Chlamydia is classified as a gram-negative bacterium. Its cell wall has the signature two-membrane structure of gram-negative organisms, with an outer membrane containing lipopolysaccharide (LPS) and an inner cytoplasmic membrane. But chlamydia is far from a typical gram-negative bug, and the reasons why have puzzled microbiologists for decades.
Why Chlamydia Is Gram-Negative
The gram stain divides bacteria into two broad camps based on their cell wall structure. Gram-positive bacteria have a thick outer layer of peptidoglycan, a mesh-like material that traps the purple dye used in the staining process. Gram-negative bacteria have a much thinner peptidoglycan layer sandwiched between two membranes, so the purple dye washes out and they pick up a pink counterstain instead.
Chlamydia fits the gram-negative blueprint. It has the characteristic double membrane, and its outer membrane contains LPS, the molecule that gram-negative bacteria are known for. In most gram-negative species, LPS is a powerful trigger of the immune system. Chlamydia’s version, however, has an unusual fatty acid structure that dampens immune signaling compared to a classic gram-negative bacterium like E. coli. Researchers believe this modified LPS is part of why so many chlamydia infections produce no symptoms at all.
The “Chlamydial Anomaly”
For years, chlamydia presented a paradox. It behaved like a gram-negative bacterium in most respects, yet scientists could not detect peptidoglycan in its cell wall. This was strange because chlamydia is sensitive to penicillin-type antibiotics, which work specifically by disrupting peptidoglycan production. It also carries the genes needed to build peptidoglycan. The contradiction was so persistent it earned its own name: the “chlamydial anomaly,” a term coined by microbiologist James Moulder.
One hypothesis proposed that chlamydia made a stripped-down, sugar-free version of peptidoglycan. But a landmark study using mass spectrometry finally resolved the debate. Researchers detected actual peptidoglycan fragments in pathogenic chlamydia species, complete with the sugar chains found in conventional peptidoglycan. The conclusion: chlamydia does produce peptidoglycan, just in such small quantities that earlier methods couldn’t pick it up. It may also restrict production to certain phases of its life cycle.
An Unusual Life Cycle
Chlamydia is an obligate intracellular bacterium, meaning it cannot survive or reproduce on its own outside a host cell. This is one of the features that makes it so different from the free-living gram-negative bacteria most people learn about in biology class. It cycles between two distinct forms.
The elementary body (EB) is the infectious form. It’s small, tough, and metabolically inactive, with a tightly compacted genome wrapped around histone-like proteins. Think of it as the bacterium’s survival capsule. The EB attaches to a host cell, triggers its own uptake, and then transforms into the reticulate body (RB), which is larger, metabolically active, and capable of dividing. The RB multiplies inside a specialized compartment called an inclusion, then eventually converts back into elementary bodies that are released to infect new cells. This two-phase cycle is why standard lab culture methods don’t work for chlamydia and why gram staining is useless as a diagnostic tool.
Why Gram Staining Can’t Diagnose Chlamydia
Even though chlamydia is technically gram-negative, you won’t find it on a gram stain in a clinical setting. The bacteria are too small, live exclusively inside cells, and don’t grow on the standard agar plates used for other bacterial infections. A gram stain of a vaginal or urethral swab might show other organisms but will miss chlamydia entirely.
The gold standard for diagnosis is a nucleic acid amplification test (NAAT), which detects chlamydia’s genetic material. These tests are highly accurate, with pooled sensitivity around 94% and specificity of 99% across cervical swabs, vaginal swabs, urine samples, and anorectal swabs. This high performance from simple specimens like urine is one reason screening has become so accessible.
Why Screening Matters
Chlamydia is the most common bacterial sexually transmitted infection in the world, with an estimated 128.5 million new cases among adults aged 15 to 49 in 2020 alone. A major reason it spreads so effectively is its stealth. Roughly 75% of women and 50% of men with chlamydia have no symptoms whatsoever. You can carry and transmit the infection for months without knowing.
Left untreated, about 10 to 15% of women with chlamydia will develop pelvic inflammatory disease (PID), an infection of the uterus, fallopian tubes, or ovaries. PID is the leading cause of preventable infertility in women in the United States, responsible for over 100,000 cases of infertility each year due to scarring and damage to the fallopian tubes. In men, untreated chlamydia can cause painful inflammation of the reproductive tract, though serious complications are less common.
How Chlamydia Is Treated
Because chlamydia has the machinery for peptidoglycan production, antibiotics that target bacterial protein synthesis or DNA replication are the primary treatment. The CDC’s current recommended regimen is doxycycline taken twice daily for seven days. A once-daily delayed-release formulation is equally effective. The main alternative is a single oral dose of azithromycin, though doxycycline has become the preferred first-line option.
Treatment is straightforward and highly effective when completed as directed. Sexual partners need to be treated at the same time to prevent reinfection, and retesting about three months after treatment is recommended since reinfection rates are high.