' facultative anaerobes ' grow either in the presence or 

 absence of oxygen. There are gradations in this respect, 

 from the strictly aerobic species, which require abundance 

 of oxygen, and will not grow in its absence, to the anaerobic, 

 which grow in the absence of free oxygen. Strict anaerobes 

 do not exist. Organisms that can live without oxvgen thrive 

 better when oxygen is present, but in very small quantity. 



