Microbial Structures and Staining Reactions 89 



of 0.5 fA, to l.O iJL in diameter, while most spirals are larger than 

 either the rods or the cocci. Some spirals have been reported to be 

 as long as 500 fx, but the general length is under 16 /x. The syphilis 

 germ, Treponema pallidum, measures 0.3 /x in width and from 6 to 

 14 fx in length. The spirochete associated with trench mouth, 

 BorreJia vincentii, is very similar in size to the syphilis organism, 

 but the nature of the spiral twists in the two organisms readily 

 differentiates them. 



SHAPE 



Although variations in morphology (size, shape, and structure) 

 of bacteria may be induced by altering the environment in which the 

 organisms are growing, in general only three principal shapes of 

 bacteria exist: spheres, called cocci, (singular coccus); rods, called 

 bacilli ( singular bacillus ) ; and twisted rods, called spirilla ( singular 

 spirillum). 



It might be well to briefly mention the Latin endings used to 

 designate singular or plural nouns in scientific terminology. Many 

 Latin nouns ending in -us form the plural by converting the word 

 ending to -i (as bacillus to bacilli). Many words ending in -a 

 become plural by changing to -ae (such as sarcina to sarcinae). 

 Other nouns ending in -um form the plural by changing the ending 

 to -a. Occasionally persons prefer the word mediums for the more 

 correct plural media, and these liberties with Latin may be adopted 

 in time and become part of the accepted terminology. 



ARRANGEMENT 



After binary fission has taken place, many bacteria separate from 

 their twins and go about their carefree way, but with some species 

 the tendency to remain attached after division has occurred results 

 in an arrangement of the cells which is a characteristic aiding in 

 the identification of the species. To best determine this typical 

 arrangement, cultures should be cultivated in liquid media for from 

 18 to 24 hours prior to being examined in a so-called hanging drop 

 preparation under the high dry objective of the compound micro- 

 scope. Thus a living culture can be studied and typical arrange- 

 ment for the species can be determined. 



