Just a quick comment about gliding motility: the last time I checked, the
mechanism was still unknown. However, latex beads on the surface of gliding
bacteria do move over the surface and may spin. Perhaps there are rotating
disks embedded in the outer membrane. If so, then gliding motility may be
a variant of flagellar motility without the flagella, just the motor and disks.
In contrast, the chemotactic sensory apparatus of the gliding myxobacterium,
Myxococcus xanthus, is highly homologous to that found in E. coli. So despite
very dissimilar modes of motility, both organisms have very similar chemotactic
systems - all the way to the "run" and "tumble" mutant behavior. By rRNA
sequence analysis, both are in the Proteobacteria (different groups though).
My point is that one must be very careful in selecting the data that will be
used for classification of organisms. Does the possibility that gliding
motility is very different from flagellar motility mean that rRNA analysis is
invalid? Does the homology between M. xanthus and E. coli mean that rRNA
based classification is correct? Of course, we know much more about M.xanthus
than just rRNA and chemotaxis - enough to legitimately place it with the Proteo
bacteria. Still don't be too quick to construct trees from just a few
characteristics.
David R. Nelson
Department of Biochemistry, Microbiology, and Molecular Genetics
University of Rhode Island
