Ureaplasma and Mycoplasma: What You Need to Know

Ureaplasma and Mycoplasma are among the smallest bacteria capable of independent life, and they share a feature that makes them genuinely unusual: they have no cell wall. That single biological quirk shapes almost everything about them, from which antibiotics work against them to why they are so difficult to pin down clinically. Some species live quietly in the urogenital tract of healthy people and never cause trouble. Others are firmly established causes of sexually transmitted infections, pneumonia, pregnancy complications, and fertility problems. The challenge for both patients and doctors is telling the harmless carriers apart from the ones doing real damage.

What Makes These Bacteria Different

Most bacteria are surrounded by a rigid cell wall, and many of the antibiotics you have heard of work by attacking that wall. Ureaplasma and Mycoplasma species lack this structure entirely. That means they are naturally immune to penicillin, amoxicillin, and every other beta-lactam antibiotic, along with several other drug classes that target the cell wall.¹ They also cannot make folic acid on their own, which renders them resistant to sulfonamides and trimethoprim, drugs commonly used for urinary tract infections.² This leaves doctors with a narrower set of weapons, mainly macrolides (like azithromycin), tetracyclines (like doxycycline), and fluoroquinolones (like moxifloxacin).

Beyond antibiotic resistance, these bacteria are tiny. They have among the smallest genomes of any self-replicating organism, meaning they rely heavily on their host for nutrients. They are parasitic in the truest sense: they attach to the surface of human cells, siphon off what they need, and can persist for long periods without killing the cell outright. This makes them capable of causing chronic, low-grade infections that fly under the radar.

The Species That Matter Clinically

The names “Mycoplasma” and “Ureaplasma” each refer to a genus containing multiple species, but only a handful cause problems in humans. Understanding which species does what is important, because lumping them together leads to confusion and overtesting.

Mycoplasma genitalium: A recognized sexually transmitted pathogen. It causes urethritis in men, cervicitis in women, and is increasingly linked to pelvic inflammatory disease and fertility problems.
Mycoplasma hominis: Found in the genital tract of many healthy adults. It sometimes contributes to bacterial vaginosis and postpartum fever, and has been associated with reduced sperm quality, but it is not routinely considered a standalone pathogen.
Mycoplasma pneumoniae: An entirely different beast. This one targets the lungs, causing bronchitis and a mild form of pneumonia sometimes called “walking pneumonia.” It is spread through respiratory droplets, not sexual contact.
Ureaplasma urealyticum: Linked to urethritis, pregnancy complications, and effects on sperm quality. More likely to cause disease than its close relative.
Ureaplasma parvum: The most commonly detected Ureaplasma species. Found in roughly a quarter of healthy young men in one study, it is usually a harmless colonizer, though it can cause problems during pregnancy.

The key distinction is between M. genitalium, which is almost always considered pathogenic when detected, and the Ureaplasma species and M. hominis, which can be found in perfectly healthy people and only sometimes cross the line into causing disease.

How Common Is Colonization in Healthy People

This is the part that trips people up. If you get tested for these organisms, there is a real chance you will test positive even if nothing is wrong. A study of asymptomatic, healthy young men found Ureaplasma parvum in 23% of them, Ureaplasma urealyticum in 12%, Mycoplasma hominis in 4%, and Mycoplasma genitalium in only 1%.³ In women, colonization rates for Ureaplasma species tend to be even higher, particularly in sexually active individuals.

A positive test result for Ureaplasma or M. hominis does not automatically mean you have an infection that requires treatment. The European STI Guidelines Editorial Board has been blunt about this: routine testing and treatment of asymptomatic people for M. hominis, U. urealyticum, and U. parvum are not recommended, because asymptomatic carriage is common and most carriers never develop disease.⁴ Testing everyone and treating every positive result drives up antibiotic resistance without clear benefit.

Mycoplasma Genitalium as a Sexually Transmitted Infection

M. genitalium stands apart from the other species because it behaves like a classic sexually transmitted infection. In men, it is strongly associated with urethritis, and symptomatic urethritis is more common than asymptomatic carriage when the bacterium is present.⁵ In women, it is linked to cervicitis and can climb into the upper reproductive tract.

Transmission rates between sexual partners are high. In one study, about 56% of male partners of women infected with M. genitalium were also infected, a rate comparable to chlamydia concordance.⁶ A separate study found that roughly 38% of male partners and 45% of female partners of infected index patients tested positive.⁵ These figures make partner notification and treatment important, just as they would be for chlamydia or gonorrhea.

