Color enhanced SEM of chlamydia, the most common bacterial sexually transmitted disease in humans. Chlamydia are obligate intercellular bacteria. After infection, chlamydia replicate in a large vesicle inside the cell. Here the vesicle has been broken open revealing chlamydia. Chlamydiae is a bacterial phylum and class whose members are obligate intracellular pathogens. They are ovoid in shape. They have a peptidoglycan-free cell wall and are susceptible to various antimicrobial agents. All known chlamydiae only grow by infecting eukaryotic host cells. They are as small as or smaller than many viruses. Chlamydiae replicate inside the host cells and are termed intracellular. Magnification unknown.