The Unlikely Villains: How Common Skin Bacteria Are Causing Serious Heart Infections
Exploring how coryneform bacteria, once dismissed as contaminants, are now recognized as serious pathogens causing infective endocarditis.
From Contaminant to Culprit
For decades, when scientists spotted coryneform bacteria in blood samples, they typically dismissed them as mere contamination from the skin. These microscopic rod-shaped organisms were considered unlikely to cause serious disease in healthy individuals. But a dramatic reassessment is now underway in medical laboratories and hospital wards worldwide.
Growing evidence reveals that these common bacteria can transform into dangerous pathogens, capable of causing a severe heart infection known as infective endocarditis—particularly in patients with prosthetic heart valves.
This article explores the fascinating story of how these underestimated microbes came to be recognized as serious threats, the cutting-edge research uncovering their secrets, and what this means for the future of diagnosing and treating this potentially fatal condition.
Meet the Coryneforms: More Than Meets the Microscope
What Are Coryneform Bacteria?
The term "coryneform" describes a diverse group of aerobic, non-sporing, irregularly shaped Gram-positive rods 6 . The name derives from the Greek word "koryne," meaning club, reflecting their characteristic curved, club-like appearance under the microscope. They're ubiquitous in nature, found in soil and water, but many species have made the human body their home, particularly our skin and mucous membranes 6 .
Within this broad category, the genus Corynebacterium is most frequently encountered in clinical infections, with over 160 identified species 4 . While Corynebacterium diphtheriae—the cause of diphtheria—has been feared for centuries, its non-diphtheria relatives were largely ignored until recent decades. Medically important species include Corynebacterium striatum, Corynebacterium jeikeium, and Corynebacterium pseudodiphtheriticum, each with distinct characteristics and pathogenic potential 1 7 .
Coryneform bacteria under microscope showing their characteristic club-shaped appearance
The Human Microbiome Connection
Coryneform bacteria are part of the normal human microbiome, living harmlessly on our skin. A recent groundbreaking study published in 2025 revealed an astonishing diversity of these bacteria at a single skin site—the underarm—identifying 215 closed genomes representing 30 distinct types, including two completely new species 8 . This incredible diversity highlights how much we have yet to learn about these common residents of our bodies.
Problems typically begin when these bacteria enter the bloodstream—often through medical procedures—and travel to the heart, where they can adhere to damaged heart valves or prosthetic materials, triggering the dangerous infection known as infective endocarditis.
Coryneform Bacteria as Pathogens: The Rise of an Underestimated Threat
The Shift in Perception
The medical community's understanding of coryneform bacteria has evolved significantly. Where these organisms were once almost universally disregarded as contaminants in blood cultures, we now know they can cause true bloodstream infections with high morbidity and mortality rates 4 . This shift resulted from improved diagnostic techniques and a growing population of vulnerable patients.
Infective endocarditis occurs when bacteria colonize the inner lining of the heart (endocardium) and heart valves, forming destructive vegetations that can break off and travel to other organs, causing strokes or abscesses. Coryneform bacteria are particularly adept at infecting prosthetic heart valves, where they can form stubborn biofilms that are difficult for both antibiotics and the immune system to eradicate 1 4 .
Risk Factors and Vulnerable Patients
Certain patient populations are particularly susceptible to coryneform endocarditis:
Prosthetic Heart Valve Recipients
Face the highest risk, with one study finding 70% of coryneform endocarditis cases occurring in this group 1 .
Compromised Immune Systems
Patients undergoing cancer treatment or living with HIV 7 .
Elderly Individuals
With multiple health conditions—the median age of patients in one study was 71 years 1 .
Medical Device Users
People with intravenous catheters or other medical devices that provide a portal of entry into the bloodstream 4 .
The clinical course can be insidious, with symptoms developing gradually over weeks, or rapidly progressive in patients with prosthetic valves, requiring urgent intervention.
A Closer Look at the Science: The Swedish Endocarditis Registry Study
To understand how researchers confirmed the significance of coryneform bacteria in endocarditis, let's examine a pivotal study that provided compelling evidence of their pathogenic role.
Methodology: Connecting the Dots
Between 2008 and 2017, researchers used the Swedish Registry of Infective Endocarditis (SRIE)—a comprehensive national database active since 1995—to identify all cases of endocarditis caused by Corynebacterium species 1 . This approach provided a population-based perspective rarely achieved in studies of uncommon infections.
