LQCL2612 - CMLE - Considerations for Work-Up of Cutaneous Non-Toxigenic Corynebacterium diphtheriae in the Clinical Microbiology Laboratory-LQCL2612

LQCL2612 - Educational Activity

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The document is a microbiology case-based module on emerging <strong>non-toxigenic Corynebacterium diphtheriae</strong> wound infections and the implications for clinical laboratory workflow and public health reporting.

A 37-year-old man with houselessness, injection drug use, chronic lower-extremity wounds, and prior MRSA skin infections presented with a necrotic, purulent right foot wound and mild leukocytosis. A wound swab Gram stain showed mixed flora including gram-positive cocci and many “diphtheroid” gram-positive bacilli in palisades. Culture grew <strong>Streptococcus pyogenes</strong>, <strong>Staphylococcus aureus</strong>, <strong>Staphylococcus epidermidis</strong>, and a coryneform gram-positive rod. Although such rods are often dismissed as skin flora, MALDI-TOF identification unexpectedly revealed <strong>C. diphtheriae</strong>, prompting escalation and confirmatory actions.

The module contrasts classic <strong>toxigenic respiratory diphtheria</strong> (pseudomembrane, high morbidity/mortality due to diphtheria toxin with potential systemic effects such as myocarditis and neuropathy) with <strong>non-toxigenic cutaneous/wound diphtheria</strong>, which is increasingly reported in the U.S. and is not prevented by diphtheria toxoid vaccination. Non-toxigenic infections commonly present as chronic/non-healing wounds and are frequently <strong>polymicrobial</strong>, often co-isolated with <strong>beta-hemolytic streptococci (especially S. pyogenes)</strong> and/or <strong>S. aureus</strong>, reflecting shared risk factors (e.g., poor living conditions, substance use, immunocompromise). While disease severity is generally lower without toxin, invasive complications (bacteremia, endocarditis, osteomyelitis) can occur.

Because <strong>C. diphtheriae is reportable</strong>, laboratories must notify public health and submit isolates for CDC testing (including the <strong>Elek test</strong> for toxin). The case isolate was confirmed as <strong>non-toxigenic biovar gravis</strong>, pan-susceptible; typical therapies include <strong>penicillins or macrolides</strong>. The lab updated procedures to <strong>identify diphtheroid GPRs</strong> when present with <strong>S. aureus and/or beta-hemolytic streptococci</strong>, adding staffing to support increased workload.

Keywords

non-toxigenic Corynebacterium diphtheriae

cutaneous diphtheria

wound infection

clinical microbiology laboratory workflow

public health reporting

MALDI-TOF identification

Elek test

polymicrobial wound culture

Streptococcus pyogenes co-infection

biovar gravis