Chlamydia trachomatis cycles between two structurally distinct cell forms during its life inside a human cell: a small, tough, infectious particle called an elementary body (EB) and a larger, fragile, actively dividing form called a reticulate body (RB). This two-stage lifestyle sets it apart from most bacteria, which look and behave more or less the same whether they are inside or outside a host. The shifts between these forms, and the elaborate structural machinery that supports them, help explain why chlamydial infections are so common, so often silent, and so difficult to clear without treatment.
The Two Cell Forms
Elementary bodies are the form that survives outside cells and initiates new infections. They are remarkably small, roughly 0.3 micrometers in diameter, which puts them near the lower size limit for bacteria. Rod-like surface projections are present even on EBs this small.
1PubMed. Structural studies of the surface projections of Chlamydia trachomatis by electron microscopy Their DNA is tightly condensed into a compact nucleoid, making them metabolically dormant. Think of EBs as spore-like delivery packages: they cannot replicate, but they are sturdy enough to survive briefly in the environment and tough enough to attach to and penetrate a host cell.
Reticulate bodies are the opposite in nearly every respect. They are larger (starting around 1.25 micrometers in diameter early in infection), metabolically active, and capable of dividing. Their DNA is relaxed and accessible for gene expression. RBs cannot survive outside the host cell and are not infectious. All chlamydial replication takes place in this form, inside a membrane-bound compartment called the inclusion. The entire developmental cycle involves converting from EB to RB, multiplying, and then converting back to EB for release.
The Peptidoglycan Puzzle
For decades, researchers struggled with a contradiction. Chlamydia’s genome encodes the machinery to build peptidoglycan, the rigid mesh that gives most bacteria their shape, yet no one could find a traditional peptidoglycan layer surrounding the organism. This became known as the “chlamydial anomaly.” The mystery was resolved when super-resolution microscopy showed that pathogenic Chlamydia species do synthesize peptidoglycan, but only as a narrow ring at the middle of the cell during division, not as a full surrounding sac.
In actively dividing RBs, this ring sits right at the division septum where the cell pinches in two. Its width is relatively constant, averaging about 139 nanometers, even as the ring’s overall diameter varies with cell size. Once a dividing RB completes separation, a small disc of peptidoglycan sometimes appears between the two daughter cells. The ring structure was found across multiple pathogenic chlamydial species, and its formation depends on the cytoskeletal protein MreB, which is better known for maintaining cell shape in rod-shaped bacteria.2PubMed Central. Pathogenic Chlamydia Lack a Classical Sacculus but Synthesize a Narrow, Mid-cell Peptidoglycan Ring, Regulated by MreB, for Cell Division The practical upshot: chlamydiae use peptidoglycan strictly for cell division, not for structural integrity of the whole envelope.
The Molecular Injection System
Studding the surface of EBs are arrays of type III secretion systems (T3SS), needle-like molecular machines that inject bacterial proteins directly into a host cell. The chlamydial T3SS has an overall architecture distinct from every other T3SS observed to date. Its basal body is more elongated, with a pronounced convex curvature, measuring about 34 nanometers long and 14 to 20 nanometers in diameter. A novel ring surrounds the needle on the outer membrane face, a feature not seen in other species.3Nature Communications. Structure of a bacterial type III secretion system in contact with a host membrane in situ
The needle itself is built from a protein called CdsF, which is concentrated in the outer membrane of EBs and is surface-exposed. CdsF can polymerize into multisubunit complexes to form needle-like projections. During active infection, CdsF appears at the inclusion membrane with a punctate distribution adjacent to RBs, indicating that the secretion system continues operating throughout the intracellular phase, not just during entry.4PubMed Central. Bioinformatic and biochemical evidence for the identification of the type III secretion system needle protein of Chlamydia trachomatis The inner-membrane ring component, CdsD, adopts an elongated shape built from three periplasmic domains and likely assembles into a 24-unit ring structure.5PubMed Central. The extended structure of the periplasmic region of CdsD, a structural protein of the type III secretion system of Chlamydia trachomatis The T3SS is the organism’s primary tool for manipulating its host, delivering effector proteins that reshape cell signaling, membrane trafficking, and immune responses.
