Mycoplasma genitalium (Mgen) and chlamydia are both bacterial infections that cause urethritis in men and cervicitis in women, and they feel almost identical. They part ways at treatment. Chlamydia clears with a short doxycycline course, while Mgen is driven by antibiotic resistance and needs a resistance-guided, two-step regimen. Only a test tells them apart.
Mycoplasma genitalium
Chlamydia trachomatis
| Item | Value |
|---|---|
| Mycoplasma genitalium | curable — Mycoplasma genitalium |
| Chlamydia | curable — Chlamydia trachomatis |
What each infection is
Mycoplasma genitalium
Mycoplasma genitalium is a bacterium with no cell wall. That matters clinically because beta-lactam antibiotics — penicillins and cephalosporins — kill bacteria by attacking the cell wall, so they're useless here CDC Mgen guidelines. It's an emerging cause of non-gonococcal urethritis (NGU) in men and cervicitis in women, and its central problem is antibiotic resistance, especially to the macrolide class (azithromycin).
Chlamydia
Chlamydia is caused by the bacterium Chlamydia trachomatis; most US genital infections come from the strains known as serovars D–K CDC fact sheet. It's the most-reported STI in the country, with about 1.65 million cases in 2023 (roughly 492 per 100,000 people), a figure that has stayed roughly flat across 2020–2023 CDC AtlasPlus, 2023. Unlike Mgen, chlamydia is routinely screened for and well understood.
Symptoms compared
Both infections are quiet. Chlamydia is famously 'silent': roughly three quarters of infected women and half of infected men have no symptoms at all. Mgen is also frequently asymptomatic, particularly in women. When symptoms do appear, they overlap heavily.
- In men, both cause urethritis: discharge from the penis and dysuria (burning with urination). Mgen urethritis is more likely to be persistent or to recur after treatment.
- In women, both cause cervicitis (inflammation of the cervix) and can produce abnormal vaginal discharge and burning on urination.
- If chlamydia spreads upward, women may notice lower abdominal or low-back pain, fever, pain during intercourse, and bleeding between periods — signs of pelvic inflammatory disease.
- Mgen cervicitis can likewise progress to PID (pelvic inflammatory disease, an infection of the upper reproductive tract that can scar the fallopian tubes and threaten fertility).
How to tell them apart
You can't tell by feel. The discharge, the burning, and the timing don't reliably point to one over the other, so a laboratory test settles it. What pushes a clinician toward Mgen is a pattern: urethritis or cervicitis that keeps coming back, or that didn't clear after a standard chlamydia regimen. That recurrence points to macrolide resistance, and Mgen is often the diagnosis hiding behind 'treatment-resistant' symptoms.
Mgen vs chlamydia at a glance
| Mycoplasma genitalium | Chlamydia | |
|---|---|---|
| Organism | Bacterium with no cell wall | Chlamydia trachomatis (serovars D–K) |
| Routine screening | No — tested when symptoms recur | Yes — standard STI screening |
| Typical symptoms | NGU, cervicitis; often persistent/recurrent | Discharge, burning; often silent |
| Best test | FDA-cleared NAAT (specific Mgen assay) | NAAT |
| First-line treatment | Two-step: doxycycline, then azithromycin or moxifloxacin (resistance-guided) | Doxycycline, single 7-day course |
| Main challenge | Macrolide resistance | Reinfection from untreated partners |
Testing
For both infections, the nucleic acid amplification test (NAAT) is the recommended method — it detects the organism's genetic material and is the optimal approach for genital and extragenital sites for chlamydia CDC chlamydia guidelines. Mgen has its own FDA-cleared NAAT that can run on a urine sample or a urethral, penile-meatal, endocervical, or vaginal swab. In practice, testing is straightforward: a urine cup, a self-collected swab, or a quick exam depending on which infection is suspected. It's often free or low-cost at health departments, Planned Parenthood, and Title X clinics. You can also get tested through a confidential service, and if you're trying to time things right after a possible exposure, here's when to test after exposure.
One caveat unique to Mgen: macrolide-resistance testing should guide therapy, but that resistance assay is not yet commercially available in the United States. That gap is why a specific M. genitalium test, not a standard STI panel, matters, and why clinicians often have to treat as if resistance is present.
Treatment compared
Chlamydia is the simpler of the two. The preferred regimen is doxycycline 100 mg by mouth twice daily for 7 days CDC STI Treatment Guidelines, 2021. Doxycycline became first-line in the 2021 guidelines partly because it clears rectal chlamydia far better than the old single dose of azithromycin — a randomized trial found a 100% cure rate with doxycycline versus 74% with azithromycin rectal CT RCT.
Mgen is more involved. Treatment is resistance-guided and always starts with doxycycline 100 mg twice daily for 7 days to lower the bacterial load first. The second step depends on resistance. If the strain is macrolide-susceptible, doxycycline is followed by azithromycin (1 g once, then 500 mg daily for 3 days). If it's macrolide-resistant — or if resistance testing isn't available — doxycycline is followed by moxifloxacin 400 mg daily for 7 days NYSDOH/Johns Hopkins guideline. The caution is warranted: macrolide-resistance mutations now exceed 50% in many areas — over 62% in one US STI clinic — so the old single azithromycin dose frequently fails. Because diagnosis and the right drug sequence are so easy to get wrong, awareness is key in mg diagnosis and treatment.
Can you have both at once?
Yes. Co-infection is common because the two share the same routes and risk factors, and because both can sit silently. A clinician treating persistent urethritis may test for Mgen even when chlamydia was already found and treated, because the lingering symptoms may be a second organism rather than a chlamydia treatment failure. Partner treatment matters for both. Giving patients medication to pass to their partners (expedited partner therapy) measurably cut persistent or repeat infection in a landmark trial, with the largest benefit seen for gonorrhea (3% vs 11%) Golden et al., NEJM. If symptoms return after treatment, don't assume failure — think about chlamydia reinfection from an untreated partner.
When to see a clinician
See a clinician promptly if you have penile or vaginal discharge, burning with urination, pelvic or testicular pain, bleeding between periods, or pain during sex — or if you've had a partner diagnosed with either infection. Get evaluated again if symptoms persist or recur after a course of antibiotics, since recurrent urethritis or cervicitis is the classic signal to test specifically for Mgen. Fever with lower abdominal pain in a woman is a red flag for PID and warrants same-week care.