32 Structure and Classification of Micro-organisms 



Messea* has suggested that the bacteria be classified, according to 

 the arrangement of the flagella, into: 



I. Gymnobacteria (forms without flagella). 

 II. Trichobacteria (forms with flagellsi). 



1. Monotricha (with a single flagellum at one end). 



2. Lophotricha (with a bundle of flagella at one end). 



3. Amphitricha (with a flagellum at each end). 



4. Peritricha (flagella around the body, springing from all parts of its 



surface). 



This arrangement is, however, less satisfactory than that of 

 Migula already given. 



Motility.— The greater number of the bacteria supplied with 

 flagella are actively motile, the locomotory power no doubt being 

 the lashing flagella. The rod and spiral micro-organisms are most 

 plentifully supphed with flagella; only a few of the spheric forms have 

 them. 



The presence of flagella, however, does not invariably imply 

 motihty, as they may also serve to stimulate the passage of currents 

 of nutrient fluid past the organism, and so favor its nutrition. The 

 flagellate bacteria are more numerous among the saprophytic than 

 the pathogenic forms. 



BaciUus megatherium has a distinct but Umited ameboid move- 

 ment. 



The dancing movement of some of the spheric bacteria seems to be the well- 

 known Brownian movement, which is a physical phenomenon. It is sometimes 

 difficult to determine .whether an organism viewed under the microscope is realty 

 motile or whether it is only vibrating. One can usually determine by observing 

 that in the latter case it does not change its relative position to surrounding 

 objects. 



In some cases the colonies of actively motile bacteria such as the 

 proteus bacilli, show definite migratory tendencies upon 5 per cent, 

 gelatin. The active movement of the bacteria composing the 

 colony causes its shape constantly to change, so that it bears a 

 faint resemblance to an ameba, and moves about from place to 

 place upon the surface of the gelatin. 



Reproduction. — Fission.- — Bacteria multiply by binary division 

 (fission). A bacterium about to divide appears larger than normal, 

 and, if a spheric organism, more or less ovoid. By appropriate 

 staining karyokinetic changes may be observed in the nuclei. 

 When the conditions of nutrition are good, fission progresses with 

 astonishing rapidity. Buchner and others have determined the 

 length of a generation to be from fifteen to forty minutes. 



The results of binary division, if rapidly repeated, are almost 

 appalling. " Cohn calculated that a single germ could produce by 

 simple fission two of its kind in an hour; in the second hour these 

 would be multiplied to four, and in three days they would, if their 

 surroundings were ideally favorable, form a mass which can scarcely 



* "Rivista d'igiene e sanata publica," 1890, 11. 



