Borrelia
Borrelia are small, obligately parasitic spirochetes characterized by their linear DNA and unique transmission methods, primarily through blood-feeding arthropods like ticks. These bacteria have a minimal genome and rely on their hosts for essential nutrients, lacking many biosynthetic pathways common in other organisms. They have a distinctive double envelope structure that makes them atypical among gram-negative and gram-positive classifications. Borrelia spp. can be challenging to culture, requiring specific conditions and nutrients, and exhibit a slow growth rate.
In terms of pathogenicity, Borrelia is known for causing significant diseases in humans, including Lyme disease and relapsing fever. Lyme disease is particularly prevalent in the Northern Hemisphere, starting with a recognizable bulls-eye rash and potentially leading to severe neurological and joint complications if untreated. Relapsing fever presents with intermittent fevers and can be more severe in cases caused by louse-borne transmission. Notably, Borrelia can evade immune responses through changes in their surface proteins, contributing to the recurring nature of some infections. Effective early treatment typically involves antibiotics such as doxycycline or amoxicillin, highlighting the importance of prompt medical intervention.
Borrelia
- TRANSMISSION ROUTE: Blood
Definition
Borrelia are small, obligately parasitic spirochetes with linear DNA (deoxyribonucleic acid). They have very small genomes that lack information for many biosynthetic pathways and thus require nutrients from their hosts. They are the only spirochetes that require a blood-feeding arthropod as a vector.
Natural Habitat and Features
Borrelia spp., like all spirochetes, have a double envelope with a peptidoglycan-based wall between the plasma membrane and the outer envelope. The outer envelope contains lipopolysaccharides similar to those of gram-negative organisms and, on staining, these bacteria appear somewhat gram-negative. However, their cell walls do not contain the same lipopolysaccharides found in gram-negative bacteria, and the arrangement of the cell wall components is different. Because of this, they are not usually classified as either gram-negative or gram-positive.
Their genomes are very small, only about 900,000 base pairs compared with 4,600,000 for Escherichia coli. They are incomplete, lacking genes for most biosynthetic enzymes, including those for the production of amino acids, nucleotides, and cofactors. They do possess numerous genes coding for the production of the wide array of lipopolysaccharides and proteins found in their outer envelopes. They also possess the enzymes for glycolysis but lack enzymes for the Krebs cycle and aerobic respiration. Thus, they obtain most of their energy by fermentation of simple sugars.
As extracellular parasites, Borrelia spp. obtain their missing nutrients from their hosts. Culturing Borrelia spp. is difficult, but the bacteria can be grown on a complex artificial medium supplemented with rabbit serum. They grow best under microaerobic conditions at 89.6° Fahrenheit (32° Celsius) and grow slowly with generation times as long as twenty-four hours.
An unusual feature is that Borrelia, unlike most other bacteria, has a linear chromosome. In addition to a linear genome, the bacterium has as many as twenty or more linear and circular plasmids, many of which are thought to be involved in virulence and host specificity. The total number of plasmid base pairs can be one-third or more of the entire number of base pairs in the organism. For example, one well-studied burgdorferi strain contains nine circular and twelve linear plasmids that total more than 600,000 base pairs, compared with the 910,000 base pairs in its linear chromosome.
Pathogenicity and Clinical Significance
Borrelia is unique among the spirochetes because it can be transmitted only by blood-feeding arthropods. Transmission of most Borrelia spp. is through various species of ticks. Ticks, small mammals, and birds serve as the main reservoirs. Humans and other large mammals usually become accidental hosts of the bacteria when they come in contact with infected ticks.
In the tick, Borrelia usually inhabits the gut and migrates to the salivary glands and, finally, the mouthparts during a blood meal. In hard ticks, the usual vector of Lyme disease, this migration is prolonged, and infection rarely occurs unless the tick has remained attached to its host for more than twenty-four hours. Soft ticks, the usual vector of tick-borne relapsing fever, can transmit the bacteria in less than one hour.
Recurrentis, the cause of louse-borne relapsing fever, is transmitted by the human body louse; thus, humans serve as its only reservoir. In the United States, burgdorferi has been isolated from the gut fleas and other Borrelia spp. have been isolated in the guts of mosquitoes in the Czech Republic and China. Thus, fleas and mosquitoes may also be able to transmit these bacteria. None of these bacteria secrete toxins; instead, they induce severe inflammatory responses that cause most of the symptoms of borreliosis.
The major diseases of humans caused by Borrelia spp. are Lyme disease and relapsing fever. Lyme disease is the most common arthropod-transmitted disease in the United States and is common throughout the entire Northern Hemisphere. Usually starting with a characteristic bulls-eye rash, it can progress, if untreated, to arthritis, neuropathy, meningitis, and even permanent paraplegia. Relapsing fever is characterized by intermittent febrile episodes with relatively long afebrile periods between. Louse-borne relapsing fever shows more severe symptoms and can become epidemic.
Borrelia spp. are often able to evade the immune system because, by turning on and off genes, they can readily change their surface proteins and lipopolysaccharides. This change in surface antigens is also the cause of the recurring fever episodes in both tick-borne and louse-borne relapsing fever.
Drug Susceptibility
In all borrelioses, early treatment is the most effective. For early Lyme disease in children, two- to four-week treatments of amoxicillin are preferred. The same treatment regimen with doxycycline is used for adults. Cefuroxime and ceftriaxone are also effective. Louse-borne relapsing fever usually responds to a single dose of tetracycline, doxycycline, or erythromycin. The same antibiotics are used for tick-borne relapsing fever for five to ten days.
Bibliography
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