Chlamydiota

The Chlamydiota (synonym Chlamydiae) are a bacterial phylum and class whose members are remarkably diverse, including pathogens of humans and animals, symbionts of ubiquitous protozoa, and marine sediment forms not yet well understood. All of the Chlamydiota that humans have known about for many decades are obligate intracellular bacteria; in 2020 many additional Chlamydiota were discovered in ocean-floor environments, and it is not yet known whether they all have hosts. Historically it was believed that all Chlamydiota had a peptidoglycan-free cell wall, but studies in the 2010s demonstrated a detectable presence of peptidoglycan, as well as other important proteins.

Among the Chlamydiota, 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 Chlamydiota are located in an inclusion body or vacuole. Outside cells, they survive only as an extracellular infectious form.

These Chlamydiota can grow only where their host cells grow, and develop according to a characteristic biphasic developmental cycle. Therefore, clinically relevant Chlamydiota cannot be propagated in bacterial culture media in the clinical laboratory. They are most successfully isolated while still inside their host cells.

Of various Chlamydiota that cause human disease, the two most important species are Chlamydia pneumoniae, which causes a type of pneumonia, and Chlamydia trachomatis, which causes chlamydia. Chlamydia is the most common bacterial sexually transmitted infection in the United States, and 2.86 million chlamydia infections are reported annually.