What is Borrelia burgdorferi?

By Dr. Tomislav Meštrović, MD, Ph.D.

Borrelia burgdorferi is a member of the spirochete phylum of bacteria, which represents an ancient evolutionary branch only distantly related to gram-positive and gram-negative bacteria. It is recognized as a causative factor of a special clinical entity known as Lyme disease which is transmitted by infected ticks.

The discovery of this microorganism and subsequent elucidation of its distinctive cellular and molecular biology is considered one of the greatest stories in the chronicles of bacterial pathogenesis research. Experiments that were conducted on Borrelia burgdorferi benefited from previous studies on the relapsing fever borreliae.

About the organism

Borrelia burgdorferi has a relatively small genome (0.91 megabases) when compared to most of the other bacteria. It contains a restricted set of housekeeping genes (needed for maintaining basal cellular function) with surprisingly few encoding metabolic and biosynthetic functions. As a result, this microorganism lacks the ability to synthesize nucleotides, amino acids and fatty acids.

That is the reason why cultivation of this microorganism in the laboratory is cumbersome and requires complex media with a panoply of essential ingredients. Improved growth of Borrelia needs synchronized collection and transport actions, adequate growth environment, as well as the implementation of a long-term culture method.

Borrelia burgdorferi also has linear plasmids with high degree of redundant genes, non-functional “pseudogenes” and non-coding sequences that are considered genomic baggage. Still, inside them several extremely important genes can be found; for example, linear plasmid lp54 which contains the gene encoding outer surface protein A (OspA).

The periplasmic flagella that have an indispensable role in cell morphology and motility are located in the periplasmic space between the outer membrane and the cytoplasm-peptidoglycan layer. The characteristic cell morphology is a result of the flagella wrapping around the protoplasmic cylinder.

Biology of infection

Borrelia burgdorferi has the ability to infect a myriad of vertebrate animals such as small mammals, birds and lizards. Ticks belonging to the genus Ixodes (most notably Ixodes scapularis or deer tick) have the main role in transmitting this microorganism between hosts and represent the only natural way of its transmission to humans.

This species has developed an unusual lifestyle that alternates between vertebrate and arthropod hosts. Furthermore, Borrelia belongs to a group of organisms that produce no known toxins or secretion systems to deliver effector molecules, yet are able to invade practically any mammalian tissue and cause disease manifestations that last for months to years.

Infection is most often seen during the late spring and summer months due to the pronounced activity of tick nymphs during that period, however cases have been described throughout the whole year. The small size of nymphs makes their detection difficult, thus enabling long feeding and transmission of the pathogen.

Lyme disease

The story of Lyme disease in Connecticut has been repeatedly told and is often cited as an example of a positive interplay between the lay community, public health workers and academic medicine. It was discovered as a cause of an outbreak of juvenile rheumatoid arthritis in 1975, and a sturdy increase in a number of reported cases has been described in the United States and other countries since then.

Immune response of the human organism to the infection is considered responsible for various clinical manifestations linked to Lyme disease. Genes that encode physiological functions for growth inside the infected host are, therefore, critical virulence determinants, as survival represents a principal component of Borrelia burgdorferi pathogenesis.

The most frequent manifestation of Lyme disease caused by Borrelia burgdorferi is erythema migrans, which is a circumscribed lesion of the skin that appears within 3 to 30 days following the tick bite. Arthritis and other musculoskeletal involvement are typical, and cardiovascular manifestations of Lyme disease consist of arrhythmia. Central nervous system can also be affected, but neurologic manifestations are highly variable.

For early Lyme disease, therapy with antimicrobial drugs such as doxycycline, cefuroxime and amoxicillin is considered effective and safe, and various professional groups are constantly updating guidelines for the management of this disease. Vaccines are still in the development, thus preventive measures include the application of repellents and avoidance of areas infested with ticks.

Further Reading

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