What Is the New STD Called and How Dangerous Is It?

The STI making headlines as the “new” sexually transmitted disease is Mycoplasma genitalium, often shortened to M. genitalium or Mgen. It is not actually new. The bacterium was first isolated from men with urethritis back in 1981, but it spent decades in relative obscurity because it is extraordinarily difficult to grow in a lab and was only reliably detectable once molecular testing became available in the 1990s.¹ What has changed is that clinicians now recognize it as a genuine threat, one that causes many of the same problems as chlamydia and gonorrhea but is harder to treat and increasingly resistant to the antibiotics thrown at it.

Why You Are Hearing About It Now

Mycoplasma genitalium belongs to a family of unusually small bacteria that lack a cell wall, which is the structure most common antibiotics target. That missing wall means entire classes of widely used drugs, including penicillin-type antibiotics, vancomycin, and sulfonamides, simply do not work against it.² For years, this organism flew under the radar because standard STI panels did not include it, and growing it in culture took weeks, sometimes more than 50 days of incubation just to see initial growth.¹ It was only after PCR-based molecular tests became practical in the early 1990s that epidemiological studies could confirm what those first researchers suspected: M. genitalium was a real cause of urethritis and other genital tract infections.³

The “new superbug” framing in media coverage is a mix of legitimate concern and sensationalism. Mgen is genuinely becoming harder to treat, and its rising antibiotic resistance is worrying. But it is not a novel organism that appeared out of nowhere. It is a pathogen that science has slowly caught up to, and public awareness is only now following.

What Symptoms Look Like

One of the more frustrating features of Mgen is that many people who carry it have no symptoms at all. In clinic-based studies, asymptomatic carriage is common, and the infection can persist silently for months. When symptoms do appear, they tend to mimic chlamydia or gonorrhea closely enough that, without specific testing, you would never know the difference.

In men, Mgen is strongly linked to non-gonococcal urethritis, the kind of urethral inflammation that shows up as discharge or burning during urination but tests negative for gonorrhea. A large pooled analysis found that Mgen infection was associated with roughly a fourfold increase in the odds of having symptomatic urethritis when chlamydia had been ruled out.⁴ In other words, if a man shows up at a clinic with classic urethritis symptoms and his chlamydia and gonorrhea tests come back negative, Mgen is a prime suspect.

In women, the picture is more concerning. Mgen has been increasingly identified in women with pelvic inflammatory disease (PID), which is an infection of the upper reproductive tract that can damage the fallopian tubes and lead to chronic pain or fertility problems. Several studies have found this association to be independent of chlamydia and gonorrhea, meaning Mgen appears capable of causing PID on its own.⁵ One prospective investigation found that Mgen-positive women had over a thirteenfold risk of endometritis, and a separate study found a sixfold increased risk of PID among Mgen-positive patients after a termination procedure.⁵ In another study comparing PID cases with controls, about 13% of women with PID had evidence of Mgen infection compared to none of the controls, and this association held even after accounting for chlamydia.⁶

Whether Mgen ultimately causes long-term infertility is not yet settled. Researchers have found elevated Mgen antibodies in women with tubal factor infertility, which is suggestive but not proof of a causal link.⁵ The evidence here is thinner than it is for chlamydia, where the connection to tubal damage is well established. But the direction of the data is concerning enough that clinicians treat Mgen-associated PID seriously.

How Common Is It

A large systematic review and meta-analysis that pooled data from studies worldwide found that in randomly selected samples from the general population, about 1.3% of people tested positive in higher-income countries and about 3.9% in lower-income countries.⁷ Prevalence was similar between women and men. Those numbers might sound low, but they are in the same range as chlamydia in many populations, and the actual figure in higher-risk groups is substantially higher. Among men who have sex with men sampled from community settings, prevalence was about 3.2%, and among female sex workers, it reached nearly 16%.⁷

A useful comparison is that among asymptomatic clinic patients, prevalence was under 1%, which is reassuring for the average person walking into a clinic without symptoms. The numbers climb sharply once you look at people who have urethritis, cervicitis, or PID. Mgen is not rare, but it is also not something the majority of sexually active adults are carrying around.

The Diagnosis Problem

You cannot diagnose Mgen the way you diagnose many other infections. Standard bacterial culture is essentially useless in clinical settings because the organism grows so painfully slowly and requires specialized media that most labs do not stock.⁸ This means molecular tests, specifically nucleic acid amplification tests (NAATs), are the only practical way to identify it. These are essentially the same type of swab-and-amplify technology used for chlamydia and gonorrhea testing, but the Mgen-specific version is not yet included on standard STI panels in most healthcare settings.

