Where Corynebacterium Normally Lives and Its Pathogenicity
Corynebacterium species are normal inhabitants of human skin, mucous membranes (including the nasopharynx and conjunctiva), and are typically non-pathogenic commensal organisms, but they can become opportunistic pathogens in immunocompromised patients or those with breached epithelial barriers.
Normal Habitat
Corynebacterium species colonize multiple body sites as part of the normal human microbiome:
- Skin: Corynebacterium is abundant on skin surfaces, particularly in the axilla and other moist areas 1
- Nasopharynx: Starting soon after birth, the nasopharynx is colonized with normal flora including Corynebacterium species 2
- Conjunctiva: Corynebacterium species are commonly found in the conjunctiva of healthy adults 3
- Mucous membranes: These organisms colonize various mucosal surfaces throughout the body 4, 5
The predominant species varies by anatomical site. For example, C. macginleyi is the predominant strain in the conjunctiva 3, while multiple species including C. tuberculostearicum colonize the skin 1.
Pathogenic Potential
Generally Non-Pathogenic with Important Exceptions
While Corynebacterium species are typically considered non-pathogenic commensals, they have emerged as clinically significant opportunistic pathogens under specific circumstances 6:
High-Risk Clinical Scenarios:
- Immunocompromised patients (diabetes, HIV, long-term steroid users) 3, 5
- Breached epithelial barriers (trauma, corneal damage, surgical wounds) 3
- Indwelling medical devices (catheters, prosthetic joints, CSF shunts) 7, 6
- Previous antibiotic exposure (selects for resistant strains) 5, 7
- Hospital-acquired infections (nosocomial transmission documented) 5
Pathogenic Species and Associated Infections
Respiratory tract infections: C. pseudodiphtheriticum, C. striatum, C. propinquum, and C. simulans can cause pneumonia, particularly in critically ill and immunosuppressed patients 4, 8
Ocular infections: C. macginleyi, C. propinquum, C. mastitidis, C. pseudodiphtheriticum, C. accolens, C. striatum, C. xerosis, and C. bovis cause keratitis and conjunctivitis 3
Bloodstream infections: C. striatum and C. jeikeium are responsible for a large portion of bacteremia cases 6
Device-related infections: C. striatum causes catheter-associated infections, including peritoneal dialysis catheter exit site infections 7
Urinary tract: C. urealyticum is associated with encrusted cystitis 6
Important Clinical Distinction
The critical exception is C. diphtheriae, which produces diphtheria toxin and causes life-threatening respiratory diphtheria. This is preventable through vaccination with diphtheria toxoid-containing vaccines 9, 10, 9. C. ulcerans can also produce diphtheria toxin and cause clinically indistinguishable diphtheria-like illness 11.
Key Clinical Pitfalls
Do not dismiss Corynebacterium as contaminants when:
- Isolated in pure culture from normally sterile sites 7
- Gram-positive rods visible on direct Gram stain with leukocyte reaction 7
- Clinical signs of infection present in high-risk patients 12, 13
- Multiple positive blood cultures or short time to positivity (≤25 hours suggests true bacteremia) 14
Antibiotic resistance considerations:
- Many strains are multidrug-resistant, particularly C. striatum and C. jeikeium 5, 6
- Ocular isolates show resistance to quinolones but remain susceptible to third-generation cephalosporins 3
- Vancomycin remains highly effective for serious infections 4, 7
- Always perform susceptibility testing on clinically significant isolates 7, 15
The pathogenic potential of Corynebacterium correlates directly with antimicrobial resistance—resistant strains demonstrate increased adhesiveness, invasiveness, and cytotoxicity 15.