What Is Staphylococcus lugdunensis & What Does It Cause?

Staphylococcus lugdunensis is a species of coagulase-negative staphylococcus that lives harmlessly on human skin most of the time but causes infections strikingly similar to those of its far more famous relative, Staphylococcus aureus. It can trigger skin abscesses, destructive heart valve infections, prosthetic joint failures, and bloodstream infections with mortality rates that rival or exceed those of S. aureus. For decades it was lumped together with other harmless-looking skin bacteria, dismissed in the lab, and sometimes misidentified entirely. Better diagnostic technology has changed that picture considerably, but the organism still catches clinicians off guard.

A Skin Commensal With a Dangerous Side

S. lugdunensis was first formally described in 1988, named after Lugdunum, the Latin name for Lyon, France, where it was identified. It normally colonizes the skin of the perineum, groin, and lower abdomen, though it turns up on other body sites too. Most people who carry it never develop an infection. When the organism does breach the skin barrier, however, the resulting disease tends to be far more aggressive than what you would expect from other coagulase-negative staphylococci.

That aggressiveness is the central paradox of S. lugdunensis. Lab tests classify it alongside relatively tame bacteria like S. epidermidis, yet its clinical behavior mirrors that of S. aureus, a pathogen that demands immediate attention.¹ This mismatch between its lab classification and its real-world destructiveness is what makes it clinically important and frequently underestimated.

Skin and Soft Tissue Infections

The most common infections caused by S. lugdunensis involve the skin and soft tissue. In a retrospective review from a major cancer center, skin and soft tissue infections accounted for the vast majority of cases, and most of those were linked to surgical or invasive procedures or implanted medical devices.² A separate case series found the organism in abscesses, surgical wound infections, and cellulitis, with the breast, abdomen, and lower limbs being the most frequent sites.³ Another series noted cystic lesions with surrounding cellulitis and periungual abscesses around the fingernails and toes, with the back and digits being common locations.⁴

In children, the pattern is similar. A pediatric review found that about two-thirds of S. lugdunensis infections in children had skin as the primary source.⁵ The same study flagged central nervous system infections as an unexpected but significant source: all three cerebrospinal fluid isolates in that series came from children with ventriculoperitoneal shunt infections, a reminder that the organism can exploit any hardware breach.

Endocarditis That Destroys Heart Valves

Infective endocarditis is the most feared complication of S. lugdunensis infection, and it is the main reason clinicians care about distinguishing this species from other coagulase-negative staphylococci. When S. lugdunensis infects a heart valve, it behaves more like S. aureus endocarditis than like the low-grade, slow-smoldering valve infections caused by S. epidermidis.

A national Swedish registry-based study found that 30-day mortality for S. lugdunensis endocarditis was about 20%, significantly higher than the 7% mortality for endocarditis caused by other coagulase-negative species and the 9% mortality for S. aureus endocarditis.⁶ Those numbers are worth pausing over: a bacterium classified alongside “harmless” skin flora was killing patients at roughly double the rate of S. aureus in the context of endocarditis.

An earlier analysis of 69 published cases of S. lugdunensis endocarditis showed native-valve disease predominating, with the mitral valve most often affected. Complications such as heart failure, abscess formation around the valve ring, and emboli to distant organs were common. Surgery was needed in roughly half of cases, and overall mortality was 42%.⁷ When the infection involved a prosthetic valve, the picture was even grimmer, with abscess formation nearly universal and mortality approaching 78% in that subset. Case reports have documented S. lugdunensis eating through the wall between heart chambers and destroying multiple valves simultaneously.⁸

Prosthetic Joint Infections

S. lugdunensis is increasingly recognized as a cause of prosthetic joint infection. A decade-long review at Mayo Clinic identified 28 episodes of S. lugdunensis prosthetic joint infection in 22 patients, with nearly 90% involving prosthetic knees.⁹ A multicenter comparison study of 88 prosthetic joint infections found that S. lugdunensis behaved with a severity quite similar to that of S. aureus in this setting, and the authors explicitly called it an emerging pathogen.¹⁰

Infections around fracture-fixation hardware follow a similar pattern. In a series of 38 such cases, about 89% required combined surgical and antibiotic therapy, with hardware removal needed in the majority. The combination of surgery plus antibiotics was the only approach significantly associated with remission.¹¹ Antibiotics alone generally were not enough when metal was involved.

