Download Spirochetes: Evolution, Genome Analyses and Physiology

J. Mol. Microbiol. Biotechnol. (2000) 2(4): 339.
JMMB Symposium
on
Introduction 339
Spirochete Physiology
Spirochetes: Evolution, Genome Analyses and
Physiology
Milton H. Saier, Jr.*
Department of Biology,
University of California at San Diego,
La Jolla, CA 92093-0116, USA
This short symposium is devoted to structural, molecular,
physiological and evolutionary aspects of spirochetes. The
spirochetes are a small cohesive group of
chemoheterotrophic bacteria, readily distinguishable from
other bacteria on the basis of their unique cell structures.
Spirochete cells are generally long and narrow, helical in
form, and surrounded by delicate flexible walls. All known
spirochetes can swim actively in liquid media by virtue of
their axial fibrils: periplasmic flagelli that underlie the cell
wall. The cells often bend or flex during movement, and
they can attain remarkable directed velocities and even
swim through highly viscous solutions (see review by Li,
Motaleb, Sal, Goldstein and Charon). The motile behavior
of these bacteria allows them to respond to chemical and
physical stimuli in their environments (see review by Lux,
Moter and Shi). These organisms comprise a relatively
close-knit group of organisms that, based on 16S RNA
analyses, diverged from other bacteria during early
evolutionary history. They exhibit unusual prokaryotic
characteristics such as linear chromosomes and a
cytoskeleton. They also are the targets of spirochetespecific bacteriophage which have co-evolved with their
hosts over evolutionary time. For this reason, the study of
phage can provide clues to the ecology and molecular
biology of their host organisms (see review by Eggers,
Casjens, Hayes, Garon, Damman, Oliver and Samuels).
Spirochetes are currently divided into nine genera (see
article by Paster and Dewhirst). While some of these genera
include free living organisms of bright colors and unusual
physiological properties, others are pathogens of humans
and animals, the causative agents of syphilis, Lyme
disease, relapsing fever, leptospirosis and peridontitis. The
pathogens responsible for these diseases exhibit
tremendous structural and physiological variability. For
example, although they are considered Gram-negative
bacteria with a double envelope membrane, many
spirochetes including Treponema and Borrelia lack outer
membrane lipopolysaccharides (LPS). Others including
Leptospira and Brachyspira do synthesize LPS, and LPS
is the principle surface antigen displayed by these
organisms, of considerable importance to diagnostics and
immunity. The review by Bulach, Kalambaheti, de la PeñaMoctezuma and Adler discusses these important antigens
from structural and biosynthetic standpoints. The
importance of vector-host interactions, particularly with
*For correspondence. Email [email protected];
Tel. (858) 534-4084; Fax. (858) 534-7108.
© 2000 Horizon Scientific Press
respect to tick-borne borreliae, is discussed by Nuttall,
Paesen, Lawrie and Wang. The pathogenic properties of
many spirochetes have provided much of the scientific
impetus for their study.
Genome analyses of two spirochetes, Treponema
pallidum and Borrelia burgdorferi , the bacterial
determinants of syphilis and Lyme disease, respectively,
have revealed a wide range of protein structures. Most
protein folds identified in these two organisms have been
shown to be common to many other organisms, but a few
spirochete-specific folds have been identified (see article
by Das, Hegyi and Gerstein). Additionally, the metabolic
capabilities of these two spirochetes are limited. For
example, they rely primarily on glycolysis for energy
generation. Transport systems in these two pathogens are
highly restricted, showing patterns of transporter types that
differ from those found in all other bacteria for which
complete genomes are currently available (see article by
Saier and Paulsen). Information obtained by analyzing the
genome of T. pallidum is analyzed in terms of the known
biological properties of this important pathogen. Genome
analyses of Borrelia species have revealed features
unusual to bacteria, often more frequently associated with
eukaryotes (see article by Casjens). Thus, housekeeping
genes are maintained primarily on the large chromosome
of B. burgdorferi while the numerous circular and linear
plasmids (or small chromosomes) of this organism bear
spirochete-specific genes that may prove to be important
for pathogenesis. They may be rapidly evolving as
compared to the chromosomal genes. These plasmids are
greatly enriched for repetitive sequences, the most obvious
of which are found in the family of cp32 plasmids (see article
by Stevenson, Zückert and Akins). Hypervariability within
these plasmids as well as the presence of multiple
paralogues undoubtedly has survival value for the bacteria
in their host organisms. These molecular features may
provide mechanisms for avoiding the animal’s immune
system.
This symposium thus summarizes important overall
aspects of spirochetes as a phylogenetic group of
organisms, but it also surveys a few key representative
pathogenic spirochetes in greater detail with respect to
specific traits. To what extent the specific findings reported
will prove to be relevant to other less well studied
spirochetes has yet to be determined. However, genome
sequences have for the first time provided detailed
information about the life processes of two of these
important and interesting microorganisms, and genome
sequencing projects for other spirochetes that will allow
comparative studies are currently in progress. It is an
exciting time in the discipline of spirochetology!
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