16 BIOLOGY AND TECHNIQUE 



tends to injure or depress physiologically the bacteria in any way, at 

 the same time tends to inhibit their motility. 



Messea 18 has proposed a classification of bacteria which is based 

 upon the arrangement of their organs of motility, as follows : 



I. Gymnobacteria, possessing no flagella. 

 II. Trichobacteria, with flagella. 



1. Monotricha, having a single flagellum at one pole. 



2. Lophotricha, having a tuft of flagella at one pole. 



3. Amphitricha, with flagella at both poles. 



4. Peritricha, with flagella completely surrounding the bac- 



terial body. 



Bacterial Spores. A large number of bacteria possesses the power 

 of developing into a sort of encysted or resting stage by a process 

 commonly spoken of as sporulation or spore formation. The forma- 

 tion of spores by bacteria depends largely upon environmental con- 

 ditions, and the optimum environment for spore formation differs 

 greatly for various species. It is usually necessary that a tempera- 

 ture of over 20 C. exist in order that spores may be formed. Un- 

 favorable factors, like acid formation, accumulation of bacterial 

 products in old cultures, or lack of nutrition, frequently seem to con- 

 stitute the stimuli which lead to sporulation. In the case of some 

 species, notably the anthrax bacillus, spores are formed only in the 

 presence of free oxygen and are therefore never formed within the 

 tissues of infected animals. It is claimed that some of the pathogenic 

 anaerobes, like B. tetani and the bacillus of malignant edema, may 

 form spores anaerobically. Nevertheless it has been observed that 

 when an absolute exclusion of oxygen is practiced in the cultivation 

 of these bacteria, vegetative forms only are seen in the cultures. 17 



The process of sporulation is by no means to be regarded as a 

 method of multiplication, since it rarely occurs that a single bacillus 

 produces more than one spore. In some species of bacteria the for- 

 mation of several spores in one individual has occasionally been ob- 

 served, but there can be no question about the fact that such a 

 condition is exceptional. 



Varieties of spores are often recognized, the so-called arthro- 

 spores and the true spores or endospores. It is seriously in doubt 

 whether the structures once spoken of as arthrospores should be 



16 Messea, Cent. f. Bakt,, I, Eef. ix, 1891. 



17 Zinsser, Jour. Exp. Med., viii, 1906, p. 542 



