The present invention provides Chlamydia organisms and compositions and methods of use in the treatment/prevention of chlamydial infection in a subject, for eliciting an immune response in a subject and for use as vectors. In one aspect, the present invention provides an isolated Chlamydia trachomatis cell comprising an amino acid substitution at Q117, e.g., Q117E, in an open reading frame of CT849 and a G216* mutation incorporated into the nucleotide sequence and/or a substitution at G322, e.g., G322R, in anopen reading frame of CT389, wherein said Chlamydia trachomatis cell has a phenotype due to said substitution at Q117 and said mutation and/or substitution at G216 or G322 of attenuated pathogenicity.
