Chlamydiota
Bacterial phylum including human pathogens and ocean-floor forms.
The Chlamydiota (also called Chlamydiae) are a phylum and class of bacteria with a wide range of lifestyles: some cause disease in humans and animals, some live as symbionts inside common protozoa, and others have been found in ocean-floor sediments and are still poorly understood. For decades, all known Chlamydiota were obligate intracellular bacteria—they can only survive and reproduce inside a host cell. A major 2020 study revealed many new Chlamydiota in deep-sea sediments, building on earlier reports of marine chlamydial sequences from the 2000s, and it remains unclear whether all of them require hosts. Among the species that infect humans, the two most significant are *Chlamydia pneumoniae*, which causes a form of pneumonia, and *Chlamydia trachomatis*, which causes chlamydia—the most frequently reported bacterial sexually transmitted infection in the United States, with about 1.6 million cases each year (2022 CDC data).
**Biology**
**Ecology and life cycle** The Chlamydiota that have been studied for a long time can only grow by infecting eukaryotic host cells. They are as small as or smaller than many viruses, ovoid in shape, and stain Gram-negative. They depend entirely on replication inside host cells, so some species are called obligate intracellular pathogens, while others are symbionts of common protozoa. Inside a cell, most of these bacteria live within a vacuole or inclusion body. While growing, they exist in a metabolically active but noninfectious form called the reticulate body. Outside a host cell, they survive only as an infectious, spore-like form called the elementary body. Because they can only grow where their host cells grow, and they follow a characteristic two-phase developmental cycle, clinically important Chlamydiota cannot be grown in standard bacterial culture media in the lab; they are best isolated while still inside host cells. The discovery of ocean-floor Chlamydiota, including a major 2020 study and earlier reports, raises the question of whether all of them have hosts.
**Peptidoglycan** Scientists have long known that Chlamydiota are sensitive to antibiotics that block peptidoglycan (PG) production, such as penicillin, yet for many years they could not find any PG in their cell walls. In 2013, *Protochlamydia amoebophila* was shown to have a PG sacculus, while *Simkania negevensis* does not. Neither organism has th
- Phylum
- Chlamydiota
- Class
- Chlamydiia
- Orders
- Chlamydiales, Parachlamydiales
- Families
- 9 (4 validly named)
- Known human pathogens
- Chlamydia trachomatis, Chlamydia pneumoniae
- Annual US chlamydia infections
- 1.6 million (2022 CDC data)
- Ocean-floor discovery
- 2000s (major 2020 study)
- Valid publication date
- 2015
Quick Facts
- Taxon
- Chlamydiia
- Subdivision
- "Similichlamydiales" / Chlamydiales
Facts from the source article.
Lore & Background
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. Chlamydial isolates cultured in the yolk sacs of embryonating eggs were obtained from a psittacosis outbreak in 1929–1930, and by the mid-20th century, isolates had been obtained from dozens of vertebrate species. The term chlamydia (a cloak) appeared in the literature in 1945, although other names continued to be used, including Bedsonia, Miyagawanella, ornithosis-, TRIC-, and PLT-agents. In 1956, Chlamydia trachomatis was first cultured by Tang Fei-fan, though they were not yet reco
Reader's Guide
The Chlamydiota are significant as a bacterial phylum that includes major human pathogens such as Chlamydia trachomatis and Chlamydia pneumoniae, responsible for widespread sexually transmitted infections and pneumonia. Their unique biology—obligate intracellular lifestyle, biphasic developmental cycle with elementary and reticulate bodies, and atypical peptidoglycan structure—has challenged traditional bacteriology. The discovery in 2020 of many additional Chlamydiota in ocean-floor environments expanded their known diversity and raised questions about host dependence. Their evolutionary history is notable: they separated from other bacteria about a billion years ago, and their relationship to cyanobacteria and plant chloroplasts has been debated. The phylum's taxonomy has evolved, with two orders (Chlamydiales and Parachlamydiales) distinguished by molecular signatures. The Chlamydiota's resistance to penicillin being bacteriostatic rather than bacteriocidal, due to a ring of peptidoglycan instead of a full sacculus, illustrates their unique cell biology. Their legacy includes ongoing research into intracellular parasitism, symbiosis, and environmental adaptation.
Did You Know?
- All Chlamydiota long known to science grow only by infecting eukaryotic host cells and are as small as or smaller than many viruses.
- In 2014, the human pathogen Chlamydia trachomatis was shown to contain peptidoglycan forming rings during cell division, not a full sacculus.
- The phylum was renamed Chlamydiota in 2022.
- Molecular signatures (CSIs and CSPs) uniquely shared by each order support the distinctness of Chlamydiales and Parachlamydiales.
Frequently Asked Questions
What exactly is Chlamydiota?
Chlamydiota is a bacterial phylum (and class) that spans an enormous ecological range, from human pathogens to symbionts inside common protozoa to enigmatic organisms pulled from ocean-floor sediments. It is organized into two orders—Chlamydiales and Parachlamydiales—and nine families, of which only four have been validly named.
What does 'obligate intracellular' mean for Chlamydiota, and why is it a big deal?
For decades, every known Chlamydiota species could survive and reproduce only inside a eukaryotic host cell, so they could not be grown on standard culture media like most other bacteria. That total dependence on a living host made their biochemistry and life cycle uniquely difficult to study.
Which Chlamydiota species infect humans, and how widespread is the problem?
The two most significant human pathogens are Chlamydia trachomatis and Chlamydia pneumoniae. In the United States alone, approximately 2.86 million chlamydia infections are reported every year, making it one of the most common bacterial sexually transmitted infections.
What changed in 2020 that shook up Chlamydiota biology?
A surge of newly described Chlamydiota turned up in deep-sea and ocean-floor environments, dramatically broadening the phylum beyond the familiar intracellular pathogens. Whether all of these newly found lineages still require a host cell, or can persist independently in sediment, remains an open question.
How is Chlamydiota organized taxonomically from top to bottom?
At the top sits the phylum Chlamydiota, which contains the class Chlamydiae. That class splits into the orders Chlamydiales and Parachlamydiales, and beneath those are nine recognized families—though only four of the nine carry a validly published name.
More in Higher-Level Taxonomy & Evolution 1-24
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