Diphtheroids are a large group of bacteria commonly found living on and within the human body. The term describes species belonging to the Corynebacterium genus that do not produce the diphtheria toxin. These organisms are typically viewed as part of the normal flora residing on the skin and mucous membranes. They are usually considered low-virulence organisms, rarely causing disease in healthy individuals, but they can become significant pathogens under specific circumstances.
Defining Diphtheroids
The term “diphtheroids” is a historical and collective name referring to non-diphtheriae Corynebacterium species (NDCs). This grouping includes dozens of bacterial species that share a similar appearance to Corynebacterium diphtheriae, the microbe responsible for diphtheria. These bacteria are classified as Gram-positive rods, meaning they retain a violet stain in a laboratory test due to their cell wall composition.
A defining feature of these bacteria is their pleomorphic morphology, meaning they change shape. They are typically straight or slightly curved rods that can appear club-shaped or in V-shapes, sometimes likened to “Chinese characters.” The difference between diphtheroids and C. diphtheriae is the lack of the gene that produces the diphtheria toxin, which causes the severe systemic effects of classic diphtheria. Because of their common presence as commensals, diphtheroids were historically often dismissed as mere contaminants when isolated from clinical samples.
When These Bacteria Become Pathogenic
Diphtheroids are opportunistic pathogens, transitioning from harmless residents to disease-causing agents only when the host’s defenses are compromised. This explains why the risk of serious infection is significantly higher in certain patient populations. A major risk factor is a weakened immune system, often due to underlying chronic diseases like diabetes or conditions requiring immunosuppressive treatments, such as chemotherapy or organ transplantation.
The bacteria gain access to deeper tissues or the bloodstream following invasive procedures, such as major surgery or the insertion of medical devices. Chronic illness and prolonged hospitalization also increase the likelihood of infection, especially with multidrug-resistant strains. Any breach in the body’s natural barriers or a substantial reduction in immune surveillance provides the opportunity for these low-virulence organisms to establish a serious infection.
Common Locations of Diphtheroid Infections
The specific environments that allow diphtheroids to thrive often dictate where the resulting infection manifests. One common presentation is a device-related bloodstream infection. Since diphtheroids colonize the skin, they easily contaminate and subsequently form biofilms on the surface of indwelling foreign materials like central venous catheters, shunts, or ports.
Diphtheroids are known to cause endocarditis, an infection of the inner lining or valves of the heart; prosthetic heart valves are particularly susceptible. The bacteria’s ability to form a protective biofilm allows them to adhere to the artificial surface and resist clearance by the immune response and antibiotics. Skin and soft tissue infections are also frequently observed, particularly in surgical wounds, ulcers, or existing skin lesions that allow the organisms to penetrate the protective epidermal layer. Certain species are recognized causes of respiratory tract infections, such as tracheobronchitis or pneumonia, primarily in patients with underlying chronic lung conditions or those requiring mechanical ventilation.
Diagnosis and Management
Diagnosing a diphtheroid infection can be challenging due to the organism’s common presence on the skin. When the bacteria are isolated from a sample, the medical team must determine if it is a true pathogen or simply a contaminant. A diagnosis of infection is typically supported by isolating the diphtheroid from a normally sterile site, such as the blood, especially if growth is heavy or occurs in multiple collected samples.
Management depends heavily on identifying the specific bacterial species and its antibiotic susceptibility profile. Diphtheroids frequently exhibit resistance to multiple classes of antibiotics, making susceptibility testing necessary before selecting appropriate treatment. If a medical device is the source of the infection, such as an infected catheter or prosthetic valve, successful treatment often requires removing the colonized device in addition to a prolonged course of antibiotic therapy.