3. Variation and Mutation of Flagellar Shape, 

 Arrangement, and Function 



Variation of Shape 



Flagella may show several shape variations which are encoun- 

 tered with variable frequency in a variety of genera. The shape 

 most usual for the flagella of a genus is designated as normal. This 

 shape is usually that of a helix with a wavelength-ampHtude ratio 

 ranging from 4:1 to 3:1. 



The most commonly encountered shape variant is the curly. 

 This shape variant has been observed in most genera of peritri- 

 chous flagellated bacteria but infrequently in polar flagellated 

 bacteria. In the enteric and related groups of bacteria, the curly 

 flagella have a wavelength close to i/^ that of the normal. The 

 ratio of wavelength to amplitude approximates 3:1 while in the 

 normal for these bacteria this ratio approximates 4:1. In other 

 groups of bacteria such as Bacillus and Clostridium the wavelength 

 of the curly flagella is about ^ that of the normal flagella. In the 

 subpolar types of Rhizobium the curly flagella have a wavelength 

 about y^ that of the normal. In one strain of Sarcina urea three 

 wavelengths were observed having a relative ratio of 3:2:1. The 

 curly flagella appear shorter and stiffer than the normal. From a 

 careful study of the two types of flagella in Proteus the actual 

 lengths of the normal and the curly flagella when stretched out 

 straight appeared to be about the same. In some strains of some 

 genera of bacteria the change from normal to curly and vice versa 

 may be induced by a change of the pH of the suspending medium. 

 Strains of Proteus, Azotobacter, Erwinia, and Bacillus have shown 

 this phenomenon. At pH 6 and below, the flagella are curly while 

 at pH 7 and above they are normal. In these genera, as well as in 

 others which do not show this pH sensitivity, both normal and 

 curly flagella may appear on the same soma. In some genera or 

 species the curly flagella appear to be stable genetic mutants. Curly 

 flagella are less efficient locomotor organs than normal flagella. A 

 pure curly strain of Salmonella Wichita, for example, showed prac- 



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