EBs Are Structurally Polarized
Cryo-electron tomography, which images frozen-hydrated cells in three dimensions, revealed that EBs are not symmetrical spheres. They have a built-in polarity. One pole features a tubular invagination of the inner membrane, while the opposite pole shows an expanded periplasmic space packed with an array of T3SS needles.6PubMed Central. Pathogen-host reorganization during Chlamydia invasion revealed by cryo-electron tomography This asymmetry likely matters for how the EB orients itself when it contacts a host cell. The T3SS-rich pole faces outward, positioned to inject effector proteins on contact, while the opposite pole may play a role in the internal reorganization that follows entry. Cryo-tomography preserves membrane features far better than older thin-section methods, which tended to flatten and distort these structures.7Microscopy. Cryo-electron tomography of Chlamydia trachomatis gives a clue to the mechanism of outer membrane changes
How Chlamydia Gets Inside
Entry into a host cell is not a passive event. EBs actively exploit the cell’s own actin machinery to force their way in. The process begins when EBs make contact with filopodia, the thin finger-like projections that many cell types extend from their surface. After this initial capture, the host cell’s actin cytoskeleton builds more complex structures, including cup-like and ruffle-like formations, around the bacterium. The entry pathway shares key features with macropinocytosis, a process cells normally use to gulp extracellular fluid. These shared features include sensitivity to the drug amiloride, uptake of surrounding fluid along with the bacterium, and recruitment of specific signaling molecules and phospholipids.8PLOS Pathogens. Chlamydia exploits filopodial capture and a macropinocytosis-like pathway for host cell entry The bacterium essentially tricks the cell into swallowing it through a pathway the cell already uses for other purposes.
Life Inside the Inclusion
Once inside, the EB sits within a membrane-bound vacuole: the inclusion. In most other bacterial infections, a vacuole like this would fuse with lysosomes and be destroyed. Chlamydia prevents that. The inclusion actively avoids lysosomes; early endocytic markers appear around the inclusion within about four hours of infection, but lysosomal markers remain absent even 20 hours later. This avoidance is an active process, driven by bacterial proteins embedded in the inclusion membrane.9PLoS ONE. Two Coiled-Coil Domains of Chlamydia trachomatis IncA Affect Membrane Fusion Events during Infection
The inclusion membrane itself is studded with chlamydial proteins called Incs (inclusion membrane proteins). Some Incs serve a structural role essential for keeping the vacuole intact. When specific Incs are absent, the inclusion membrane becomes fragile, ruptures prematurely, and releases bacteria into the cytoplasm. The host cell detects this breach and responds by triggering its own death through apoptosis.10PubMed Central. Absence of specific Chlamydia trachomatis inclusion membrane proteins triggers premature inclusion lysis and host cell death So maintaining the inclusion’s integrity is not optional; it is a survival requirement for the pathogen.
Chlamydia also positions the inclusion near the host cell’s endoplasmic reticulum (ER) and establishes membrane contact sites between the two. The lipid transfer protein CERT is recruited to the inclusion as early as eight hours after infection and colocalizes with ER-resident proteins at the inclusion membrane.11PLoS Pathogens. The Lipid Transfer Protein CERT Interacts with the Chlamydia Inclusion Protein IncD and Participates to ER-Chlamydia Inclusion Membrane Contact Sites Through this intimate contact, the bacterium siphons lipids, particularly sphingomyelin precursors, from the host ER to build its own membranes. Chlamydia cannot synthesize most lipids on its own, so this theft is critical for growth.
