Mycoplasma genitalium
Mycoplasma genitalium, often shortened to Mgen or MG, is one of the smallest known bacteria and a sexually transmitted infection most people have never heard of. It spreads through vaginal and anal sex and usually causes no symptoms, so it often goes undetected while quietly inflaming the urethra or cervix. In some people it leads to urethritis, cervicitis, or pelvic inflammatory disease. Here is what mycoplasma genitalium is, how it spreads, the problems it can cause, and how it is treated.
- Curable
- Yes
- Resistance-guided two-step regimen: doxycycline first, then azithromycin (if macrolide-sensitive) or moxifloxacin (if resistant or unknown)
- Macrolide resistance
- 50–62%
- In US STI clinic populations; single-dose azithromycin monotherapy now fails in the majority of cases
- Routine screening
- Not recommended
- CDC does not recommend screening asymptomatic people; test those with symptoms of NGU, cervicitis, or treatment failure
- Asymptomatic in women
- Frequent
- Cervicitis and PID can occur without symptoms, making the complication risk real even without a warning sign
Understanding mycoplasma genitalium
What is mycoplasma genitalium?
Mycoplasma genitalium is a slow-growing bacterium that lacks a cell wall entirely, the target that penicillins and cephalosporins attack, making the entire beta-lactam antibiotic class useless against it. This structural quirk, combined with a decade of widespread azithromycin use for urethritis and chlamydia, has created a resistance crisis: macrolide-resistance mutations have been detected in 50–62% of MG isolates at US STI clinics, which means the antibiotic that once cleared most NGU with a single dose now fails more often than it works.
The infection is classified as an emerging STI because it was only identified as a distinct pathogen in 1981 and added to CDC treatment guidelines relatively recently. Prevalence data are limited, MG is not a nationally notifiable infection, but it is estimated to cause 15–25% of non-chlamydial NGU in men, and it is increasingly implicated in recurrent or treatment-refractory cervicitis in women. Most infected people, particularly women, are asymptomatic; in men, persistent or recurring urethral symptoms after standard NGU treatment are the signature presentation.
The central problem in managing MG is that macrolide-resistance testing, the test that would tell a clinician which branch of treatment to use, is not commercially available in the United States. Clinicians must therefore decide whether to treat with the azithromycin arm (effective if the infection is macrolide-sensitive) or default directly to the moxifloxacin arm (effective but broader-spectrum). The CDC's 2021 guidelines endorse a two-step approach that starts with doxycycline to reduce the bacterial load before layering the second antibiotic, which improves cure rates regardless of resistance status.
Untreated, MG in women can ascend from the cervix to the fallopian tubes and uterus, causing pelvic inflammatory disease (PID) with roughly twofold increased risk of infertility and elevated risk of preterm delivery. In men, the main consequence is persistent or recurring NGU: uncomfortable and disruptive, and a signal to test for MG if standard doxycycline or azithromycin has already failed. Neither sex should delay evaluation when symptoms persist or recur after a treated episode of NGU or cervicitis.
Symptoms
What are the symptoms of mycoplasma genitalium?
The majority of people with MG, particularly women, have no symptoms at all. In studies, MG is detected in cervicitis without the patient reporting discharge or pelvic discomfort. This means complications (PID, infertility) can develop silently. Asymptomatic women are not routinely screened; the infection is found when symptoms prompt testing or when a partner is diagnosed. When symptoms occur in men, they typically develop 2–5 weeks after exposure. In women, symptoms of cervicitis or PID may be absent entirely, or so mild they are attributed to other causes. The lag between infection and clinical presentation (if it appears at all) means exposure history is often unhelpful in timing a diagnosis.
