Among the Chlamydiae, all of the ones long known to science grow only by infecting eukaryotic host cells. They are as small as or smaller than many viruses. They are ovoid in shape and stain Gram-negative. They are dependent on replication inside the host cells; thus, some species are termed obligate intracellular pathogens and others are symbionts of ubiquitous protozoa. Most intracellular Chlamydiae are located in an inclusion body or vacuole. Outside cells, they survive only as an extracellular infectious form. These Chlamydiae can grow only where their host cells grow, and develop according to a characteristic biphasic developmental cycle. Therefore, clinically relevant Chlamydiae cannot be propagated in bacterial culture media in the clinical laboratory. They are most successfully isolated while still inside their host cells.