One complication: M. genitalium and chlamydia do not produce dramatically different symptoms in women, making it difficult to distinguish between them based on symptoms alone. When a woman has cervicitis or urethritis and standard chlamydia and gonorrhea tests come back negative, M. genitalium is a leading suspect.

Reproductive Health and Fertility

Both Mycoplasma and Ureaplasma species have been linked to fertility problems in men and women, though the strength of the evidence varies by species.

For women, the most concerning organism is M. genitalium. It is an emerging cause of pelvic inflammatory disease independent of chlamydia and gonorrhea, with one prospective study suggesting over a thirteenfold increased risk of endometritis in infected women.⁷ Because pelvic inflammatory disease can scar the fallopian tubes, it raises the risk of tubal factor infertility. Women with tubal factor infertility have been found to carry elevated antibodies against M. genitalium, suggesting that past infections may have caused permanent damage even after the bacteria cleared.⁸

For men, the evidence points to Ureaplasma urealyticum and Mycoplasma hominis affecting sperm quality. Men with U. urealyticum in their semen showed lower sperm concentrations, reduced forward motility, and more abnormal sperm shapes compared to uninfected men.⁹ A meta-analysis of 11 studies found that infertile men harboring M. hominis had significantly lower sperm concentration, progressive motility, viability, and normal morphology than uninfected men.¹⁰ U. urealyticum was also detected more frequently in infertile men than in fertile men, and concordance of U. urealyticum between partners was higher in infertile couples (about 32%) than in fertile couples (about 12.5%).¹¹

Whether treating these infections in men actually improves fertility outcomes is less clear-cut. The associations are there, but controlled treatment trials are still limited. For couples struggling with unexplained infertility, testing for Ureaplasma and Mycoplasma species in both partners is increasingly part of the workup at fertility clinics, even if the decision to treat remains case-by-case.

Pregnancy and Newborn Complications

Ureaplasma species take on a different level of clinical significance during pregnancy. There is growing evidence that intrauterine infection with these organisms is a meaningful risk factor for preterm birth, and Ureaplasma parvum appears to be the most common bacterium found in intra-amniotic infections.¹² Animal models have confirmed a causal link: inoculating the amniotic cavity with Ureaplasma species triggers preterm labor, and the bacteria can be detected in the placenta, fetal membranes, and reproductive tissues, provoking local inflammation.

For extremely premature infants, Ureaplasma colonization is associated with a higher rate of bronchopulmonary dysplasia, a chronic lung disease of prematurity. One study found that the incidence of bronchopulmonary dysplasia was about 25% in Ureaplasma-positive preterm infants compared to 6% in Ureaplasma-negative infants.¹³ The proposed mechanism involves a sustained inflammatory response in the immature lung that disrupts normal development of the air sacs and stimulates excessive scar-tissue formation.¹⁴ Ureaplasma infection has also been associated with intraventricular hemorrhage in preterm newborns.¹⁵

The clinical question of whether antibiotic treatment of Ureaplasma during pregnancy prevents these outcomes is still being worked out. Animal data suggest that clarithromycin given after intra-amniotic Ureaplasma inoculation can prevent preterm birth and neonatal death,¹² but translating animal findings to standard prenatal care requires more clinical trials. For now, screening and treatment decisions in pregnancy tend to be guided by obstetric history and symptoms rather than blanket testing.

Mycoplasma Pneumoniae and the Respiratory Side

Mycoplasma pneumoniae is the odd one out in this family. It has nothing to do with the urogenital tract. Instead, it lives in the airways and is a major cause of community-acquired pneumonia, especially in children, teenagers, and young adults.¹⁶ The pneumonia it causes tends to be milder than typical bacterial pneumonia, which is why it earned the nickname “walking pneumonia,” but it can still make people genuinely miserable with weeks of coughing, fatigue, and fever.

What makes M. pneumoniae particularly interesting is how much damage it can do beyond the lungs. The severity of the lung disease seems tied to the strength of the immune response rather than the bacteria themselves, and that same immune overreaction can spill into other organ systems.¹⁷ Extrapulmonary complications include skin rashes, liver dysfunction, pancreatitis, and a range of neurological problems from encephalitis to acute cerebellar ataxia.¹⁸ Joint inflammation can also occur: reactive arthritis following M. pneumoniae infection has been documented, and in at least one case it progressed to chronic spondyloarthropathy in a genetically susceptible patient.¹⁹ These complications are uncommon, but they highlight that M. pneumoniae can affect virtually any organ system.