In men
- Urethral discharge: mucoid, watery, or purulent; may be subtle and only apparent when expressed
- Painful or burning urination (dysuria)
- Urethral itching or tingling, particularly in the morning
- Symptoms that recur weeks after completing a standard NGU antibiotic course, the hallmark of MG or macrolide-resistant MG
- No symptoms in some men, MG can be a silent urethral colonization
In women
- Often none, most women with MG are asymptomatic even when cervicitis is present on examination
- Abnormal vaginal discharge: watery or mucopurulent, often indistinguishable from chlamydia discharge
- Pelvic pain or lower abdominal cramping, when MG has ascended and caused PID
- Bleeding between periods or after sexual intercourse (intermenstrual or post-coital bleeding), from cervical inflammation
- Painful urination: less common than in men, can mimic a UTI
If you have been treated for NGU or cervicitis and symptoms return within weeks, MG, particularly a macrolide-resistant strain, should be specifically tested for. Standard NGU panels and point-of-care tests do not detect MG; a specific NAAT is required.
Left untreated
Why mycoplasma genitalium is worth catching early
Pelvic inflammatory disease (PID) in women
MG can ascend from the cervix through the endometrium to the fallopian tubes, causing PID with pelvic pain, fever, and uterine or adnexal tenderness. Subclinical or "silent" PID, with minimal symptoms, is particularly concerning because the inflammation and scarring proceed without prompting treatment. Repeated episodes compound the scarring risk.
Infertility and ectopic pregnancy risk
Studies show roughly a twofold increased risk of tubal-factor infertility in women with MG antibodies, comparable to the risk from untreated chlamydia. Fallopian tube scarring from MG-related PID also raises the risk of ectopic pregnancy, which can be life-threatening. Treatment before complications develop is the most effective prevention.
Preterm delivery
MG detected during pregnancy is associated with higher rates of preterm birth and preterm labor. Testing pregnant people with symptoms of cervicitis or recurrent urethritis is appropriate; treatment in pregnancy is limited by antibiotic safety concerns (moxifloxacin is contraindicated; azithromycin is used for macrolide-sensitive strains).
Persistent or recurrent urethritis in men
Repeated episodes of NGU, particularly post-gonorrhea or post-chlamydia, significantly impact quality of life and may signal an MG infection cycling between partners. Treatment failure with azithromycin alone is now the rule rather than the exception for MG; escalation to moxifloxacin is frequently required.
Treatment
How is mycoplasma genitalium treated?
Resistance-guided two-step regimen always starts with doxycycline (to reduce bacterial load), then layers a second antibiotic based on likely resistance: azithromycin for presumed macrolide-sensitive strains, moxifloxacin for macrolide-resistant or unknown strains. Single-dose azithromycin alone is no longer recommended.
Treat partners
All sex partners from the past 60 days should be evaluated and tested; presumptive treatment is recommended if the partner cannot be tested.
In pregnancy
Moxifloxacin is contraindicated in pregnancy. For macrolide-sensitive strains, azithromycin (after doxycycline) is acceptable. Consult an infectious disease specialist for macrolide-resistant MG in pregnancy.
Re-test after treatment
Test of cure is recommended 3–4 weeks after completing therapy in all patients, given the significant treatment-failure rate. Re-treat with the alternative step-2 arm if initial therapy fails.
Resistance note: 50–62% of MG strains in US STI clinic populations carry macrolide-resistance mutations, driven by years of azithromycin monotherapy. Because macrolide-resistance testing is not commercially available in the US, the two-step doxycycline-then-moxifloxacin approach is now the practical default in many clinical settings.
Prevention
How to prevent mycoplasma genitalium
-
Consistent condom use
Condoms used correctly and consistently reduce transmission risk. Because MG spreads through the same mucosal contact as chlamydia and gonorrhea, condom use at every act of sex with a partner of unknown status is the primary prevention measure.
-
Partner notification and concurrent treatment
Treating all recent partners at the same time prevents re-infection, the "ping-pong" cycle where treated and untreated partners re-infect each other repeatedly. Even asymptomatic partners should be evaluated and treated.
-
Avoid azithromycin monotherapy for NGU
Single-dose azithromycin given empirically for NGU selects for macrolide-resistant MG even in patients who test negative for MG, resistance emerges in the residual MG that was present below the detection threshold or acquired shortly after treatment. The current CDC recommendation is doxycycline 7 days as the first-line NGU regimen precisely because it has lower selection pressure for MG resistance.
