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. Most intracellular chlamydiae are located in an inclusion body or vacuole. Outside of cells they survive only as an extracellular infectious form. Chlamydiae can grow only where their host cells grow. Therefore, chlamydiae cannot be propagated in bacterial culture media in the clinical laboratory. Chlamydiae are most successfully isolated while still inside their host cell.