B. recurrentis is the causative agent of relapsing fever, whichmanifests as one or more relapses of fever after the subsidence of primary paroxysm of fever.
recurrentis shows the following morphological features:
· B. recurrentis has an unequal spiral-shaped structure withone or both the ends pointed. It measures 8–20 mm in length and 0.2–0.4 mm in breadth.
· It possesses 15–20 endoflagella per cell and 5–10 loose uneven spiral coils. The spiral coils are coarser and more irregular than those of treponemes and leptospires.
· It is Gram negative. It can be stained by Wright and Giemsa stain for demonstration by microscope. It is actively motile in fresh blood preparation, moves in forward and backward waves, and exhibits cork-screw like motility.
Borrelia is microaerophilic. The bacteria grow at optimum tem-perature of 28–30°C. They have complex nutritional require-ments, hence are difficult to grow on artificial media. They can grow in chorioallantoic membrane of chick embryo. Mice or rats are highly susceptible to infection. Primary isolation of bacteria from clinical specimens can be made by intraperi-toneal inoculation of specimens in these laboratory animals. After inoculation, borreliae appear early in the blood and also are found in the brain for a longer time.
Antigenic variation is a unique property exhibited by Borrelia in humans. DNA rearrangement in linear plasmid present in Borrelia appears to be responsible:
· For antigenic variations shown by the bacteria and
· For causing relapse, characteristically seen in the relapsing fever caused by the bacteria.