14 APPLIED BACTERIOLOGY 



Spirilla are subdivided into ' comma ' bacilli or vibrios, 

 spirilla in the more restricted sense, and spirochsetse. The 

 vibrios usually form strings of cells which strongly resemble 

 spirilla; the spirochsetse are distinguished for their flexi- 

 bility. 



• Leptotricheae. — These form rodlets and longish threads, 

 ■which show a distinction between the base and apex of the 

 filaments, growing out from a thinner base to a broader apex. 



The three most important classes are : (1) Beggiatoa ; 

 (2) Crenothrix; (3) Leptothrix. 



(1) Beggiatoa. — These form long motile threads, consist- 

 ing of colourless cells, and are distinguished by the presence 

 of strongly refracting granules of sulphur. They occur in 

 sulphur springs and in dirty water. 



(2) Crenothrix. — These form simple threads, the separate 

 cells of which surround themselves with a distinct sheath, 

 and then change themselves by segmentation at their ends 

 into roundish spores. The threads are motionless, and, 

 especially in their younger stages, group themselves into 

 little patches. 



(3) Leptothrix. — Threads with or without sheaths. 

 Division not very numerous or well marked. The cells are 

 devoid of sulphur. 



Cladotricheae. — Forms consisting of threads which pos- 

 sess pseudo-branches ; the separate cells are provided with 

 sheaths. Spore formation not yet demonstrated. This 

 class has only one division, namely Cladothrix. They are 

 found in dirty water. 



Conditions of Growtli of the Bacteria. — The bacteria, like 

 the higher organisms, cannot live and multiply unless they 

 have proper nourishment and conditions of growth. As 

 the bacteria do not contain chlorophyll, they are not able to 

 avail themselves of the carbon existing in the air as carbon 

 dioxide (carbonic acid gas), but are dependent for their 



