Lyme Disease

Borrelia burgdorferi belongs to the phylum Spirochaetes. The members of this phylum are long, thin, helically coiled bacteria that have flagella ( axial filaments ) running lengthwise between the peptidoglycan layer and the outer membrane. Movement of the flagellum produces a screw-like motion that propels the organism.

The phylum Spirochates contains a single class (Spirochaetes), a single order (Spirochaetales), and three families: Brachyspiraceae, Leptospiraceae, and Spirochaetaceae.

The Leptospiraceae family includes the genus Leptospira , to which belong the causative agents of the disease Leptospirosis, a disease that usually targets the kidneys, liver, and central nervous system.

The Spirochaetaceae family includes the genus Treponema and the genus Borrelia . Treponema pallidum is the causative agent of the sexually-transmitted disease syphilis. The three members of the Borrelia genus— Borrelia burgdorferi sensu stricto, Borrelia garinii , and Borrelia afzelii —are collectively known as Borrelia burgdorferi sensu lato, and are the causative agents of Lyme disease.

Borrelia cells average 0.2 to 0.5 µm by 4 to 18 µm, and have fewer coils than Leptospira . The periplasmic flagella originate from either end of the spirochete (where they are anchored to the cytoplasmic membrane) and wind around the protoplasmic cylinder, imparting both motility and shape to the organism—in contrast to other bacteria, in which the peptidoglycan layer determines the shape.

The role of flagella in imparting Borrelia 's helical shape was established by inactivation of the flaB gene, which encodes the major flagellar filament protein, FlaB. This produced bacteria that lacked periplasmic flagella, were non-motile and rod-shaped.

Whereas the motility of externally-flagellated bacteria is hindered in viscous substances, that of spirochetes is enhanced, and about 6% of the chromosomal genome encodes proteins involved in motility and chemotaxis.

The genome of Borrelia burgdorferi consists of a single linear chromosome and several plasmids, both linear and circular. To date—as of January 2005—only the genome of Borrelia burgdorferi sensu stricto B31 strain has been fully sequenced.

Distribution of cellular functions of E. coli and B. burgdorferi genes
Category	B. burgdorferi genes (%)
Intermediary metabolism	4.9%
Biosynthesis of small molecules	3.1%
Macromolecule metabolism	22.2%
Cell Structure	37.0%
Cellular processes	7.4%
Other functions	5.6%
Unknown functions	19.8%

[1]

B. burgdorferi contains a single linear chromosome of approximately 900 kb, and about 90% of it is comprised of coding sequences. Most of the genes encoded by the chromosomal genome are homologous to genes of known function.

The extra-chromosomal genome of B. burgdorferi B31 consists of 12 linear plasmids and nine circular plasmids that total 610 kb in size.

There are two linear plasmids in B. burgdorferi that are absolutely necessary for persistent infection of a mammalian host. These plasmids, known as lp25 and lp28-1, are relatively unstable in culture, and are commonly lost after a few generations of in vitro growth. Bacteria that have lost either of these two plasmids remain capable of in vitro growth, but lose their ability to cause persistent infection even in immunocompromised mice. The lp25 plasmid contains a gene, pncA , which encodes a nicotinamidase whose function is most likely the biosynthesis of NAD; by all appearances its activity is dispensable growth in vitro , but crucial for growth within a host. Transforming the lp25- spirochetes with pncA on a shuttle vector replaces the requirement of lp25 in vivo . Likewise, reintroduction of the entire lp25 plasmid (by transformation) into lp25- spirochetes successfully rescues infectivity. [2]

An unusual feature of B. burgdorferi is a series of related 32-kb circular plasmids, termed cp32s. These have been found to be prophage genomes, and it is believed that they play a role in the horizontal transfer of DNA among spirochetes that share a common geographical and ecological niche. [3, 4]

The nutritional requirements for Borrelia are significantly more complex than those of other spirochetes like Leptospira . These include, but are not limited to: glucose, amino acids, long-chain fatty acids, N-acetylglucosamine, and several vitamins. When cultivated in artificial media, Borrelia can exhibit as much as a twofold increase in generation time (when compared to growth in mice). [5]

The Outer Surface Proteins (Osps) of B. burgdorferi are lipoproteins that play an important role in interacting with interstitial and cellular components of insect and mammalian hosts. OspA, the most studied of the Osps, is expressed on spirochetes in unfed nymphs and adult ticks, as well as in culture. OspA mediates adherence to the cells of the tick midgut, which presumably allows spirochetes to avoid endocytosis by tick gut cells during digestion of the blood meal. The ability of Borrelia to regulate expression of OspA indicates that it also plays a role in detachment from the midgut, which allows the bacteria to enter the mammalian host when the tick takes a second bloodmeal.

During tick feeding, Borrelia in the midgut upregulate expression of another outer surface protein, OspC, and begin to move toward the salivary glands. This evident correlation suggests that OspC might play a role in transmission. Once it has entered the mammalian host, Borrelia downregulates OspA and exhibits variable OspC upregulation patterns. Although B. burgdorferi possesses only one copy of the ospC gene, sequences vary significantly from one strain to the next, which accounts for the observed antigenic variation between OspC proteins. The host immune system plays an important role in selecting for certain strains by eliminating the immunodominant ones.