Bacilos Gram-positivos

Corynebacterium spp. (difteroides)

Pleomorphic Gram-positive bacilli in palisade / Chinese-letter arrangement

Identification & Growth

Identification

Catalase-positive, non-motile
C. urealyticum: strongly urease-positive, slow growth

Growth Conditions

Blood agar, 35 °C, 24–48 h; C. urealyticum may need 48–72 h
C. diphtheriae: tellurite agar (black colonies) and Loeffler medium

Clinical Significance

A skin contaminant in most cases
Clinically relevant with prosthetic material, catheters, alkaline urinary infection (C. urealyticum) and diphtheria

Intrinsic Resistance

Fosfomycin in several species; polymyxins and aztreonam have no activity

Bench Alerts

Only regard as significant in pure culture, sterile-site material, or on repeat isolation
Vancomycin is the predictable option for multidrug-resistant strains

Clinical Notes

Clinical presentations

Most species are part of skin microbiota and are frequently considered contaminants in routine cultures.
Corynebacterium jeikeium causes bacteremia and device infection in neutropenic patients and long-term catheter carriers.
Corynebacterium diphtheriae, when toxigenic, causes respiratory diphtheria with pharyngeal pseudomembrane and cardiac/neurologic complications.

Specimens and collection

Oropharyngeal/nasopharyngeal swab beneath the pseudomembrane is needed for culture and toxigenicity testing when diphtheria is suspected.
Blood cultures from catheterized, neutropenic patients should allow adequate incubation time, as some species grow more slowly.
Isolation from a skin specimen should be clinically correlated before being considered a causative infectious agent.

Epidemiology and at-risk populations

Diphtheria is rare in populations with adequate vaccine coverage (diphtheria toxoid) but persists in regions with low vaccination coverage.
Neutropenic oncology patients and prosthesis/catheter carriers are the main risk group for invasive infection by non-diphtheriae species.
Asymptomatic skin colonization by Corynebacterium spp. is universal, complicating interpretation of skin and wound cultures.

Resistance and therapeutic implications

Corynebacterium jeikeium is frequently multidrug-resistant, with preserved susceptibility typically only to vancomycin.
Diphtheria treatment requires diphtheria antitoxin combined with antibiotic therapy (penicillin or erythromycin) to eradicate the carrier state.
Susceptibility testing should be performed only when clinical significance is established, as profiles vary widely among species.

Bench and reporting notes

Pleomorphic Gram-positive rods arranged in palisades or "Chinese letters" pattern guide genus-level identification.
Growth on Tinsdale or tellurite agar, with blackened colonies, is used for C. diphtheriae screening.
The report should explicitly indicate the need for toxigenicity testing (PCR or Elek test) to confirm clinical significance in suspected diphtheria.

Sources

Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.

FAQ: Frequently Asked Questions

How to identify Corynebacterium spp. (difteroides) in the lab?

Corynebacterium spp. (difteroides) is identified through catalase-positive, non-motile, c. urealyticum: strongly urease-positive, slow growth.

What are the intrinsic resistances of Corynebacterium spp. (difteroides)?

This organism is naturally resistant to fosfomycin in several species; polymyxins and aztreonam have no activity. These drugs should not be reported as susceptible.

Where is Corynebacterium spp. (difteroides) commonly found?

It is typically associated with a skin contaminant in most cases, clinically relevant with prosthetic material, catheters, alkaline urinary infection (c. urealyticum) and diphtheria.

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