Abstract: The invention involves the discovery that Chlamydia sp. strains can be cured of their plasmids by treatment with novobiocin, and that plasmid-deficient strains are defective in infecting cells under standard conditions, but can infect cells if centrifuged onto the host cells. But it is found that plasmid-deficient strains with wild-type infection efficiency under standard conditions can be isolated as mutants from parent plasmid-deficient strains with low infectivity by selecting for infection under standard conditions. Both the less infective and the highly infective plasmid-deficient strains were able to infect mice with little or no pathological symptoms, and both reduced the pathology in mice later challenged with the parental wild-type disease-causing Chlamydia strain. Thus, plasmid-deficient Chlamydia are effective vaccine strains. The invention provides a process for isolating a plasmid-deficient strain of Chlamydia sp., a process for developing a plasmid-deficient strain of Chlamydia sp.

Abstract: The invention involves the discovery that Chlamydia sp. strains can be cured of their plasmids by treatment with novobiocin, and that plasmid-deficient strains are defective in infecting cells under standard conditions, but can infect cells if centrifuged onto the host cells. But it is found that plasmid-deficient strains with wild-type infection efficiency under standard conditions can be isolated as mutants from parent plasmid-deficient strains with low infectivity by selecting for infection under standard conditions. Both the less infective and the highly infective plasmid-deficient strains were able to infect mice with little or no pathological symptoms, and both reduced the pathology in mice later challenged with the parental wild-type disease-causing Chlamydia strain. Thus, plasmid-deficient Chlamydia are effective vaccine strains. The invention provides a process for isolating a plasmid-deficient strain of Chlamydia sp., a process for developing a plasmid-deficient strain of Chlamydia sp.