How RBs Divide, and Why It Is Debated
For over 40 years, the textbook view was that RBs divide by binary fission, the straightforward splitting-in-two that most bacteria use. Three-dimensional electron microscopy analysis supports this, showing that dividing RBs split relatively symmetrically without detectable budding.12Nature Communications. Replication-dependent size reduction precedes differentiation in Chlamydia trachomatis However, a separate study challenged this assumption by observing asymmetric membrane expansion from one pole of the cell, a process resembling budding seen in certain unusual bacteria (Planctomycetes). In that work, the outer membrane protein MOMP was polarized to one end of the cell, and new membrane grew outward from the MOMP-positive pole.13PLOS Pathogens. Polarized Cell Division of Chlamydia trachomatis
This disagreement has not been fully resolved. Some researchers suspect the division mode may differ depending on the stage of infection: polarized budding could predominate during the first few divisions after EB-to-RB conversion, while symmetric binary fission may take over during the exponential growth phase. What is clear is that Chlamydia lacks FtsZ, the cell-division protein that nearly all other bacteria use to build a constriction ring, which makes its division machinery unusual regardless of which mode turns out to be dominant.
Shrinking Before Converting Back
One of the more surprising discoveries about chlamydial development is that RBs get progressively smaller as they divide. Mean RB volume drops roughly sixfold over the course of infection: from about 1.01 cubic micrometers at 12 hours post-infection down to about 0.16 cubic micrometers at 32 hours, when conversion back to EBs is underway. This happens because RBs divide before they double in size, a pattern with no close parallel in other well-studied bacteria.12Nature Communications. Replication-dependent size reduction precedes differentiation in Chlamydia trachomatis
This size reduction appears to be the trigger for converting back to EBs. The current model proposes that RBs carry an internal inhibitory signal that prevents differentiation. With each division, the signal is diluted. Once an RB reaches a minimum size threshold after roughly three to five divisions, the inhibitor drops below a critical concentration and the cell becomes capable of converting into an EB.14bioRxiv. Cell Type Development in Chlamydia trachomatis Follows a Program Intrinsic to the Reticulate Body Because individual RBs vary in size, they do not all reach this threshold at the same time, which is why conversion is asynchronous within a single inclusion: some RBs are still dividing while others have already become EBs.
Size is not the only cue. The redox environment inside the inclusion also matters. Elevated oxidizing conditions push RBs to express EB-associated genes earlier and produce EBs sooner, while artificially buffering against oxidation delays conversion.15PubMed Central. Altering the redox status of Chlamydia trachomatis directly impacts its developmental cycle progression A periplasmic protease called Tsp is also required; knocking it down produces EBs with abnormal morphology, directly linking this enzyme to the structural changes of secondary differentiation.16PubMed Central. The Periplasmic Tail-Specific Protease, Tsp, Is Essential for Secondary Differentiation in Chlamydia trachomatis
DNA Packaging Drives the Shape Change
One of the most visible structural differences between RBs and EBs is the state of their DNA. In RBs, the chromosome is diffuse and actively transcribed. In EBs, it is condensed into a tight, electron-dense nucleoid. This compaction is carried out by two small histone-like proteins, Hc1 and Hc2, which are similar to eukaryotic histone H1. Hc1 is expressed only during the late stages of the developmental cycle, coinciding with the physical reorganization of RBs into EBs.17PubMed. Chlamydia trachomatis developmentally regulated protein is homologous to eukaryotic histone H1 Expressing Hc1 in E. coli produces the same kind of chromatin condensation seen in late-stage chlamydiae, confirming that it is sufficient to drive nucleoid compaction.18PubMed Central. The chlamydial EUO gene encodes a histone H1-specific protease
These histone-like proteins function as global regulators of chromatin structure and gene expression.19PubMed Central. Regulation of the Chlamydia trachomatis histone H1-like protein Hc2 is IspE dependent and IhtA independent When they condense the DNA, gene transcription shuts down globally, which is consistent with the metabolic dormancy of EBs. The reverse process at the start of infection, when EBs convert to RBs, involves degrading or removing these histones to relax the chromosome and switch genes back on. The protease EUO, which cleaves histone H1, is thought to be part of this decompaction step.
