Chlamydia trachomatis, an obligate intracellular human pathogen, is one of three bacterial species in the genus Chlamydia. C. trachomatis is a Gram-negative bacteria, therefore its cell wall components retain the counter-stain safranin and appear pink under a light microscope. In humans both sexes can display urethritis, proctitis (rectal disease and bleeding), trachoma, and infertility following infection with Chlamydia trachomatis.