Borrelia burgdorferi

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Borrelia burgdorferi
Scientific classification

B. burgdorferi is a motile, two-membrane, spiral-shaped bacteria which is found mostly in North America and Europe. It's named after American scientist Willy Burgdorfer, who in 1982 isolated and determined the bacteria to be the causative agent of Lyme disease. Lyme disease is the most common tick borne disease in the US. [1]

Genome structure

B. burgdorferi has a small complex genome containing linear DNA, accompanied by up to 20 circular as well as linear plasmids. A study in 1997 found the linear DNA to be composed of 910,725 base pairs (average G+C content of 28.6%), and at least 17 linear and circular [[plasmid]s with a combined size of more than 533,000 base pairs. The small genome size of B. burgdorferi is associated with an apparent absence of genes for the synthesis of amino acids, fatty acids, enzyme cofactors, and nucleotides. The lack of biosynthetic pathways also explains why growth of B. burgdorferi in vitro requires serum-supplemented mammalian tissue-culture medium.

The chromosomes contains 853 genes encoding a basic set of proteins for DNA replication, transcription, translation, solute transport and energy metabolism, but no genes for cellular biosynthetic reactions. Of 430 genes on 11 plasmids, most have no known biological function.The biological significance of the multiple plasmid-encoded genes is not clear, although they may be involved in antigenic variation or immune evasion.

DNA Molecule Summary

	Number	Percent
Total Number of all DNA molecules:	18	100.00%
Total Size of all DNA molecules:	1443725bp	100.00%
Total number of linear DNA bases:	910,725bp	63.1%
Number of chromosomal G+C bases:	256551bp	28.5%

Chromosomal Gene Summary

	Number	Percent
Total genes:	853	100.00%
Protein coding genes:	793	93%
Genes assigned a role category:	502	59%
Genes not assigned a role category:	51	6.26%
Hypothetical genes:	102	12%

Cell structure and metabolism

B. burgdorferi are long gram negative spirochetes (some researchers however claim that the organism does not exhibit properties of either gram positive or gram negative bacteria). Stained cells are visible under by dark-field or phase-contrast microscopy, but not visible by bright field microscopy. They are flexible helical cells and are much longer than they are wide. They usually have a length of 20-30um but a width of only 0.2-0.3um. The organism is motile with both rotational and translational movements, and the coiling of cells is regular. On the average, seven periplasmic flagella are located at each cell end, and these flagella overlap at the central region of the cell. There are multiple layers of membrane that surround the protoplasmic layer, cytoplasmic membrane, and the enclosed cytoplasmic contents.

The spirochetes have an optimal growth temperature between 34 to 37°C. The generation time of at optimal temperature is 11 - 12 hours. The organism is microaerophilic; they require oxygen to survive, however, at low concentrations only. Too much or lack of oxygen can be harmful to them. They also happen to be catalase negative; they are unable to break down hydrogen peroxide.

B. Burgdorferi are classified as being chemoorganotropic, and derive energy by metabolizing organic molecules. They cannot synthesize their own amino acids, fatty acids, enzyme cofactors, and nucleotides. It's possible to cultivate the organism in vitro, but it requires a very complex growth medium since it lacks the ability to synthesize various necessary compound (see genome structure). The medium used to grow Borrelia burgdorferi is called Barbour-Stoenner-Kelly (BSK) medium. It contains over thirteen ingredients in a rabbit serum base.

Ecology

The optimal growth temperature is 34 to 37"C, and the organism has a generation time of 11 to 12 h at 35°C. Their life cycle is directly linked to their host, and they depend on arthropod vectors (ticks) to infect mammals. They reside in infected mammals as well as in the gut of ticks. They multiply when there is enough blood ingested by the tick to support bacterial growth.

The vector tick, Ixodes scapularis, undergoes a two year life cycle beginning when the female drops from her animal host to lay eggs in the soil during the spring. Very small larvae emerge during the summer and attach to a passing small animal (such as mouse) to feed on its blood. If the small animal is infected with the spirochete, the larva become infected as they feed. the fed larvae drop back to the ground and molt into nymphs. the nymphs wait until next year's warm weather to search for another host to feed on before becoming adult ticks. They prefer the white-footed mouse, but will take any warm-blooded host that happens along. At this point, the infected nymphs can infect new animal hosts (including human). The fed nymph falls back to earth to molt, and emerge as an adult tick. The adult has one more blood feeding before it mates by late spring, and infected nymphs carry the infection into adulthood. Male ticks die following mating, while female ticks die after egg laying, and thus, completing the life cycle (Daniels and Falco 1989; Kantor 1974).

Pathology

Borrelia Burgdorferi is the is the causative agent of Lyme disease, and is transmitted to mammalian hosts by tick vectors. The bite of an infected nymph or adult tick can only transmit Lyme disease after it attaches to the host for thirty six to forty eight hours. This is due to the initial lag time, and the generation time of B. Burgdorferi, which happens to be 12 hours. Spirochetes waiting in the mid-gut of the tick only begin to multiply as the tick starts its blood meal. Then they migrate to the salivary glands where they are discharged into the host with the tick's saliva. It takes several hours before a large enough infectious dose of spirochetes can penetrate the new host.

References