Chlamydia and Mycoplasma genitalium are both bacteria that infect the urethra and cervix, and they cause overlapping symptoms like discharge, burning, and pelvic pain. But they're different organisms with different treatments. Chlamydia is far more common and reliably cleared by doxycycline; M. genitalium is an emerging, drug-resistant cause of stubborn urethritis that needs a specific test and a two-step regimen. Ureaplasma is a related organism rarely treated as a true pathogen.
Mycoplasma genitalium
Chlamydia trachomatis
| Item | Value |
|---|---|
| Mycoplasma genitalium | curable — Mycoplasma genitalium |
| Chlamydia | curable — Chlamydia trachomatis |
What each one is
Mycoplasma genitalium
Mycoplasma genitalium is a tiny bacterium with no cell wall. That structural quirk matters enormously for treatment, because beta-lactam antibiotics (penicillins and cephalosporins) attack the cell wall and therefore do nothing against it CDC, 2021. It's an increasingly recognized cause of non-gonococcal urethritis in men and cervicitis in women, and its defining clinical problem is antibiotic resistance, especially to macrolides like azithromycin. Building awareness is key in mg diagnosis and treatment, because it's often missed on standard STI panels.
Chlamydia
Chlamydia is caused by the bacterium Chlamydia trachomatis; most US genital infections come from serovars D through K CDC Fact Sheet. It's the most-reported STI in the country, roughly 1.65 million cases in 2023, about 492 per 100,000 people, a rate that's held fairly steady across 2020 through 2023 CDC AtlasPlus, 2023. For a fuller picture of how it spreads and what it does long-term, see our overview of chlamydia.
Symptoms compared
Both infections are quiet far more often than people expect. With chlamydia, roughly three quarters of infected women and half of infected men have no symptoms at all, so it spreads efficiently. When symptoms do appear in women, they include abnormal vaginal discharge and burning on urination; if the infection climbs into the upper reproductive tract, women may notice lower abdominal or low-back pain, fever, pain during intercourse, and bleeding between periods.
Mycoplasma genitalium produces a similar picture: in men, urethral discharge and dysuria (painful urination) that's often persistent or keeps coming back after treatment; in women, cervicitis (inflammation of the cervix) that can progress to pelvic inflammatory disease. Like chlamydia, it's frequently asymptomatic, particularly in women. The symptoms overlap too much to tell these apart by feel, so a test is what settles it.
How to tell them apart
Clinicians don't separate these by symptom. They separate them by test result and clinical pattern. A few discriminating features guide the thinking:
- Recurrent or treatment-resistant urethritis points toward M. genitalium. If a man was treated for non-gonococcal urethritis and the discharge or burning returns, M. genitalium is a leading suspect.
- Standard STI panels usually catch chlamydia but may not include M. genitalium. A negative routine panel doesn't rule out M. genitalium unless that specific test was ordered.
- Response to doxycycline differs. Chlamydia is reliably cured by the doxycycline course; M. genitalium often is not, which is why it needs a second drug.
- M. genitalium has no cell wall, so any penicillin or cephalosporin a patient may have taken for something else has no effect on it. That's a clue when symptoms persist despite prior antibiotics.
Side-by-side comparison
| Feature | Chlamydia (C. trachomatis) | Mycoplasma genitalium |
|---|---|---|
| How common | Most-reported US STI (about 1.65 million cases, 2023) | Emerging; under-tested and underdiagnosed |
| Typical illness | Urethritis, cervicitis; often silent | Non-gonococcal urethritis, cervicitis; often persistent/recurrent |
| Cell wall | Atypical; beta-lactams not used | No cell wall — beta-lactams useless |
| Best test | NAAT (urine or swab) | FDA-cleared NAAT (urine or swab) |
| First-line treatment | Doxycycline (100 mg twice daily for 7 days) | Resistance-guided two-step (doxycycline first, then azithromycin or moxifloxacin) |
| Main treatment challenge | Partner treatment and reinfection | Macrolide resistance |
Testing
Nucleic acid amplification testing (NAAT) is the recommended method for both organisms. It detects the bacterium's genetic material and is highly sensitive CDC. For chlamydia, NAAT is optimal for genital and extragenital (throat and rectal) sites. For M. genitalium, FDA-cleared NAATs run on urine or on urethral, penile-meatal, endocervical, or vaginal swabs; current guidance is to test men with recurrent non-gonococcal urethritis and women with recurrent cervicitis rather than everyone NYSDOH/Johns Hopkins.
There's a real-world gap with M. genitalium. Macrolide-resistance testing should guide therapy, but it isn't commercially available in the US, so clinicians often have to treat as if resistance is present. In practice, getting tested means giving a urine sample, collecting your own swab, or a quick exam depending on which infection is suspected, and it's free or low-cost at health departments, Planned Parenthood, and Title X clinics. You can review your options and get tested, and if you're counting days since a possible exposure, see when to test after exposure so you don't test too early for a reliable result.
Treatment compared
Chlamydia treatment is straightforward. The preferred regimen is doxycycline 100 mg orally twice daily for seven days. This replaced the older single dose of azithromycin after a randomized trial showed doxycycline cleared rectal chlamydia far more reliably, 100% cure with doxycycline versus 74% with azithromycin, which drove the 2021 guidelines to make doxycycline first-line Rectal CT RCT.
Mycoplasma genitalium is harder. It now requires a resistance-guided, two-step approach that always starts with doxycycline to lower the bacterial load. If the strain is macrolide-susceptible, that's followed by azithromycin (1 g once, then 500 mg daily for three days). If it's macrolide-resistant, or resistance testing simply isn't available, doxycycline is followed by moxifloxacin 400 mg daily for seven days MMWR RR-4, 2021. The old single azithromycin dose fails so often because macrolide-resistance mutations now exceed 50% in many areas, and over 62% in one US STI clinic, so a specific M. genitalium test matters for choosing the right second drug.
For either infection, partners need treatment too. In a landmark randomized trial, giving patients medication to deliver to their partners (expedited partner therapy) cut persistent or repeat infection, with the biggest benefit for gonorrhea (3% vs 11%) Golden et al., NEJM. When partners go untreated, people get reinfected within months.
Can you have more than one at once?
Yes. Because these bacteria share the same sexual routes and the same target tissues, co-infection is common; someone can carry chlamydia and M. genitalium together, sometimes alongside gonorrhea. That's one argument for comprehensive testing rather than treating a single suspected bug. If only chlamydia is found and treated but M. genitalium is also present, symptoms can persist and look like "treatment failure" when it's really an untreated second infection.
A note on Ureaplasma
Ureaplasma species are relatives of M. genitalium that also lack a cell wall and can be detected in the genital tract. Unlike chlamydia and M. genitalium, Ureaplasma is frequently part of normal genital flora and is not routinely treated as a true sexually transmitted pathogen in most people. It isn't part of standard screening, and a positive result alone, without symptoms or another explanation, usually doesn't warrant antibiotics. If you've been told you have Ureaplasma, talk with your clinician about whether it's actually causing your symptoms.
When to see a clinician
See a clinician if you have new or persistent genital discharge, burning with urination, pelvic or testicular pain, bleeding between periods, or pain during intercourse, and especially if symptoms come back after you were treated, the classic flag for M. genitalium. Anyone with a new partner or a partner who tested positive should be evaluated even without symptoms, since both infections are silent so often. Untreated chlamydia and M. genitalium can both lead to pelvic inflammatory disease (infection of the uterus, tubes, and ovaries that can cause infertility and chronic pain), so don't wait out persistent symptoms.