Interstrain Gene Transfer inChlamydia trachomatisIn Vitro: Mechanism and Significance

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Abstract
The high frequency of between-strain genetic recombinants ofChlamydia trachomatisamong isolates obtained from human sexually transmitted infections suggests that lateral gene transfer (LGT) is an important means by whichC. trachomatisgenerates variants that have enhanced relative fitness. A mechanism for LGT inC. trachomatishas not been described, and investigation of this phenomenon by experimentation has been hampered by the obligate intracellular development of this pathogen. We describe here experiments that readily detected LGT between strains ofC. trachomatisin vitro. Host cells were simultaneously infected with an ofloxacin-resistant (Ofxr) mutant of a serovar L1 strain (L1:Ofxr-1) and a rifampin-resistant (Rifr) mutant of a serovar D strain (D:Rifr-1). Development occurred in the absence of antibiotics, and the progeny were subjected to selection for OfxrRifrrecombinants. The parental strains differed at many polymorphic nucleotide sites, and DNA sequencing was used to map genetic crossovers and to determine the parental sources of DNA segments in 14 recombinants. Depending on the assumed DNA donor, the estimated minimal length of the transferred DNA was ≥123 kb in one recombinant but was ≥336 to ≥790 kb in all other recombinants. Such trans-DNA lengths have been associated only with conjugation in known microbial LGT systems, but natural DNA transformation remains a conceivable mechanism. LGT studies can now be performed with diverse combinations ofC. trachomatisstrains, and they could have evolutionary interest and yield useful recombinants for functional analysis of allelic differences between strains.