Bloodstream Infections

When S. lugdunensis reaches the bloodstream, the outcomes look worryingly close to S. aureus bacteremia rather than the relatively benign course of most coagulase-negative staph bacteremias. A comparative study found that 7-day mortality was similar between S. lugdunensis and S. aureus bacteremia (about 8% versus 7%), while S. epidermidis bacteremia had a much lower mortality of around 0.5%. Rates of metastatic lesions, where the infection spreads to seed distant organs, were also comparable between S. lugdunensis and S. aureus (roughly 17% versus 20%), and both were far higher than S. epidermidis.¹²

The rate of endocarditis among patients with S. lugdunensis bloodstream infections is also similar to the rate among patients with S. aureus in the bloodstream.¹³ The practical takeaway is that infectious-disease specialists now recommend treating S. lugdunensis bacteremia with the same seriousness as S. aureus bacteremia: echocardiography to check for valve involvement, repeat blood cultures to confirm clearance, and a full treatment course rather than a quick stop of antibiotics.

Why Labs Have Struggled to Identify It

Much of the trouble with S. lugdunensis historically came down to how microbiology labs handled coagulase-negative staphylococci. For years, the quick way to sort staph in the lab was a slide test for clumping factor, a surface protein that causes bacteria to clump in the presence of fibrinogen. A positive slide meant S. aureus; a negative one meant “not S. aureus, probably not important.” The problem is that S. lugdunensis also produces clumping factor. It tests positive on slide tests that were designed to identify S. aureus, but negative on the confirmatory tube coagulase test. Labs that skipped the tube test and relied solely on the slide result ended up calling it S. aureus.¹⁴ Labs that did both tests sometimes swung the other direction, correctly noting the negative tube coagulase result and labeling the organism as a generic, unimportant coagulase-negative staph, not worth identifying to species level.

The introduction of mass spectrometry-based identification technology in clinical microbiology labs over the past decade has changed the picture dramatically. When one lab began routinely identifying all coagulase-negative staphylococci to species level using this technology, they found S. lugdunensis appearing far more often than historical records suggested.¹⁵ Another institution saw S. lugdunensis urinary tract infections jump from one case in a five-year span to 38 cases in the next five years, coinciding precisely with the adoption of the new identification method. The organism had not suddenly become more common; the lab simply started recognizing it.¹⁶

Antibiotic Susceptibility and Emerging Resistance

One piece of relatively good news: S. lugdunensis remains much more susceptible to antibiotics than S. aureus. In a large study of 540 clinical isolates, about 75% were susceptible to penicillin G, and the vast majority were susceptible to all other antibiotics tested.¹⁷ A separate evaluation of 112 isolates using gene-based testing confirmed a similar pattern, with about 67% penicillin-susceptible.¹⁸ For penicillin-susceptible strains, plain penicillin is an effective and narrow-spectrum choice, which is unusual for any staphylococcal infection in 2025.

That said, methicillin resistance is appearing. Oxacillin resistance mediated by the mecA gene is emerging in some geographic areas.¹⁹ Case reports of methicillin-resistant S. lugdunensis have been published, including one hospital-acquired bloodstream infection where the organism was initially misidentified as methicillin-resistant S. aureus because lab staff assumed any resistant staph with a positive slide test had to be MRSA.²⁰ A neonatal intensive care unit case illustrated the practical consequences: when methicillin-resistant S. lugdunensis was identified, the infection-control team implemented contact isolation and screening protocols similar to those used for MRSA. The neonate was found to be colonized in the nose, throat, and perineum, though her parents were not carrying the organism.²¹