Who is most at risk
Who is most at risk for mycoplasma genitalium?
- Men with recurrent or persistent NGU
- The most common clinical scenario prompting MG testing: standard NGU regimens fail MG at high rates, making recurrent urethritis after treatment the key flag.
- 15–25% of non-chlamydial NGU cases
- MSM, men who have sex with men
- Particularly at risk for rectal MG, which requires rectal swab testing and is frequently asymptomatic. Higher partner turnover rates also increase exposure probability.
- People with prior STI
- A history of other STIs signals the sexual exposures that also transmit MG, and the antibiotic courses used to treat chlamydia/NGU may have selected for MG resistance.
Stats & rates
How common is mycoplasma genitalium in the U.S.?
M. genitalium prevalence estimates range from 1–2% of sexually active adults in the general population to 15–25% of men with non-chlamydial NGU presenting to STI clinics. It disproportionately affects people with recurrent or treatment-refractory urethritis or cervicitis. Not a nationally reportable condition; no official US incidence figures are published.
- 50–62%
- Macrolide resistance rate in US STI clinic populations, the key driver of treatment failure
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Good to Know
Mycoplasma genitalium FAQs
Common questions about mycoplasma genitalium, answered.
What is Mycoplasma genitalium and why haven't I heard of it?
M. genitalium is a bacterium discovered in 1981 that is increasingly recognized as a significant cause of urethritis and cervicitis. It wasn't included in most STI guidelines until recently because it was hard to culture and tests weren't widely available. Now that NAAT testing exists, it's being found in 15–25% of men with non-chlamydial NGU. It's not yet a nationally reportable infection, so it doesn't appear in the CDC's headline statistics, but that doesn't mean it's rare.
Why does standard NGU treatment fail for MG?
Standard single-dose azithromycin was used for NGU for years, and M. genitalium was repeatedly exposed to it without being reliably cleared. This created strong selection pressure for resistant strains. 50–62% of MG in US STI clinic populations now carry macrolide-resistance mutations. A single 1 g azithromycin dose that cures chlamydia easily often does nothing to MG, while selecting for even more resistant bacteria.
Can MG cause infertility?
Yes, particularly in women. Studies show roughly a twofold increased risk of tubal-factor infertility in women with evidence of past MG infection, similar to the risk from untreated chlamydia. MG causes PID, and the fallopian tube scarring from repeated or untreated episodes is the mechanism. Treating MG early, especially when cervicitis or PID is present, is the most effective way to reduce this risk.
What is the two-step treatment and why does it start with doxycycline?
Doxycycline is given first (100 mg twice daily for 7 days) not because it cures MG on its own, it only clears 30–40% of MG infections alone, but because it significantly reduces the bacterial load before the second antibiotic is applied. A lower bacterial burden means fewer resistant organisms to mutate and escape when azithromycin or moxifloxacin is added, which meaningfully improves cure rates. It's the sequence that matters, not doxycycline alone.
How long does it take for symptoms to go away after treatment?
Discharge and painful urination typically begin to improve within a few days of starting doxycycline, though the full treatment course must be completed. The second-step antibiotic (azithromycin or moxifloxacin) should still be taken even if symptoms have resolved. A test of cure 3–4 weeks after finishing the full regimen is recommended for all patients given the significant failure rate, symptoms clearing is not the same as the infection clearing.
Medically Reviewed · Updated
Reviewed by Dr. Ngozi Adeyemi, MD, MPH · Infectious Disease & Epidemiology
Board-certified in infectious disease with a focus on STI epidemiology and public-health screening programs. Leads testing, diagnosis and the data-driven 'state of STDs' reporting. Our editorial guidelines →
2 Sources
Data & references
- CDC, Mycoplasma genitalium (STI Treatment Guidelines, 2021)https://www.cdc.gov/std/treatment-guidelines/mycoplasmagenitalium.htm
- NYSDOH/Johns Hopkins, M. genitalium Management (NCBI Bookshelf)https://www.ncbi.nlm.nih.gov/books/NBK583532/
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