MOVEMENTS - Cyanobacteria - page 283

and Harayama, 1983; Alexandre and Zhulin, 2001) suggesting that there are many ways to sense
chemicals in bacteria.
I. TYPES OF MOVEMENTS
Three types of motility are noted in cyanobacteria, i.e. (A) Gliding (B) Swimming and (C)
Twitching.
A) Gliding motility
i) Occurrence : Unicellular cyanobacteria are generally immotile with the exception of certain marine
Synechococcus strains (Castenholz, 1973; Brock, 1978) and Synechococcus sp. strain PCC 6910 (Waterbury
and Rippka, 1989) which exhibit gliding motility. Members of Oscillatoriaceae (species of Oscillatoria ,
Lyngbya , Phormidium and Spirulina ) and Nostocaceae ( Anabaena sp.) show gliding motion. Gliding
is a slow uniform and highly coordinated movement that involves revolution around the long axis
of the fi laments during which the fi laments do not show contractions or any sort of alterations in
morphology. However, Anabaena sp. shows lateral and bending movements. The speed at which
these organisms travel has been noted to be 10 µm s -1 . Another characteristic feature of gliding is
that the trichomes leave behind tracts of mucilage as they move ahead (Hoiczyk, 2000).
ii) Structures associated with gliding
a) S-layer and Fibrils : Haften and Castenholz (1971) showed the existence of a “locomotory machinery”
in Oscillatoria princeps . This is assembled in parallel fi brils with diameters of 5 to 8 nm which are
aligned in a helical array just beneath the outer membrane. The arrangement of the fi brils and
helical path of the fi laments are together taken as an indication that these are associated with
motility (Halfen, 1973). Studies on the envelope structure of four gliding fi lamentous cyanobacteria
by freeze substitution, freeze fracturing and negative staining revealed a complex external surface
layer consisting of two structural elements, i.e. a tetragonal crystalline S-layer attached to the outer
membrane and a group of parallel, helically arranged surface fi brils (of 8 to 12 nm in diameter;
with serrated appearance in cross section) spaced at 14 nm with each other on the top of the S-layer
(Hoiczyk and Baumeister, 1995). Of the four cyanobacteria studied, the revolution of fi brils is in a
clockwise direction in the two Phormidium sp. while they are in a counter-clock direction in O. princeps
and Lyngbya aeruginosa (Hoiczyk and Baumeister, 1995). The serrated fi brils appear to correspond
to the fi brils described earlier except for their location (Halfen, 1973, 1979). In contrast, Adams et al .
(1999) reported the existence of a complex array of fi brils located between the peptidoglycan layer
and the outer membrane of several Oscillatoria spp. The arrangement of each fi bril in relation to the
outer membrane and the peptidoglycan layer is such that the outer membrane makes contact with
the peptidoglycan layer between each fi bril (Adams et al ., 1999). Both the S-layer and fi brillar layer
present above the outer membrane reported in the studies of Hoiczyk and Baumeister (1995) were
not observed by Adams et al . (1999) in the Oscillatoria strains examined by them.
Considerable differences exist in literature on the diameter of the fi brils observed by various
workers. The fi brils located on the surface of peptidoglycan layer reported by Halfen and Castenholz
(1971) measured 5 to 8 nm in diameter (Halfen, 1973, 1979) whereas those observed by Adams et al .
(1999) were 25 to 30 nm in diameter. However, the observations of Hoiczyk and Baumeister (1995)
closely corresponded with those of Halfen and Castenholz (1971) who reported a diameter of 8 to