7.7: Bacterial Phylum Chlamydiae

The phylum Chlamydiae or Chlamydiota is composed of a single order, Chlamydiales. This phylum consists entirely of obligate intracellular parasites that infect eukaryotic hosts. While human pathogens within this group have been studied extensively, the phylum encompasses many species capable of interacting with various eukaryotic organisms. Members of Chlamydiae are typically small cocci, approximately 0.5 μm in diameter, and exhibit a distinctive developmental cycle. As is characteristic of many obligate parasites and symbionts, their genomes are reduced in size, ranging from 0.55 to 1 Mbp.

Species in this phylum are highly specialized for invading and colonizing eukaryotic cells, with different species targeting a diverse array of hosts. For instance, Parachlamydia acanthamoebae infects free-living amoebae, particularly those in the genus Acanthamoeba. During amoebal infection, Parachlamydia undergoes the typical developmental cycle observed in other chlamydial species. Many Chlamydiae can multiply or persist within free-living amoebae, suggesting that these hosts play a significant role in the survival and distribution of Chlamydiae in natural environments. Analyses of 16S rRNA gene sequences reveal a high diversity of Chlamydiae in the environment, indicating their widespread presence and the potential existence of numerous undiscovered natural hosts. Although P. acanthamoebae naturally infect amoebae, it can also infect humans, albeit less effectively than species adapted to human hosts.

The most thoroughly studied human pathogens are in the genera Chlamydia and Chlamydophila. These include species such as Chlamydophila psittaci, which causes psittacosis; Chlamydia trachomatis, responsible for trachoma and other human diseases; and Chlamydophila pneumoniae, associated with certain respiratory conditions. Psittacosis is primarily an avian epidemic that occasionally spreads to humans, resulting in pneumonia-like symptoms. Trachoma, a debilitating eye disease characterized by corneal vascularization and scarring, is a leading cause of blindness worldwide. Other strains of C. trachomatis infect the genitourinary tract and are among the most common sexually transmitted infections.

The chlamydias are among the most biochemically constrained bacteria known, with genomes approximately 1 Mbp in size. Their biosynthetic capabilities are even more limited than those of the rickettsias, another group of obligate intracellular parasites. Notably, the genome of C. trachomatis lacks the gene encoding FtsZ, a protein considered essential for septum formation during cell division in most prokaryotes. Additionally, specific genes in C. trachomatis appear to have been acquired from eukaryotic hosts through horizontal gene transfer, potentially enhancing their ability to survive and proliferate within host cells.

The evolutionary success of Chlamydiae lies in their efficient survival strategy, which involves exploiting host resources and producing resistant cell forms for transmission. The chlamydial life cycle involves two distinct cell types: (1) elementary bodies, which are small, dense, and specialized for infectious transmission, and (2) reticulate bodies, which are larger, less dense, and function as the replicative form. Elementary bodies are resistant to desiccation and facilitate airborne transmission, making them particularly effective invaders of the respiratory system. In contrast, reticulate bodies are non-infectious and multiply within host cells via binary fission, generating a substantial inoculum for subsequent infections.

Reticulate bodies transition into elementary bodies following multiple divisions, released when the host cell disintegrates. These elementary bodies then infect nearby cells, perpetuating the cycle. Reticulate bodies have been observed to divide with generation times of 2–3 hours, a rate significantly faster than rickettsias. Unlike the rickettsias, however, chlamydias are not transmitted by arthropods but rely primarily on airborne dispersal.