Bacterial locomotion

Locomotion or motility is important characteristic of bacteria. Bacterial locomotion is of three types: Flagellar, Spirochaetal and Gliding movement. The word motility, movement and locomotion are used synonymously.

Flagellar motility:
This type of motility is caused by flagella, cell surface appendages. Flagellum has typical structure; it is embedded in cell wall by S ring or stator (hook) and basal body or motor. M ring is attached to the flagellum and acts like a rotor (shaft). P and L are also present and work like bearings or bushers. Basal body is powered by proton energy, which is movement of ions between M and S rings. Transformation of proton energy into work operates flagella in clockwise and counterclockwise directions. Depending upon location of flagella, bacteria can swim smoothly, reverse the movement backward or forward or tumble. Peritrichously flagellated bacteria bear flagella all over the surface move by tumbling or anticlockwise swimming. Polar flagella (mono, bi or multipolar) are present at the ends of cell and bacteria move in one direction and as well as in reversal. Flagellar motility is present in Pseudomonas, Vibrio, Spirillum, Azospirillum, Klebsiella, Salmonella, Proteus and etc.

Spirochaetal movement:
Spirochaetal movement is seen in all genera of bacterial group (V), 'The Spirochetes' of Bergey's Manual of Determinative Bacteriology. Important genera include, Spirochaeta, Cristispira, Treponema, Borrelia and Leptospira. Spirochetes are helical bacteria. They have flagella like axial filament buried in space between inner and outer membranes of cell wall. Axial filament is composed of 2 or more fibrils which are embedded in inner membrane and acts like basal body or motor. Spirochetes can perform flexing, swimming, creeping or spinning types of movements. Imagination of motion of flexible helical rod in air will give you an exact idea about spirochetal movement.

Gliding movement:
Like spirochetes, gliding motility is represented by special bacteria, 'The Gliding Bacteria' group (II) of Bergey's Manual of Determinative Bacteriology. Bacteria move by gliding on the surface! They do not have flagellar structures either internally or externally but they secrete slimy substance like snails during locomotion. The exact mechanism of gliding locomotion is still unknown but some scientists have suggested presence of fimbriae like appendages at the poles of glider cell. The generation of contractile waves or surface tension or pushing by secreted slime was also proposed as possible mechanisms of gliding. Principle glider genera are Myxococcus, Archangium, Cystobacter, Melittangium, Stigmatella, Polyangium, Nannocystis, Chondromyces, Cytophaga, Flexithrix, Herpetosiphon, Beggiatoa, Saprospira, Thioplaca, Leucothrix, Alysiella, Achroonema and cyanobacterium Oscillatoria.

Laboratory detection and assay:
Motility can be directly observed under light microscope by hanging drop in cavity slide or wet mount preparation. It is important to determine true and false motility microscopically. Truly motile bacteria will show propelling action towards definite direction, as if they are pushing themselves with efforts! Nonmotile bacteria also appear to be motile because of bombardment of liquid medium particles or air currents. Motility of nonmotile bacteria is zigzag and directionless. This movement of nonmotile bacteria is actually a Brownian movement; even dead bacteria seem to be moving because of this movement. Craigie's tube or capillary tube can be used by placing them in broth culture for observation of directed movement of bacteria towards chemical or physical gradients with time. All motile bacteria show movement towards chemical or physical gradients. This phenomenon is known as tactic response. Chemical or physical gradient can be attractant or repellent and accordingly, tactic response would be positive or negative. Presence of gradients is sensed by special receptors of bacteria. Thus swimming towards certain glucose concentration present in the medium would be positive chemotactic or chemotaxis. Similarly, motile bacteria exhibit phytotaxis (light intensity) and magnetotaxis (magnetic particles) responses. Motile bacteria are assayed on semisolid agar or broth medium for chemotaxis and are very important in species identification and classification.

Importance of bacterial locomotion:

Chemotactic behavior and survival:
Motility confers bacteria an ability to change direction. This is important when bacteria require moving away or towards repellents or attractants respectively. It avoids unfavorable conditions of habitat and offers protection. It is thus important in the survival and offers to choose favorable environment containing positive stimuli, light, gravity or chemicals for bacteria.

Root colonization:
Root colonization is perquisite for establishment of bacteria in the rhizosphere region. Motile bacteria are effective root colonizers and can swim towards root exudates or other nutrient gradients earlier than nonmotile bacteria. Pseudomonads and Azospirilla are very efficient in attachment and subsequent root colonization of their host plants.
Pathogenesis: Most human pathogenic bacteria (Campylobacter, Salmonella and Vibrio) and saprophytes or opportunists (Escherichia) are motile and motility is important for attachment and colonization of cell wall of intestine and other vital organs.

Motile versus nonmotile:
Some bacteria like Acinetobacter spp. show twitching or jumping type of motility even though flagella are absent in them. These bacteria show the twitching particularly on semisolid media and also present chemotactic response. Twitching motility is thought to be because of piliated cell surface. It is the favorite topic of interest and research that why some bacteria are nonmotile? It has been found that in some bacterial genera that nonmotile species are equally efficient like their motile species. These nonmotile bacteria also possessed flagellar appendages; but basal body or motor function was found to be impaired or paralyzed. Reason for their efficiency even in absence of motility however could not be explained.

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