CHARACTERIZATION OF TRIMETHOPRIM-RESISTANT AND SULPHISOXAZOLE-RESISTANT CHLAMYDIA-TRACHOMATIS

Trimethoprim and sulphisoxazole were used as selective agents in culture to isolate, by a stepwise procedure, a series of Chlamydia trachomatis L2 populations resistant to the cytotoxic effects of the drugs. Two trimethoprim-resistant populations, L2Tri(R)-60 and L2Tri(R)-100, and one sulphonamide-resistant population, L2Sulf(R)-100, were characterized in more detail. In addition to being resistant to trimethoprim, L2Tri(R)-60 was cross-resistant to methotrexate, sensitive to sulphisoxazole and displayed a ribonucleotide auxotrophy similar to that of its parental wild type, C. trachomatis L2. Surprisingly, L2Tri(R)-100 and L2Sulf(R)-100 appeared phenotypically identical. Both mutants were highly resistant to trimethoprim, sulphisoxazole, and methotrexate. In contrast to wild-type C. trachomatis L2, these populations were sensitive to 5-fluorouracil. L2Tri(R)-100 and L2Sulf(R)-100 were incapable of taking pyrimidine ribonucleotides from the host cell and no longer synthesized thymidine nucleotides de novo. The pyrimidine requirement of these mutants was met by salvaging host-cell uracil and thymidine, a property which can account for their drug-resistance characteristics. L2Tri(R)-100 and L2Sulf(R)-100 could also salvage adenine and guanine. Using L2Tri(R)-100 as a starting stock, a mutant population resistant to the cytotoxic effects of trimethoprim and 5-fluorouracil (L2Tri/5-FU) was selected. L2Tri/5-FU was resistant to 5-fluorouracil because it had regained the capacity to take pyrimidine ribonucleotides from the host cell.