This creates a frustrating gap. A person could walk into a clinic with textbook urethritis symptoms, get tested for the “usual suspects,” receive negative results, get prescribed antibiotics empirically, and never learn that Mgen was the actual cause. If the empiric antibiotics happen to work, the problem resolves. If they do not, especially if the strain is resistant, the infection persists and the person may unknowingly pass it on. The difficulty of growing Mgen in the lab also means that standard drug-susceptibility testing, where you expose the organism to various antibiotics and see which ones kill it, is rarely performed.⁹

Why Antibiotic Resistance Is the Real Danger

This is where the story gets genuinely alarming. Because Mgen’s missing cell wall rules out penicillin-type drugs from the start, treatment options were already limited to macrolide antibiotics (like azithromycin) and fluoroquinolones (like moxifloxacin). Over the past two decades, resistance to both has climbed steeply. A large surveillance study in France analyzed over 1,600 Mgen-positive specimens and found that macrolide resistance mutations were present in roughly 35% to 43% of samples in metropolitan France between 2018 and 2020, with rates even higher in men (over 50%).¹⁰ Fluoroquinolone resistance mutations were less common but still substantial, around 15% to 16% in the same population.¹⁰

The parallel with gonorrhea is hard to miss. Gonorrhea has already demonstrated the capacity to develop resistance to every class of antibiotic previously recommended for first-line treatment, including extended-spectrum cephalosporins, raising the possibility of truly untreatable infections.¹¹ Mgen is following a worryingly similar trajectory, with the added disadvantage that it started with fewer antibiotic options in the first place.

The geographic variation is worth noting. In the French study, overseas territories had dramatically lower resistance rates, about 6% for macrolide resistance compared to 35% or more in mainland France.¹⁰ This kind of disparity reflects local prescribing patterns and antibiotic pressure. Regions where azithromycin is heavily used for STIs tend to have higher Mgen resistance, which is a textbook example of how overuse drives resistance.

How Treatment Works When the Drugs Still Work

The most promising approach to treating Mgen today is resistance-guided therapy, where a molecular test not only detects the organism but also checks for specific mutations that predict macrolide resistance. If the test shows no resistance mutations, a macrolide antibiotic is used first. If resistance mutations are present, the patient skips straight to a second-line drug like moxifloxacin. A prospective evaluation of this approach found cure rates above 90% for both groups: about 95% for infections without macrolide resistance mutations and about 92% for those with resistance mutations.¹² Equally important, this strategy dramatically reduced the selection of new macrolide resistance, with resistance developing in only about 3% of susceptible infections.¹²

The problem is access. Resistance-guided testing is not yet widely available in most healthcare settings. Where it is not available, clinicians are left guessing which antibiotic to use, and guessing wrong means the infection persists and may develop further resistance. Test-of-cure follow-up and partner notification are also critical but inconsistently practiced. Molecular typing studies have shown that what looks like a treatment failure can sometimes actually be a reinfection from an untreated partner, underscoring the importance of testing and treating sexual contacts.¹³

The Link to HIV

Beyond the direct damage Mgen causes in the genital tract, there is growing evidence that infection may increase a person’s vulnerability to HIV. The mechanism is thought to involve the inflammatory response Mgen triggers in mucosal tissue, which recruits the very immune cells that HIV preferentially targets. A study of African women found a greater than twofold independent increase in HIV acquisition risk among those infected with Mgen, and estimated that roughly 9% of new HIV infections in the study population were attributable to Mgen.¹⁴ Researchers have proposed that the same mechanism could operate in rectal tissue, potentially increasing HIV susceptibility in men who have sex with men, though more research is needed to confirm that specifically.¹⁵

This interaction between Mgen and HIV is not unique. Other STIs that cause mucosal inflammation, including herpes and gonorrhea, have similar associations. But it adds urgency to diagnosing and treating Mgen in populations with high HIV exposure, where even a modest increase in susceptibility can translate into a meaningful number of preventable HIV infections.

Should You Get Tested

Given how concerning all of this sounds, you might assume that everyone should be screened for Mgen at their next clinic visit. Current guidelines say otherwise, and for good reason. A study comparing two testing strategies across a statewide health system found that testing all patients who presented for STI screening resulted in antibiotic overuse, because many positive results came from people who were asymptomatic and might not have needed treatment.¹⁶ The researchers recommended against adding Mgen to the standard emergency department STI panel and instead endorsed following CDC guidelines, which focus testing on people with symptoms or with specific risk factors.

The logic here is that treating Mgen in someone who has no symptoms and may clear the infection on their own contributes to antibiotic pressure without clear benefit. That pressure is exactly what drives resistance. For now, the consensus approach is to test for Mgen when someone has urethritis, cervicitis, or PID that is not explained by chlamydia or gonorrhea, or when a sexual partner has been diagnosed. Routine screening in the absence of symptoms is not currently recommended for the general population.

Prevention and Condoms

Mgen is transmitted through sexual contact, and condoms appear to provide meaningful protection, though the evidence base is smaller than for chlamydia or gonorrhea. An Australian contact-tracing study found that less than 100% condom use for penile-vaginal sex with a regular partner roughly doubled the risk of Mgen infection compared with consistent use, though the result did not quite reach statistical significance.¹⁷ A separate observational study among women in China found that genital mycoplasma infection occurred less often when condoms were used.¹⁸ The direction of the data is consistent: condoms help, even if the exact degree of protection against Mgen is harder to quantify than it is for other STIs.