Why These Bacteria Are Hard to Eliminate

Beyond their built-in resistance to cell-wall-targeting antibiotics, Mycoplasma and Ureaplasma species have another trick: they are exceptionally good at evading the immune system. Their outer surfaces are studded with lipoproteins that provoke a strong immune response, but these surface proteins can change their structure through a process called antigenic variation.²⁰ Essentially, the bacteria shuffle the appearance of their outer coat so that by the time your immune system mounts a defense against one version, the bacteria are already wearing a different one.²¹ This creates a moving target that allows chronic, persistent infections to take hold.

And then there is the growing problem of antibiotic resistance among the drugs that actually do work. M. genitalium is the poster child for this. Azithromycin used to be the standard first-line treatment, but mutations in the 23S ribosomal RNA gene have made macrolide resistance increasingly common. Moxifloxacin, the fluoroquinolone used as a second-line option, was once considered reliable, but mutations in the parC and gyrA genes that may confer fluoroquinolone resistance have now been identified.²² When both first-line and second-line drugs start to fail, treatment options narrow considerably. This is one reason why resistance-guided therapy, where the bacteria are tested for specific resistance mutations before choosing an antibiotic, is becoming the recommended approach for M. genitalium in several countries.

Testing and When It Makes Sense

Traditional culture methods for Ureaplasma and Mycoplasma are slow and can be unreliable. These organisms are fastidious, meaning they need specialized growth media, and cultures can be overgrown by other bacteria in the sample. Molecular tests, particularly quantitative PCR, have become the preferred diagnostic tool. PCR can simultaneously identify the species, differentiate between U. urealyticum and U. parvum, and provide a sense of bacterial load, all in a single test and even when other bacteria would have overwhelmed a culture plate.²³

But just because a test exists does not mean it should always be ordered. For M. genitalium, testing makes clear sense in anyone with persistent or recurrent urethritis or cervicitis that has not responded to standard chlamydia treatment. It also makes sense as part of a workup for pelvic inflammatory disease when the usual suspects are negative. For Ureaplasma and M. hominis, though, the picture is murkier. Testing in asymptomatic individuals leads to a flood of positive results that usually represent harmless colonization, and treating those positive results with antibiotics does more harm than good by fueling resistance.⁴

Where testing is more justified for Ureaplasma is in specific clinical scenarios: couples undergoing fertility evaluation, women with recurrent pregnancy loss or preterm labor, and very premature infants being assessed for lung disease. In those contexts, the result can guide a meaningful treatment decision rather than simply flagging a common colonizer.

Ureaplasma and Urinary Stones

One lesser-known connection is the link between Ureaplasma urealyticum and kidney stones. U. urealyticum produces an enzyme called urease, which breaks down urea into ammonia and raises the pH of the surrounding environment. This alkaline shift promotes the formation of struvite stones, a type of mineral deposit in the urinary tract composed of magnesium, ammonium, and phosphate.²⁴ Struvite stones are already associated with other urease-producing bacteria, but U. urealyticum is sometimes overlooked as a culprit because standard urine cultures may not detect it. If you have recurrent struvite stones and no obvious bacterial source on routine culture, this organism is worth considering.

The Vaginal Microbiome Connection

Mycoplasma hominis frequently shows up in discussions of bacterial vaginosis, a condition characterized by a shift in the vaginal microbial community away from protective lactobacilli and toward a mix of anaerobic bacteria. M. hominis is one of several organisms whose numbers increase during bacterial vaginosis, alongside Gardnerella, Prevotella, Atopobium, and others.²⁵ Whether M. hominis actively drives the condition or simply thrives in the altered environment remains debated. In practice, treating bacterial vaginosis with standard regimens (metronidazole or clindamycin) typically reduces M. hominis numbers along with the other overgrown bacteria, so it rarely requires separate, targeted treatment in that context.

Ureaplasma species are also part of the normal vaginal flora in many women and do not appear to play a central role in bacterial vaginosis the way M. hominis does. Their clinical relevance during routine gynecological care is low unless specific symptoms or pregnancy complications raise the question.

1
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2
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3
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4
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8
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9
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10
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11
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12
PubMed Central. Intra-Amniotic Infection with Ureaplasma parvum Causes Preterm Birth and Neonatal Mortality That Are Prevented by Treatment with Clarithromycin
13
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14
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15
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16
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17
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18
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20
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