Lyme Disease Morphology

lyme disease morphologyLyme disease morphology is the study of the specific organisms that cause the disease including their particular structures and the relationships between such structures. Before looking at the morphology of these organisms it may help to examine Lyme disease bacteria in terms of taxonomy and classification. Lyme disease bacteria are known collectively as Borrelia burgdorferi sensu lato and understanding fundamental taxonomy makes it easier to discuss these and other bacteria that cause Lyme disease and other, similar conditions.

Phylum is the rank in taxonomy below kingdom and above class. Borrelia are part of the bacteria kingdom, the phylum Spirochaetes, and the class Spirochaetes. They are then classified as of the order Spirochaetales and the family Spirochaetaceae. Bacteria that belong to the phylum Spirochaetes are long, thin, bacteria with flagella and a helically coiled shape. The flagella are filaments, in this case axial, that run between the spirochaetes’ outer membrane and their peptidoglycan layer. This particular structure is what allows the bacteria to move as the flagellum create a screw-like effect to propel the organism.

The phylum Spirochaetes consists of a single class (Spirochaetes), a single order (Spirochaetales), and three families: Brachyspiraceae, Leptospiraceae, and Spirochaetaceae. Borrelia is the genus (a taxonomic rank one below the family – spirochaetaceae), and the species of Borrelia which are known to cause Lyme disease include B. afzelii, B. burgdorferi sensu stricto, and B. garinii, although other species are also considered implicated in the disease’s development (such as B. bavariensis).

What’s So Special About Spirochaetes?

Other members of the order of spirochaetales include leptospiraceae with the genus Leptospira (the causative agents of Leptospirosis, a disease affecting the kidneys, liver, and central nervous system). The gemus Treponema and the species Treponema pallidum also belong to the spirochaetaceae family and this species of bacteria is the causative agent of syphilis. The research carried out into syphilis has had considerable bearing on our knowledge of Lyme disease, making it possible to develop treatment strategies and detection methods based on existing protocols for a similar pathogen.

More recent research has investigated the induction of atypical forms of Borreli burgdorferi such as a cystic, or coccoid, form. It is possible that Borrelia in cystic form would have the capacity to create a latent infection, similar to that which occurs in syphilis, before prompting a relapse as spirochaetes are then regenerated post-treatment with a Lyme disease antibiotic. Conducting such research into Lyme disease morphology is clearly vital in ensuring appropriate detection and treatment of the condition for all patients.

Structure and Morphology

Unlike other bacteria, the shape and the motility of Borrelia are governed by the fact that the periplasmic flagella attached to the cytoplasmic membrane at either end of the spirochaete are wound around the proptoplasmic cylinder. Borrelia bacteria have fewer coils than Leptospira and average 0.2-0.5µm by 4 to 18 µm. In other bacteria it is the peptidoglycan layer which determines the shape, not these filaments which run lengthwise through Borrelia cells. Research investigating the movement of Borrelia discovered the role of the flagella by inactivating a gene (flaB) that encodes the major flagellar filament protein FlaB. In doing this the bacteria lost their periplasmic flagella, became rod-shaped and also became immobile.

The importance of understanding the structure and morphology of Borrelia bacteria is highlighted by the recognition that the mobility of spirochaetes is enhanced in viscous substance in contrast to the motility of those bacteria which are externally-flagellated. About 6% of the chromosomal genome is involved in encoding those proteins in the bacteria which are responsible for motility and chemotaxis. Knowledge of the proteins involved in movement of bacteria can help clinicians investigate ways of reducing the likelihood of disease transmission as well as demonstrating the significance of factors in the saliva of ticks which contributes to the dissemination of Lyme disease bacteria.

borrelia burgdorferi spirochaete

What Do We Know About Lyme Disease Morphology?

Not all of the bacteria which cause Lyme disease have had their full genome sequenced which has ramifications for the development of vaccines against Lyme disease that work in the US and Europe and the ability to detect and treat infection with different hybrid Borrelia bacteria. The Borrelia burgdorferi sensu stricto B31 genome has been mapped however and it has a single linear chromosome with several plasmids (linear and circular). B. burgdorferi’s single chromosome is approximately 900kb, with 90% of it taken up by coding sequences for genes with a known function.

There are twelve linear plasmids and nine circular plasmids comprising 610kb in the extra-chromosomal genome of B. burgdorferi B31. Key linear plasmids in the bacteria are Ip25 and Ip28 as these are required for persistent infection of mammalian hosts. These two plasmids are somewhat unstable in culture however and when grown in vitro they are frequently lost after just a few generations. This does not mean that they cannot continue to grow, just that they become incapable of causing persistent infection in mammals.

A particular gene found in the Ip25 linear plasmid (pncA) is responsible for encoding nicotinamidase which is involved in the biosynthesis of NAD. The removal of this gene appears to have little effect in terms of growth in vitro but has been found to be crucial for growth within a host environment. Another key feature in Borrelia burgdorferi and the morphology of Lyme disease is the series of circular plasmids, known as cp32s, which are thought to be prophage genomes. These 32kb plasmids are involved in the transfer of DNA between spirochaetes with a shared geographical and ecological origin or niche.

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