How It Evades the Immune System

Part of what makes S. lugdunensis so tenacious once it establishes an infection is its ability to hide inside your own immune cells. Researchers have shown that macrophages, the white blood cells whose job is to engulf and destroy bacteria, fail to kill ingested S. lugdunensis. The bacteria survive for extended periods inside these cells without replicating, essentially sitting quietly in a compartment that should have dissolved them. The organism’s cell wall carries a chemical modification that protects it from lysozyme, one of the key enzymes macrophages use to break down bacterial walls. When that modification is knocked out experimentally, survival inside macrophages drops.²²

The practical implication is that macrophages may serve as a hiding place from which the bacteria can later disseminate to other body sites. This could help explain why S. lugdunensis infections sometimes seem to clear and then relapse, or why bloodstream infections seed distant organs at rates comparable to S. aureus. The organism also deploys a regulatory system called agr that controls the production of toxins, including hemolytic peptides that punch holes in red blood cells and a protease called lugdulysin.²³ Strains associated with endocarditis show enhanced ability to bind von Willebrand factor, a blood protein involved in clotting, and form thicker biofilms, both of which make them harder to dislodge from heart valves.²⁴

Lugdunin and the Antibiotic Discovery Angle

In a twist that has drawn interest well beyond infectious-disease medicine, S. lugdunensis itself produces an antibiotic. In 2016, researchers discovered that certain strains secrete a compound called lugdunin, a cyclic peptide that kills a broad range of other bacteria, including methicillin-resistant S. aureus and vancomycin-resistant enterococci, by disrupting their cell membranes.²⁵ Lugdunin-producing strains were found to be common among the major genetic lineages of S. lugdunensis, with production rates ranging from roughly 53% to 83% depending on the lineage.

The compound does more than just directly poison rival bacteria. When applied to human skin cells or mouse skin, lugdunin boosts the skin’s own antimicrobial defenses by increasing the production of natural antimicrobial peptides and recruiting immune cells like monocytes and neutrophils. It also works synergistically with those host-produced peptides, meaning the combined effect is larger than the sum of its parts.²⁶ This multi-layered protection against S. aureus, combining direct killing, immune activation, and synergy with other skin defense molecules, has generated enthusiasm about lugdunin as a potential new topical treatment for S. aureus skin infections, particularly in an era of rising antibiotic resistance.

Recent work has also found that S. aureus strains with defects in their energy metabolism, including the hard-to-treat small colony variants that cause chronic infections, are especially susceptible to lugdunin.²⁷ Lugdunin has not yet reached clinical trials, but it represents one of the more promising leads to emerge from studying the human skin microbiome.

A Genetically Isolated Organism

S. lugdunensis is an oddity among staphylococci at the genomic level. Most staph species have what geneticists call an open pan-genome, meaning different strains vary substantially from each other and freely swap genes with their neighbors. S. lugdunensis is the opposite. A comparative genomic study found that its pan-genome is closed: strains are remarkably similar to one another, with very few new genes appearing even when large numbers of genomes are compared. The species has a high level of genetic identity between strains and multiple built-in barriers to picking up foreign DNA, including restriction enzymes that chew up incoming genetic material, CRISPR-based defense systems, and toxin-antitoxin systems that enforce genetic stability.²⁸

This genetic insularity has practical consequences. It likely explains why S. lugdunensis has been slow to pick up antibiotic-resistance genes compared with S. aureus or S. epidermidis, which readily acquire resistance through gene transfer. It may also explain why the organism’s virulence tools vary by genetic lineage in a structured way rather than being scattered randomly across strains. Analysis of one virulence-related gene cluster showed eight distinct genetic arrangements that tracked reliably with the organism’s major clonal groups.²⁹ Whether this closed genome will continue to protect S. lugdunensis from acquiring resistance in a world saturated with antibiotic pressure is an open question, but for now it remains one of the more treatable members of its genus.