細菌門：クラミジア

Chlamydiae are small cocci approximately 0.5 μm in diameter belonging to a single order, Chlamydiales. They possess reduced genomes ranging from 0.55 to 1 Mbp, reflecting their adaptation as obligate intracellular parasites. This genome reduction is even more extreme than in rickettsias, limiting their biosynthetic capabilities and making them highly dependent on host cell resources for survival and replication.

Chlamydiae are obligate intracellular parasites infecting diverse eukaryotic hosts including free-living amoebae and humans. Parachlamydia acanthamoebae naturally infects amoebae in the genus Acanthamoeba, while Chlamydia trachomatis and Chlamydophila species are well-studied human pathogens. Their ability to persist within amoebae suggests these organisms play a significant role in Chlamydiae survival and distribution in natural environments.

Chlamydia trachomatis causes trachoma, a leading cause of blindness characterized by corneal scarring, and sexually transmitted infections affecting the genitourinary tract. Chlamydophila pneumoniae is associated with respiratory conditions, while Chlamydophila psittaci causes psittacosis, an avian disease that occasionally spreads to humans causing pneumonia-like symptoms.

Elementary bodies are small, dense, non-infectious cells specialized for transmission between host cells and resistant to desiccation, facilitating airborne dispersal. Reticulate bodies are larger, less dense, and function as the replicative form, multiplying within host cells via binary fission with generation times of 2–3 hours. This two-stage cycle allows efficient exploitation of host resources and production of infectious particles.

Elementary bodies are phagocytosed by host cells and transform into reticulate bodies, which multiply through binary fission. After multiple divisions, reticulate bodies transition back into elementary bodies. Upon host cell disintegration, these elementary bodies are released to infect nearby cells, perpetuating the cycle and generating a substantial inoculum for subsequent infections.

Chlamydiae possess highly reduced genomes with limited biosynthetic capabilities, making them dependent on host resources. Notably, Chlamydia trachomatis lacks the FtsZ gene essential for septum formation in most prokaryotes. Specific genes appear acquired from eukaryotic hosts through horizontal gene transfer, enhancing their ability to survive and proliferate within host cells, which can be studied through modern molecular taxonomy.

Unlike rickettsias, which are transmitted by arthropods, chlamydias rely on airborne dispersal for transmission. Reticulate bodies divide significantly faster than rickettsias with generation times of 2–3 hours. Both groups are obligate intracellular parasites with reduced genomes, but chlamydias employ a distinctive two-stage developmental cycle involving elementary and reticulate body transformations.