Mgen isn't in most routine STI panels — yet it's increasingly common and increasingly resistant. Here's why it deserves more attention.

Mycoplasma Genitalium (Mgen) — The Emerging STI

Mycoplasma genitalium (Mgen) is a small bacterium that's been around for decades but only formally recognized as an STI in the past 15-20 years. It causes urethritis in men, cervicitis and PID in women — and antibiotic resistance is rising fast.

Most STI panels don't test for it. Most clinicians don't reflexively test. This is changing, but slowly.

Here's the practical picture.

The short answer

What it is: A small bacterium (no cell wall) similar to but distinct from chlamydia and gonorrhea
Prevalence: Roughly 1-3% of general population; higher in sexually active populations and STI clinic patients
Symptoms: Often asymptomatic. When symptomatic: urethritis in men, cervicitis and PID in women
Testing: Not in most routine STI panels; requires specific NAAT test
Treatment: Doxycycline followed by moxifloxacin (resistance is increasing)
Concerning: Antibiotic resistance to first-line drugs is rising rapidly

What Mgen looks like

In men

Urethritis — most common presentation
- Mild-to-moderate urethral discharge
- Burning with urination
- Often called "non-gonococcal urethritis" (NGU) historically
Often asymptomatic
Can persist for months without symptoms

In women

Cervicitis
- Increased vaginal discharge
- Bleeding between periods or after sex
- Often subtle
PID (pelvic inflammatory disease)
Often asymptomatic

Rectal and pharyngeal infection

Rectal infection in MSM — sometimes symptomatic
Pharyngeal infection — rare, poorly characterized

Why most STI panels miss it

Mgen wasn't formally classified as a pathogenic organism until 1980, and the first commercial PCR test in the US was only approved in 2019. Reasons it's still missed:

Slow uptake of testing by labs
Most "STI panels" run a fixed bundle (HIV, syphilis, chlamydia, gonorrhea) — Mgen isn't standard
Cost / lab capacity considerations
Asymptomatic cases not detected without testing

But: about 15-30% of "persistent urethritis" or "treatment-resistant chlamydia" cases turn out to be Mgen.

Who should be tested for Mgen

CDC recommends testing for Mgen in specific situations:

Persistent or recurrent urethritis in men after treatment for chlamydia/gonorrhea
Persistent or recurrent cervicitis in women
PID if standard treatment fails
Symptomatic patients with negative chlamydia and gonorrhea tests
Sexual partners of confirmed Mgen cases

CDC currently does NOT recommend routine Mgen screening in asymptomatic people — but this may evolve.

Testing

NAAT (PCR)

Same kind of test as for chlamydia/gonorrhea but specific to Mgen
Specimens: urine (men), vaginal swab (women), rectal swab if applicable
Available through some commercial labs (Aptima M. genitalium test from Hologic, etc.)
Results in 1-7 days

What to ask for

"Can you test me for Mycoplasma genitalium? I'm having persistent symptoms."

If your clinic doesn't have it, sexual health clinics and specialty labs often do.

Treatment — and the resistance problem

Mgen treatment has gotten complicated because of resistance:

Current recommended regimen (CDC, 2021 guidelines)

First: Doxycycline 100 mg twice daily for 7 days
Then: Moxifloxacin 400 mg daily for 7 days (if resistance testing not available)

Or if susceptibility testing is available:

Macrolide-susceptible: Doxycycline + azithromycin
Macrolide-resistant: Doxycycline + moxifloxacin

Why two drugs?

Doxycycline reduces bacterial load but doesn't reliably eradicate Mgen
The second agent (azithromycin or moxifloxacin) is the curative drug
Sequential treatment helps overcome resistance

Resistance landscape

Macrolide resistance (azithromycin): 50-80% in many regions
Fluoroquinolone resistance (moxifloxacin): rising; 10-30% in some studies
Some "extensively resistant" Mgen is essentially untreatable with current regimens

If standard treatment fails

Susceptibility testing if available
Pristinamycin or other rescue regimens
Infectious disease consultation

After treatment

Test of cure: Recommended at 3-4 weeks for Mgen
Partner treatment is required
Avoid sex for 7 days after treatment AND until partner is treated

Partner notification

Sex partners from the past 60 days should be:
- Tested
- Treated empirically if testing isn't readily available
All partners need treatment together to prevent reinfection

Connection to other STIs

Mgen often co-occurs with:

Chlamydia
Gonorrhea
Trichomoniasis

Mgen has been associated with:

Increased HIV acquisition risk (modest)
Adverse pregnancy outcomes (preterm birth, spontaneous abortion, possibly)
Infertility in women (some evidence)

What this means for testing strategy

If you have urethritis or cervicitis symptoms and:

First test (chlamydia + gonorrhea) is negative: Ask about Mgen testing
First test positive, treatment given, symptoms persist: Ask about Mgen testing
You have ongoing unprotected sex with multiple partners: Annual Mgen screening might be reasonable in high-prevalence settings

Common confusions

"Is Mgen the same as Mycoplasma hominis or Ureaplasma?" No — these are different organisms. Mgen specifically is the well-validated pathogen. M. hominis and Ureaplasma can also cause genital symptoms but are less well-defined pathogens (often part of normal flora).

"Is Mgen serious?" It causes real symptoms and complications (PID, etc.), and resistance is concerning. But individual cases are usually treatable.

"Will doxycycline I'm taking for acne prevent Mgen?" No — acne dosing is sub-therapeutic for Mgen treatment. Therapeutic Mgen treatment requires specific dosing and a second drug.

"Is Mgen a 'new' STI?" Not new — recognized since 1980 — but newly testable and newly appreciated.

Bottom line

Mgen is:

Real STI with real consequences (urethritis, cervicitis, PID)
Under-tested — not in most basic panels
Resistance-prone — current treatment is complicated and may fail
Treatable with appropriate drugs and resistance testing when available
Worth asking about if you have persistent or recurrent symptoms that didn't resolve with standard treatment

The single most important point: if you've been treated for "chlamydia" or "NGU" and symptoms persist, ask about Mgen specifically. Most STI panels don't catch it.