The research team undertook meticulous laboratory work to verify their findings:
Species Reidentification
Using Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS), an advanced technology that provides rapid and accurate bacterial identification 1 .
Antibiotic Susceptibility Testing
Using Etest strips to determine the minimum inhibitory concentration (MIC) of various antibiotics 1 .
Comparative Analysis
With more common endocarditis-causing pathogens like Staphylococcus aureus and streptococci 1 .
Key Findings: Revealing the Patterns
The study analyzed 30 episodes of Corynebacterium endocarditis confirmed over the 10-year period. The results painted a clear picture of a distinct clinical entity:
| Characteristic | Finding | Significance |
|---|---|---|
| Median Age | 71 years | Primarily affects elderly patients |
| Gender Distribution | 77% male | Strong male predominance |
| Prosthetic Valve Involvement | 70% | Strong association with artificial valves |
| In-Hospital Mortality | 13% | Substantial despite treatment |
| Surgical Intervention Rate | 50% | Often requires combined medical and surgical treatment |
Distribution of Corynebacterium Species Causing Endocarditis 1
Prosthetic Valve Involvement Comparison 1
Perhaps most strikingly, when compared to endocarditis caused by other bacteria, Corynebacterium species were significantly more likely to involve prosthetic valves than either Staphylococcus aureus or non-beta-hemolytic streptococci (14% and 26% respectively) 1 .
The Antibiotic Resistance Challenge: Navigating Treatment Options
The increasing antibiotic resistance of coryneform bacteria adds another layer of complexity to managing these infections. Unlike decades past, when penicillin might have sufficed, today's clinicians face multidrug-resistant strains that demand carefully tailored therapy.
Resistance Patterns: A Shifting Landscape
The Swedish study revealed concerning resistance profiles among Corynebacterium isolates 1 :
| Antibiotic | Susceptibility Pattern | Clinical Implications |
|---|---|---|
| Vancomycin | Active against all isolates | Remains reliable for empirical treatment |
| Penicillin G | Resistance common | Not suitable without susceptibility testing |
| Gentamicin | Variable resistance | Requires individual susceptibility testing |
| Rifampicin | Generally active | Often used in combination therapy |
These patterns vary by species, with C. jeikeium typically exhibiting the broadest resistance, while other species may remain susceptible to multiple antibiotic classes.
The Treatment Approach
Successful management of coryneform endocarditis typically involves 1 7 :
Prolonged Antibiotic Therapy
Often 4-6 weeks of intravenous treatment to ensure complete eradication of the infection.
Combination Therapy
Frequently using vancomycin as a backbone, combined with other antibiotics based on susceptibility testing.
Surgical Intervention
Required in approximately 50% of cases to remove infected hardware or drain abscesses.
Specialist Involvement
Infectious disease specialist involvement to guide complex treatment decisions.
The high surgical rate reflects the tenacity of these infections, particularly when prosthetic materials are involved and biofilms have formed.
Conclusion: An Evolving Story with Life-Saving Implications
The journey of coryneform bacteria from dismissed contaminants to recognized pathogens illustrates medicine's evolving nature. What was once overlooked now demands our attention, especially as medical advances—such as prosthetic valve implantation—create new niches for these adaptable microbes to exploit.
Ongoing research continues to uncover the complex interactions between these bacteria and their human hosts, from their role as skin commensals to their transformation as invasive pathogens. For clinicians, the message is clear: not all coryneforms are created equal, and accurate species identification combined with antimicrobial susceptibility testing can mean the difference between treatment success and failure.
For the broader scientific community, these organisms represent a fascinating example of how improved diagnostic technologies can reshape our understanding of disease. As one researcher aptly noted, we are just beginning to appreciate the "extensive variation" that exists within this genus, even within a single individual 8 . This diversity undoubtedly holds many secrets waiting to be uncovered—secrets that may one day lead to better ways to protect the most vulnerable patients from these unlikely villains of the microbial world.
Key Takeaways
- Coryneform bacteria are emerging pathogens, not just contaminants
- They disproportionately affect patients with prosthetic heart valves
- Accurate species identification is crucial for appropriate treatment
- Multidrug resistance complicates therapeutic approaches
- Combined medical and surgical management is often necessary
Future Directions
- Improved rapid diagnostic methods
- Better understanding of biofilm formation
- Development of targeted therapies
- Enhanced preventive strategies for high-risk patients
- Continued surveillance of resistance patterns