Which one of the following microorganisms uses antigenic variation as a major means of evading host-defences -

Ans. is 'b' i.e., Borrelia recurrentis . "Borrelia readily undergoes antigenic variations in vivo and this is believed to be the reason for the occurrence of relapses in the disease". . Antigenic variations are caused by DNA rearrangements in linear plasmids present in borrelia. This rearrangement occurs within "Vmp genes" located in the plasmid. Vmp genes encode variable major proteins (VMPs) found on the spirochete's outer membrane surface. . This ability to vary surface antigen allows bacteria to survive for a longer time because antibody produced in response to infection by one strain of pathogen will not protect against subsequent attacks from a different strain of that bacterium.