36 Structure and Classification of Micro-organisms 



Bacillus megatherium has a distinct but limited ameboid 

 movement. 



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 really 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 tenden- 

 cies upon 5 per cent, gelatin. The active movement of 

 the bacteria composing the colony causes its shape con- 

 stantly to change, so that it bears a faint resemblance to an 

 ameba, and moves about from place to place upon the sur- 

 face of the gelatin. 



Size. Bacteria are so minute that a special unit has 

 been adopted for their measurement. This is the micro- 

 millimeter (//), or one-thousandth part of a millimeter, 

 equivalent to the one-twenty-five-thousandth (-jTriinr) of an 

 inch. 



The size of bacteria varies from a fraction of a micro- 

 millimeter to 20 or even 40 micromillimeters. 



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 nutri- 

 tion are good, fission progresses with astonishing rapidity. 

 Buchner and others have determined the length of a gener- 

 ation 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 be reck- 

 oned in numbers." " Fortunately for us," says Woodhead, 

 " they can seldom get food enough to carry on this appalling 

 rate of development, and a great number die both for want 

 of food and because of the presence of other conditions 

 unfavorable to their existence." 



Sporulation. When the conditions for rapid multiplica- 

 tion by fission are no longer good, many of the organisms 

 guard against extinction by the formation of spores (Fig. i). 



