Chlamydia is a genus of bacteria that must live and replicate inside the cells of a host organism, making them obligate intracellular pathogens. This dependence on the host cell is a defining characteristic, shaping their structure, life cycle, and success as disease-causing agents.
The Biphasic Developmental Cycle
Chlamydia possesses a life cycle that alternates between two distinct forms. This cycle begins when an infectious particle, the elementary body (EB), attaches to and enters a host cell. Once inside, the EB is enclosed within a protective membrane-bound vacuole called an inclusion. This inclusion separates the bacterium from the host cell’s cytoplasm, creating a safe environment for development.
Within this protected space, the EB develops into a larger, non-infectious form called the reticulate body (RB). The RB is the replicative stage of the bacterium, and it begins to divide, using nutrients and energy from the host cell to multiply. The developmental cycle is asynchronous, meaning different bacteria within the same cell can be at different stages.
After a period of rapid replication, the RBs begin to reorganize and condense back into the smaller, denser EBs. The inclusion becomes filled with these newly formed infectious particles. The cycle, which takes approximately 36 to 72 hours to complete, culminates in the release of the new EBs from the host cell to infect neighboring cells.
Structure of the Elementary Body
The elementary body (EB) is the form of Chlamydia adapted for survival outside of a host cell. It is small, measuring between 0.25 and 0.4 micrometers in diameter, and has a dense, spore-like structure. Its primary function is to initiate infection by attaching to and entering a new host cell.
The resilience of the EB is largely due to its rigid outer membrane. This stability is achieved through extensive cross-linking of proteins rich in the amino acid cysteine. Proteins like the major outer membrane protein (MOMP), OmcA, and OmcB form strong disulfide bonds, giving the EB the structural integrity needed to withstand the harsh conditions of the extracellular environment.
Structure of the Reticulate Body
In contrast to the dense elementary body, the reticulate body (RB) is structured for replication within the host cell. It is larger and less dense, measuring between 0.5 and 1.5 micrometers in diameter. Inside the protective inclusion, it becomes metabolically active, dedicating its resources to growth and division.
The RB’s structure reflects its function as the replicative form of the bacteria. Its cell membrane is more fragile because it lacks the extensive disulfide bond cross-linking found in the EB. This less rigid structure facilitates the transport of nutrients from the host cell’s cytoplasm into the RB, providing the necessary building blocks and energy for multiplication.
The Atypical Cell Wall
A defining feature of all Chlamydia species, in both their elementary and reticulate forms, is their unusual cell wall. Although classified as gram-negative bacteria, they are unique in that they lack a peptidoglycan layer. This is an uncommon trait among bacteria.
The chlamydial outer membrane contains several components. The Major Outer Membrane Protein (MOMP) is a surface protein that functions as an adhesin, helping the bacterium attach to host cells. The cell wall also contains lipopolysaccharide (LPS), a molecule common to gram-negative bacteria that can trigger inflammatory responses in the host.
The absence of peptidoglycan has clinical implications. Many common antibiotics, such as penicillin, work by disrupting the synthesis of peptidoglycan, thereby weakening the bacterial cell wall. Since Chlamydia does not have this layer, it is naturally resistant to these types of antibiotics, necessitating different treatment strategies for infections.