Plasmid requirements for infection of ticks by Borrelia burgdorferi

Borrelia burgdorferi strain B31 MI commonly loses one or more of its complement of 21 extrachromosomal plasmids during normal handling procedures and during genetic manipulations. Certain plasmid losses cause an inability or reduction in the ability of spirochetes to infect mice. In the current study, nine strains of spirochetes with varying plasmid profiles were used to identify plasmids necessary for nymphal tick infection. Nymphal ticks were artificially fed the nine spirochete strains as well as the parental strain containing a full complement of plasmids. The capillary fed nymphs were allowed to feed on mice for at least 63 h and then examined for the presence of spirochetes in their guts and salivary glands. All spirochete strains tested were able to infect ticks guts, but to different degrees. We determined that the plasmids 1p5, 1p28-1, and cp9 were not required for infecting tick guts, whereas loss of 1p25 and 1p28-4 was associated with reduced gut infectivity. A reduction in the ability of spirochetes to invade salivary glands was seen in bacteria that did not have 1p28-1, whereas cp9 was not required for salivary gland infection. This study has pinpointed specific plasmids whose absence is deleterious to infecting nymphal tick guts and salivary glands.