Getting Out of the Host Cell
At the end of the developmental cycle, the inclusion is packed with newly formed EBs that need to escape. Chlamydia uses two mutually exclusive exit strategies. The first is straightforward lysis: the inclusion membrane breaks, followed by the nuclear envelope, and finally the plasma membrane, killing the host cell. Protease activity is required for the inclusion to rupture, and calcium signaling drives the final plasma membrane breakdown.20PubMed Central. Mechanisms of host cell exit by the intracellular bacterium Chlamydia
The second strategy, called extrusion, is more subtle. A portion of the inclusion pinches off, pushes outward still wrapped in host cell membrane, and detaches from the cell like a bud. The host cell survives, at least temporarily. Extrusion requires actin polymerization, myosin II motor activity, and Rho GTPase signaling. An inclusion membrane protein called MrcA recruits a calcium channel (ITPR3) and a calcium sensor (STIM1) to the inclusion surface. Disrupting MrcA or depleting ITPR3 reduces extrusion, as does chelating intracellular calcium.21PLoS Pathogens. Chlamydia trachomatis inclusion membrane protein MrcA interacts with the inositol 1,4,5-trisphosphate receptor type 3 (ITPR3) to regulate extrusion formation By keeping the host cell alive, extrusion may allow the infection to spread more quietly, without triggering the inflammatory signals that accompany cell death.
Persistence and Aberrant Bodies
Under stress, RBs can enter a third morphological state that complicates the neat EB-RB binary: the aberrant body (AB). ABs are enlarged, non-dividing RBs that are viable but cannot be cultured. They arise in response to various insults, including exposure to penicillin, interferon-gamma from the immune system, iron starvation, nutrient deprivation, or co-infection with herpes viruses. When the stressor is removed, ABs can revert to normal RBs and ultimately produce infectious EBs.22The Journal of Infectious Diseases. Chlamydia trachomatis Persistence In Vitro: An Overview This reversible persistence is thought to play a role in chronic and recurrent infections, allowing the organism to hunker down during antibiotic treatment or immune attack and resume its cycle once conditions improve.
Immune Evasion Through Surface Chemistry
The outer surface of Chlamydia contributes directly to its ability to dodge immune detection. Lipopolysaccharide (LPS), a molecule found on the surface of most gram-negative bacteria, usually triggers a powerful inflammatory response when detected by host immune receptors. Chlamydial LPS, however, has an unusual structure that evades both the canonical and noncanonical innate immune sensing pathways. This structural adaptation may explain why chlamydial infections are so frequently asymptomatic: the immune system simply does not mount the alarm it would for a typical gram-negative pathogen.23PubMed Central. Chlamydia trachomatis Lipopolysaccharide Evades the Canonical and Noncanonical Inflammatory Pathways To Subvert Innate Immunity
Serovar Differences in Developmental Timing
Not all strains of C. trachomatis develop at the same pace. Genital serovars (D through K and L1 through L3) complete their developmental cycle in about 36 to 44 hours. Ocular serovars (A, B, and C), which cause trachoma, lag considerably behind. The difference comes from a longer lag phase at the beginning, encompassing both entry and initial EB-to-RB differentiation, and a slower generation time during the replicative phase.24The Journal of Infectious Diseases. Different Growth Rates of Chlamydia trachomatis Biovars Reflect Pathotype These timing differences likely reflect adaptation to different tissue environments. The columnar epithelium of the genital tract and the conjunctival epithelium of the eye present different challenges, and strains have evolved different developmental programs to match.
How the Local Environment Shapes Growth
The microenvironment of the host tissue also influences the developmental cycle independently of strain differences. Oxygen levels, for example, matter more than you might expect for an intracellular pathogen. Hypoxic conditions, which are physiologically relevant in the female genital tract, promote chlamydial growth. Measurements of inclusion-forming units, bacterial DNA quantity, and inclusion size all increase under low oxygen, with the effect mediated through the host cell’s PI3K-AKT signaling pathway and changes in the cell’s own metabolic balance.25PubMed. Hypoxia promotes Chlamydia trachomatis L2/434/Bu growth in immortal human epithelial cells via activation of the PI3K-AKT pathway and maintenance of a balanced NAD(+)/NADH ratio This means the bacterium may actually grow more efficiently in the relatively oxygen-poor environment where genital infections naturally occur, a wrinkle that standard laboratory culture conditions, which use ambient oxygen, do not fully capture.