Biology Reference
In-Depth Information
C
HAPTER
5
C
HAPTER
MOVEMENTS
I. TYPES OF MOVEMENTS
283
A) Gliding motility
B) Swimming
C) Twitching movements
II. PHOTOTAXIS
293
A) Action spectra and nature of photoreceptors
B) Pix-genes and mechanism of phototaxis
Motility confers on the microorganisms the ability to survive in their natural habitats. In order to
cope up with environmental conditions of excess or defi ciency of light/nutrients (or chemicals) the
microorganisms exhibit necessary adjusting movements. The fi rst organ of motility that attracted
the attention of bacterial motility is fl agella. Ever since the discovery of bacteria it has served as
an important taxonomic trait to distinguish bacterial species. Motility due to fl agella is present
in both eubacteria and archaebacteria. The bacterial fl agella are composed of three parts, a basal
body, the hook and the fi lament. The basal body acts as the motor, the hook joins the fi lament to
the basal body and the fi lament acts as the propeller. The fi lament is composed of many thousands
of molecules of a single protein known as fl agellin and assumes a diameter of 20 nm. There are
certain subtle differences between the organization of the proteins of the motor of eubacteria and
archaebacteria. Moreover, the polar and lateral fl agella differ in their structure while the former is
sheathed and thicker the latter is unsheathed and thinner. In spirochetes the fl agella are unusually
located internally in the periplasmic space. Type IV pili (Tfp), junctional pore complex (JPC), ratchet
structure and contractile cytoskeleton are the other motility structures present in eubacteria (Bardy
et al
., 2003). The movements associated with Tfp are known as twitching movements.
Pseudomonas
aeruginosa
and
Neisseria gonorrhoeae
are the classical examples that exhibit twitching movements.
The myxobacterium
Myxococcus xanthus
exhibits two types of gliding motion, i.e. social gliding
