Corynebacterium is a large genus of bacteria best known for including the species that causes diphtheria, but most of its members are harmless residents of your skin, nose, and throat. These are club-shaped, gram-positive bacteria (meaning they have a thick cell wall that stains purple under a microscope) that show up across medicine as everything from normal body flora to emerging hospital-acquired infections.
Shape, Structure, and Basic Biology
The name Corynebacterium comes from the Greek word for “club,” which describes the bacteria’s distinctive shape: slightly swollen at one end, like a tiny club or bowling pin. Under a microscope, they often appear in irregular arrangements, sometimes described as resembling Chinese letters or a picket fence. They are classified as gram-positive bacilli (rod-shaped bacteria), and they test positive for the enzyme catalase, which helps distinguish them from other bacterial groups.
These bacteria don’t form spores, and they don’t move on their own. Some species need lipids (fats) to grow well in the lab, which reflects their preference for oily or moist environments on the human body. Because many Corynebacterium species look similar to each other on initial lab tests, identifying the exact species often requires specialized culture media and biochemical testing.
A Normal Part of Your Skin
Most Corynebacterium species are commensals, meaning they live on you without causing harm. They are especially abundant in moist, enclosed areas of the body: the armpits, groin, and between the toes. A landmark study mapping 20 different skin sites found that while oil-loving bacteria (like Cutibacterium) dominate oily areas such as the face, Corynebacterium species thrive in these damper zones.
Far from being freeloaders, some species actively protect you. Corynebacterium striatum, for instance, can dial down the virulence of Staphylococcus aureus (a common cause of skin infections) by interfering with the communication system S. aureus uses to coordinate attacks on tissue. In mouse models, co-infection with C. striatum reduced S. aureus bacterial counts compared to S. aureus alone. Another species, Corynebacterium accolens, produces a lipase enzyme that breaks down skin oils into free fatty acids. Those fatty acids directly inhibit the growth of Streptococcus pneumoniae, a major cause of pneumonia and ear infections. Children with higher levels of nasal Corynebacterium were found to be free of S. pneumoniae colonization, suggesting a real protective effect.
The Dangerous One: Corynebacterium diphtheriae
The most notorious member of the genus is Corynebacterium diphtheriae, the cause of diphtheria. What makes this species dangerous isn’t just the infection itself but a powerful toxin produced by certain strains. This toxin permanently disables a molecule called elongation factor 2, which cells need to build proteins. Without functioning protein production, cells die. The result is widespread tissue destruction.
The hallmark of respiratory diphtheria is a thick, gray coating of dead tissue called a pseudomembrane that forms in the nose and throat. This membrane can grow large enough to block the airway entirely, which is the most immediately life-threatening complication. Other symptoms include mild fever, sore throat, swollen neck glands (sometimes dramatically swollen, called “bull neck”), and general weakness.
If the toxin enters the bloodstream, it can damage organs far from the throat. The heart muscle is particularly vulnerable, and toxin-related heart damage (myocarditis) is a leading cause of death in diphtheria cases. Nerve damage and kidney failure are also possible complications.
How Vaccination Changed the Picture
Diphtheria was once a leading killer of children. Widespread vaccination has made it rare in most of the world. The standard childhood vaccine series (DTP, which also protects against tetanus and pertussis) reached 85% global coverage among one-year-olds in 2024, up from 72% in 2000. In countries with high vaccination rates, diphtheria cases are now exceptionally uncommon. Outbreaks still occur in regions with low immunization coverage or among populations displaced by conflict and poverty.
Non-Diphtheria Species Causing Infections
Over the past two decades, other Corynebacterium species have gained attention as causes of serious infections, particularly in hospitals. Species like C. jeikeium, C. striatum, C. urealyticum, and C. amycolatum are now recognized as opportunistic pathogens, meaning they take advantage of weakened immune defenses or breached skin barriers to cause disease.
These infections typically affect people who are already hospitalized: patients recovering from surgery, those in intensive care, or people with conditions like diabetes. Chronic non-healing wounds, post-surgical sites, and diabetic foot ulcers are common locations. In one study of hospital-acquired Corynebacterium infections, C. amycolatum was the most frequently isolated species (20% of cases), followed by C. striatum (16%). All cases came from inpatients, and the majority were from surgical wards.
C. jeikeium and C. urealyticum stand out because they can cause infections even in people with intact immune systems, making them true pathogens rather than purely opportunistic ones. C. urealyticum is particularly associated with urinary tract infections and can create calcium-based deposits on urinary catheters.
Antibiotic Resistance Is a Growing Concern
One reason non-diphtheria Corynebacterium infections are increasingly problematic is multidrug resistance. C. striatum has become especially concerning. In a Chinese study of 260 C. striatum isolates from respiratory infections, 96.2% were multidrug-resistant, showing resistance to three or more antibiotic classes. Resistance rates to commonly used antibiotics were striking: 93.5% of isolates resisted quinolones and tetracyclines, and 85.2% resisted lincosamines. Over three-quarters of the strains carried two or more resistance genes.
The good news is that virtually all tested isolates remained sensitive to vancomycin and linezolid, with 100% sensitivity reported across multiple studies. Vancomycin, either alone or combined with other agents, is currently considered the most effective treatment for multidrug-resistant Corynebacterium infections. Analysis of patient outcomes found that using vancomycin or linezolid reduced overall mortality rates.
How Corynebacterium Is Identified in the Lab
When diphtheria is suspected, throat swabs are cultured on blood agar containing potassium tellurite, which selectively allows Corynebacterium to grow while suppressing other throat bacteria. A specialized medium called Tinsdale agar helps distinguish C. diphtheriae from other species: the diphtheria bacterium produces a distinctive brown or black halo around its colonies on this medium.
Confirming that a strain actually produces the dangerous toxin requires a separate test called the Elek test. A strip of filter paper soaked in diphtheria antitoxin is embedded in agar, and the suspect bacteria are streaked across it. If the bacteria produce toxin, a visible line of precipitate forms where the toxin and antitoxin meet. This distinction matters because not all C. diphtheriae strains produce toxin, and only toxin-producing strains cause the severe form of the disease.