4. Bacterial Evolution with Respect to Flagellation 



The available evidence indicates that bacterial evolution is from 

 polar monotrichous organisms to peritrichous organisms and, 

 finally, to atrichous organisms. Polar flagellation is most efficient 

 for locomotion through a liquid medium and such bacteria are 

 best adapted to an aquatic habitation. All strictly autotrophic 

 bacteria, and most water types, both fresh water and marine, have 

 shown only polar flagellation, if any. Current ideas on the evolu- 

 tion of the earth are that in the early stages the surface was com- 

 pletely covered with water. In this environment the polar flagel- 

 lated bacteria were evolved. Peritrichous flagellated bacteria are 

 more efficient in locomotion through a medium denser than water 

 and, as more and more land appeared, the peritrichous types 

 evolved in the soil. With the appearance of animals and plants 

 some bacteria became parasitic with an environment in which 

 flagella served no useful purpose and were only a hindrance in that 

 • they required food and energy to be produced and to function. 

 Under these conditions the bacteria evolved into atrichous types. 

 Soil, and even water, rich in bacterial food could also render flagella 

 superfluous. 



Flagellar variations and mutations which have been observed 

 in the laboratory invariably have been changes from polar flagel- 

 lation to peritrichous flagellation, never the reverse, and from peri- 

 trichous to atrichous, rarely the reverse. An unequivocal instance 

 is a polar flagellated strain of Lophomonas which spontaneously 

 mutated to a peritrichous type. Strains of Aeromonas produce 

 peritrichous cells in young cultures, and indirect evidence indicates 

 that one Aeromonas culture produced a stable peritrichous mu- 

 tant. There are some indications that the subpolar flagellated 

 Rhizobium species of soy bean, lima bean, lupine, etc. are evolving 

 into peritrichous types. The Rhizobium species from pea, garden 

 bean, alfalfa, clover, etc. have peritrichous flagella, indicating per- 

 haps a longer period of association with plants and rich soil. 



The evidence for the peritrichous to the atrichous type of 

 lution is very suggestive. The mutation of laboratory cultures f 

 peritrichous to atrichous is common experience. A majority of tl 

 bacteria strictly parasitic and pathogenic for animals are atrichous. 



18 



evo- 

 rom 

 le 



