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.

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.

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; when growing in a cell, they survive in a metabolically active but noninfectious form called the reticulate body. Outside cells, they survive only as an infectious, spore-like form called the elementary body.

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.

In 2020 many additional Chlamydiota were discovered in ocean-floor environments, and it is not yet known whether they all have hosts.

Scientists have long known that Chlamydiota are susceptible to antibiotics that target the production of peptidoglycan (PG) such as penicillin, yet have for a long time failed to find any PG in their cell walls. In 2013, Protochlamydia amoebophila was shown to have a sacculus made of PG while Simkania negevensis does not. There is no FtsZ gene, which is previously believed to be essential for cell division in the presence of PG, in either of them. In 2014, the human pathogen Chlamydia trachomatis was shown to contain PG in its intracellular stage, apparently forming rings. In 2016, the role of PG in Chlamydia was clarified using more data: it does not make a whole sacculus around the cell like usual bacteria and Protochlamydia do, but instead produces a thin ring of PG down the middle during cell division. MreB controls the production of the ring, taking up the role that FtsZ would've performed. This explains why penicillin is bacteriostatic and not bacteriocidal to Chlamydia.

The elemental bodies of Chlamydiaare characterized by the presence of a tough cell wall. This wall is not made of PG, but instead consists of a network of proteins.

Chlamydia-like disease affecting the eyes of people was first described in ancient Chinese and Egyptian manuscripts. A modern description of chlamydia-like organisms was provided by Halberstaedrrter and von Prowazek in 1907.