Bacteria are mostly simple in form and exhibit one of three
basic structures: bacillus (plural, bacilli) straight and rodshaped,
coccus (plural, cocci) spherical-shaped, and spirillus
(plural, spirilla) long and helical-shaped, also called
spirochetes. Spirilla bacteria generally do not form associations
with other cells and swim singly through their environments.
They have a complex structure within their cell
membranes that allow them to spin their corkscrew-shaped
bodies which propels them along. Some rod-shaped and
spherical bacteria form colonies, adhering end-to-end after
they have divided, forming chains (see figure 34.2). Some
bacterial colonies change into stalked structures, grow
long, branched filaments, or form erect structures that release
spores, single-celled bodies that grow into new bacterial
individuals. Some filamentous bacteria are capable of
gliding motion, often combined with rotation around a
longitudinal axis. Biologists have not yet determined the
mechanism by which they move.