Diphtheroids are a diverse group of bacteria frequently encountered on the human body, acting as a natural part of the skin’s microbial community. These organisms are typically harmless residents, coexisting peacefully when the body’s defenses are intact. They possess the capacity to transform into problematic agents, particularly when the skin barrier is compromised. This transition to an opportunistic colonizer is a significant focus in clinical settings regarding wound care and infection control. Understanding their biology reveals why they are often dismissed as simple contaminants, yet also recognized as potential causes of serious infections.
Identifying Diphtheroids
The term “diphtheroids” is a descriptive designation used in microbiology for aerobic, non-spore-forming, Gram-positive bacilli that resemble Corynebacterium diphtheriae. This group encompasses a wide variety of species, primarily belonging to the genus Corynebacterium, excluding the toxin-producing strain. Under a microscope, these bacteria are characterized by their pleomorphic, irregularly shaped rod structure, often appearing club-shaped or arranged in distinctive palisade or “Chinese letter” patterns.
Diphtheroids are common inhabitants of the skin and mucous membranes, thriving particularly in moist areas like the armpits, between the toes, and within the nasal passages. Their prevalence often leads to them being dismissed as mere contaminants during clinical testing. However, their consistent presence provides the reservoir from which they can potentially launch an infection.
Commensal Role on Healthy Skin
Diphtheroids function as crucial members of the resident microbiota, contributing to the overall ecological balance of the cutaneous environment. Their presence is generally beneficial, reinforcing the skin’s natural defenses against invading pathogens. This beneficial relationship is maintained through competitive exclusion.
By occupying available niches and consuming local nutrients, diphtheroids prevent harmful, transient bacteria from establishing a foothold. The high density of these resident bacteria makes it difficult for pathogens to adhere to skin cells and multiply. Furthermore, certain Corynebacterium species produce antimicrobial substances that directly inhibit the growth of other microorganisms.
These bacteria also influence the local chemical environment. They metabolize components of sweat and sebum, contributing to the slightly acidic pH of the skin, typically around 5.5. This acidic environment is unfavorable for the survival and proliferation of many pathogenic species, helping to maintain skin homeostasis.
Shift to Opportunistic Wound Colonization
The shift from a harmless commensal to an opportunistic pathogen occurs when the natural skin barrier is breached, such as through a wound, surgical incision, or the insertion of a medical device. This disruption provides diphtheroids direct access to underlying tissues and the bloodstream, enabling them to cause infection. Risk factors accelerating this transition include severe immunosuppression, chronic illnesses like diabetes, and prolonged hospitalization.
Once inside a wound environment, certain pathogenic diphtheroid species, notably Corynebacterium striatum and Corynebacterium jeikeium, are recognized as significant causes of complicated infections. C. striatum has emerged as a nosocomial pathogen, frequently associated with hard-to-heal peripheral wounds, including foot ulcers and venous leg ulcers.
A major virulence factor is the ability of diphtheroids to form biofilms. This complex matrix adheres to surfaces, shielding the bacterial community from immune cells and significantly reducing antibiotic effectiveness. Foreign bodies, such as central venous catheters, provide an ideal surface for biofilm formation, making device-related infections common.
Clinical Diagnosis and Treatment
Diagnosing a clinically significant diphtheroid infection requires careful interpretation, moving past the assumption that the organism is simply a contaminant. Clinicians must correlate laboratory findings with clear evidence of infection, such as fever, inflammation, or the isolation of heavy growth from a normally sterile site. Microscopic examination showing numerous inflammatory cells alongside the Gram-positive bacilli strongly suggests a true infection.
For confirmed infections, laboratory identification of the specific species is performed using biochemical tests or specialized assays, followed by antibiotic susceptibility testing. Susceptibility testing is necessary due to the frequent presence of multidrug resistance in pathogenic strains, which often exhibit resistance to common antibiotics like macrolides and co-trimoxazole.
Most strains remain susceptible to vancomycin, making it a frequent choice for initial, broad-spectrum treatment, especially in serious cases involving indwelling devices. Effective management relies on antimicrobial therapy guided by susceptibility results, combined with general wound care strategies, including debridement to remove necrotic tissue and the biofilm matrix.