2. Shape and Arrangement of Flagella 



Flagellar Shape 



The most common shape of bacterial flagella is a fairly uniform 

 helix with a pitch characteristic of the species. Since the flagella 

 are very thin the helix is flattened when they dry on the slide and 

 they appear to be wavy. The distance from one wave crest to the 

 next, which is termed the wavelength, may be slightly different 

 from the pitch of the original helix, but measurements made on 

 stained flagella and on flagella in moist preparation (dark field) 

 have shown little difference. The amplitude of the waves is com- 

 parable to the diameter of the original helix. The exact relation- 

 ship of these two is still somewhat uncertain but the amplitude 

 seems definitely greater than the diameter of the helix. With most 

 bacteria the flagellar shape is quite uniform and constant, but 

 with a few bacteria the flagellar shape is quite irregular. 



Measurement of flagellar wavelength and amplitude is most 

 conveniently and accurately done by means of a filar micrometer. 

 A fixed scale micrometer is less accurate but gives good mean 

 values where many flagella are measured. Since some flagella are 

 bound to become damaged and distorted when they dry on the 

 slide, such flagella should not be measured. Figure 1 illustrates 

 the way in which wavelength and amplitude are defined. 



Flagellar Arrangement 



The old terminology describing the arrangement of the flagella 

 on the bacterial soma is rather inadequate. The following ter- 

 minology will be used throughout this Atlas: 



Polar, (a) Monotrichous: Predominantly a single flagellum at 

 one or both poles. The base of the flagellum usually parallel to the 

 long axis of the soma, (h) Multitrichous:^ Predominantly two or 



1 Since this classification is based primarily on flagellar arrangement the 

 term lophotrichous is not included. Where this term is used in the Atlas 

 it refers to polar multitrichous flagellation with flagella of relatively long 

 wavelength and typically having less than one complete wave. 



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