There has also been interest in whether doxycycline taken after unprotected sex, a strategy called doxy-PEP, could prevent Mgen along with other bacterial STIs. Randomized trials have shown consistent efficacy of doxy-PEP in reducing chlamydia and syphilis in men who have sex with men and transgender women, with variable effects on gonorrhea depending on local resistance patterns.¹⁹ Whether doxy-PEP reduces Mgen is less clear, and some researchers worry that widespread doxycycline use could accelerate resistance in Mgen and other organisms. The current thinking favors implementing doxy-PEP for populations with proven benefit while monitoring resistance patterns closely.²⁰

Mgen and Pregnancy

Given the association between Mgen and upper genital tract infections, it is natural to worry about effects during pregnancy. The data here is actually more reassuring than you might expect. Among pregnant women, the prevalence of Mgen is low, under 1% in the meta-analysis that pooled data from multiple studies.⁷ And a study of young urban pregnant women found that Mgen was not independently associated with spontaneous abortion after adjusting for other risk factors.²¹ There may be other pregnancy complications worth investigating, but the current evidence does not support the idea that Mgen is a major driver of pregnancy loss the way some media coverage has implied.

What Makes This Different from the STIs You Know

If you have heard of chlamydia, gonorrhea, and syphilis, you might wonder what Mgen adds to the picture. The honest answer is that Mgen occupies an uncomfortable middle ground. It is not as immediately dangerous as untreated syphilis, which progresses through well-defined and severe stages. It is not as common as chlamydia. And it does not cause the acute, unmistakable symptoms that often drive people with gonorrhea to seek treatment quickly. What makes Mgen distinctly worrisome is the combination of its ability to silently cause upper reproductive tract damage in women, its natural resistance to the most commonly prescribed antibiotics, and its accelerating acquired resistance to the few drugs that do work against it.

For the average sexually active adult with a regular partner, Mgen is unlikely to be a pressing daily concern. But for clinicians managing patients with persistent or recurrent urethritis and cervicitis, it is increasingly the diagnosis they reach after ruling out everything else. And for public health officials watching antibiotic resistance trends, Mgen is one of the clearer examples of how a relatively obscure pathogen can become a genuinely difficult problem when treatment options narrow faster than new drugs arrive.

1
The Journal of Infectious Diseases. The Unique Microbiology and Molecular Pathogenesis of Mycoplasma genitalium
2
PubMed Central. Turning the Tide against Antibiotic Resistance by Evaluating Novel, Halogenated Phenazine, Quinoline, and NH125 Compounds against Ureaplasma Species Clinical Isolates and Mycoplasma Type Strains
3
PubMed Central. Mycoplasma genitalium: an emerging sexually transmitted pathogen
4
PubMed. Mycoplasma genitalium is associated with symptomatic and asymptomatic non-gonococcal urethritis in men
5
PubMed Central. Mycoplasma genitalium: an emerging cause of pelvic inflammatory disease
6
PubMed Central. Associations between Mycoplasma genitalium, Chlamydia trachomatis, and pelvic inflammatory disease
7
PubMed Central. Prevalence of Mycoplasma genitalium in different population groups: systematic review and meta-analysis
8
PubMed Central. Latest Advances in Laboratory Detection of Mycoplasma genitalium
9
PubMed. Challenges of in vitro propagation and antimicrobial susceptibility testing of Mycoplasma genitalium
10
PubMed. Prevalence of macrolide and fluoroquinolone resistance-associated mutations in Mycoplasma genitalium in metropolitan and overseas France
11
PubMed Central. Emergence of multidrug-resistant, extensively drug-resistant and untreatable gonorrhea.
12
PubMed Central. Outcomes of Resistance-guided Sequential Treatment of Mycoplasma genitalium Infections: A Prospective Evaluation
13
PubMed Central. Molecular Typing of Mycoplasma genitalium-Positive Specimens Discriminates between Persistent and Recurrent Infections in Cases of Treatment Failure and Supports Contact Tracing
14
PubMed. The association between Mycoplasma genitalium and HIV-1 acquisition in African women
15
Clinical Microbiology and Infection. The contribution of Mycoplasma genitalium to the aetiology of sexually acquired infectious proctitis in men who have sex with men
16
PubMed Central. Comparison of two testing strategies for Mycoplasma genitalium in emergency department patients across a statewide health system
17
Emerging Infectious Diseases. Mycoplasma genitalium Infection in Adults Reporting Sexual Contact with Infected Partners, Australia, 2008–2016
18
Contraception. Prevalence survey on condom use and infection of urogenital mycoplasmas in female sex workers in China
19
PubMed Central. A Review of Doxycycline Post-Exposure Prophylaxis and Its Implications for Antimicrobial Resistance and the Human Microbiome
20
PubMed. Antimicrobial Resistance Concerns Should not Limit Doxycycline Postexposure Prophylaxis Implementation for Bacterial Sexually Transmitted Infection Prevention
21
PubMed Central. Mycoplasma genitalium among Young, Urban Pregnant Women