Cellulitis Caused by Mixed Infection with Group A Streptococci and Non-Toxigenic tox-Gene-Bearing Corynebacterium...

Cutaneous diphtheria has re-emerged in Europe, often presenting as skin-limited disease. Uncertainty around diagnosis, infection control, and toxigenicity testing can delay appropriate management.

A 62-year-old woman experiencing homelessness presented with a 2-week history of a painful lower-leg eruption. This evolved from vesicles to bullae and shallow ulcers with a characteristic green-grey pseudomembrane, unresponsive to oral flucloxacillin. Swabs of the ulcer grew Corynebacterium diphtheriae and concurrent group A β-haemolytic streptococci, while throat swabs were negative. The national reference laboratory confirmed a non-toxigenic, tox-gene-bearing strain by tox polymerase chain reaction with a negative Elek test. After failing to respond to intravenous flucloxacillin, the patient was treated empirically with intravenous ceftriaxone, followed by oral azithromycin 500 mg once daily for 10 days, which led to complete clinical resolution.

This case highlights that skin-limited C. diphtheriae can mimic common skin infections and coexist with other pathogens. Key management priorities include (1) early lesion culture with specific request for C. diphtheriae and tox-gene testing; (2) prompt notification to public health authorities; and (3) implementation of appropriate infection-control precautions. For non-toxigenic cutaneous disease, a macrolide or penicillin-based antibiotic course is typically sufficient, and diphtheria antitoxin is generally not required.