Infectivity Acts asIn VivoSelection for Maintenance of the Chlamydial Cryptic Plasmid

Abstract

ABSTRACTChlamydia trachomatiscontains a conserved ∼7.5-kb plasmid. Loss of the plasmid results in reduced glycogen accumulation, failure to activate TLR2, and reduced infectivity. We hypothesized that reduced infectivity functions as a means of selection for plasmid maintenance. We directly examined the biological significance of the reduced infectivity associated with plasmid deficiency by determining the relative fitness of plasmid-deficient CM972 versus that of wild-typeC. muridarumNigg in mixed inoculain vitroandin vivo.C. muridarumNigg rapidly out-competed its plasmid-cured derivative CM972in vitrobut was not competitive with CM3.1, a derivative of CM972 that has reverted to a normal infectivity phenotype.C. muridarumNigg also effectively competed with CM972 during lower and upper genital tract infection in the mouse, demonstrating that strong selective pressure for plasmid maintenance occurs during infection. The severity of oviduct inflammation and dilatation resulting from these mixed infections correlated directly with the amount ofC. muridarumNigg in the initial inoculum, confirming the role of the plasmid in virulence. Genetic characterization of CM972 and CM3.1 revealed no additional mutations (other than loss of the plasmid) to account for the reduced infectivity of CM972 and detected a single base substitution in TC_0236 in CM3.1 that may be responsible for its restored infectivity. These data demonstrate that a chlamydial strain that differs genetically from its wild-type parent only with respect to the lack of the chlamydial plasmid is unable to competein vitroandin vivo, likely explaining the rarity of plasmid-deficient isolates in nature.