Chlamydia. Coloured scanning electron micrograph (SEM) of a colony of Chlamydia sp. bacteria. This genus of spherical, gram-negative bacteria includes several species that parasitize humans. The bacteria are obligate parasites, incapable of living outside the cells of their host. It is one of the most common sexually transmitted diseases C. trachomatis causes the disease lymphogranuloma venereum along with two types of eye disease (trachoma and inclusion conjunctivitis). In addition it is involved in many cases of non-specific urethritis (NSU). Chlamydia are among the smallest of bacteria, being about the size of large viruses. Magnification x 20000 at 10cm wide.