Opinion Paper

From neglect to necessity: The case for routine work-up of nontoxigenic Corynebacterium diphtheriae in clinical microbiology

Kgaogelo R. Masemola

Received: 03 Mar. 2025; Accepted: 04 Aug. 2025; Published: 17 Sept. 2025

Copyright: © 2025. The Author(s). Licensee: AOSIS.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Introduction

Toxigenic Corynebacterium diphtheriae is well known for causing classical respiratory diphtheria, a life-threatening disease controlled by widespread toxoid vaccination.^1,2 Nontoxigenic C. diphtheriae strains (those lacking diphtheria toxin production), on the other hand, have received less attention.^1,3 However, nontoxigenic C. diphtheriae infections are being increasingly reported globally in recent times.⁴ For years, clinical laboratories often dismissed corynebacteria from non-respiratory sites as ‘diphtheroids’ (skin or mucosal commensals/contaminants), and did not identify them to species level.⁵ This practice has led to under-recognition of infections by nontoxigenic C. diphtheriae.¹ With improvements in identification methods and accumulated case reports, nontoxigenic C. diphtheriae is now understood as an emerging pathogen capable of causing invasive disease, even in immunised populations.^5,6 The aim of this article is to underscore why ‘diphtheroids’ should no longer be routinely overlooked as insignificant commensals dismissed as contaminants, and to encourage laboratories and clinicians to recognise and appropriately manage nontoxigenic C. diphtheriae infections.⁵

Global perspective

Nontoxigenic C. diphtheriae infections have been increasingly reported since the late 20th century.^1,4 These infections often occur in socioeconomically marginalised groups and settings (Table 1) where the organism can colonise damaged skin.⁷ In the 1980s and 1990s, outbreaks and case clusters were documented in various regions.^4,8 By the mid-1990s, experts were already warning of the resurgence of C. diphtheriae and the rise of nontoxigenic C. diphtheriae in hospital settings.⁴ Studies and surveillance reports have indicated that nontoxigenic C. diphtheriae infections are rising, often linked to intravenous drug use, homelessness, and skin infections.^7,9

African perspective

African cases of nontoxigenic C. diphtheriae infections are less well documented, likely because of underreporting.^1,2 Despite this, there is growing recognition that these infections also occur in Africa and can be severe.¹ South Africa has reported sporadic invasive cases over the years such as an outbreak of five fatal nontoxigenic endocarditis cases in 2021.¹ Unlike Europe and North America, where nontoxigenic C. diphtheriae surveillance is more documented, African epidemiological data remain sparse. The true burden of disease is therefore unknown.

Clinical manifestations and disease spectrum

Nontoxigenic C. diphtheriae infections can present in a variety of ways. Cutaneous diphtheria manifests as chronic, non-healing ulcers, or sores, which often serve as reservoirs for transmission.^10,14 Invasive bloodstream infection and endocarditis are the most severe form of nontoxigenic C. diphtheriae disease, with a high mortality rate.^1,8 Septic arthritis and osteomyelitis are less common, but are serious infections which often occur in the context of trauma or injection drug use.¹⁶ Nontoxigenic strains do not cause classic respiratory diphtheria, but they can colonise the throat and cause sore throat or tonsillitis.⁶ Other rare manifestations of nontoxigenic C. diphtheriae are pneumonia, urinary tract infections (especially in catheterised patients), and other localised infections.^3,15

Patient outcomes

The mortality associated with nontoxigenic C. diphtheriae infections varies depending on the type and severity of the infection, as well as patient-specific factors.³ In a study in Seattle, 9 (20%) of 44 patients with C. diphtheriae detections had death documented in the medical records.³ Nontoxigenic C. diphtheriae infective endocarditis has a mortality rate ranging from 35% to 66%.³ In one cluster of endocarditis cases, the 30-day mortality rate was 80%.¹ Three of the four endocarditis patients in the Seattle study died from endocarditis-related complications within 1 week of presentation.³

Laboratory diagnosis of Corynebacterium diphtheriae

The laboratory work-up for C. diphtheriae is typically undertaken when there is a strong clinical suspicion of diphtheria or in the context of an outbreak. For example, during the 2015 KwaZulu-Natal diphtheria outbreak, laboratories in South Africa were advised to incorporate selective media for the isolation of C. diphtheriae when processing throat swabs.² However, in the absence of these defined scenarios, coryneform bacteria are seldom investigated beyond being classified as ‘diphtheroids’ or ‘Coryneform bacteria’, with many isolates dismissed as contaminants.⁵

Incidental identification of C. diphtheriae has increased with the widespread adoption of advanced technologies such as Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS), which provides rapid species-level identification.³ In cases where further work-up is pursued, laboratory protocols typically involve:

• Gram staining to identify pleomorphic, Gram-positive bacilli with a club-shaped morphology, often arranged in angular ‘Chinese character’ formations.^8,9,13
• Standard culture media, including 5% sheep’s blood agar, chocolate agar, and MacConkey agar, are inoculated, with additional biochemical testing performed on suspect colonies.^8,13 Some laboratories opt to subculture onto selective media such as Cystine tellurite blood agar or Tinsdale medium, where C. diphtheriae produces characteristic black colonies with brown halos.¹⁷
• Genus and species identification is then confirmed using commercial systems such as Analytical Profile Index assays or MALDI-TOF MS, with isolates in some countries being referred to national reference laboratories for confirmation and toxin determination.¹⁷
• The modified Elek test is used for phenotypic expression, while polymerase chain reaction detects the tox gene.¹⁷

Traditionally, antimicrobial susceptibility testing is not routinely performed. The European Committee on Antimicrobial Susceptibility Testing body has published guidelines for susceptibility testing of C. diphtheria and C. ulcerans.¹⁸

Barriers and practical implications

While this article is framed as an advocacy argument for the routine species-level identification of Corynebacteria, the author fully acknowledges that there are practical and cost considerations to be kept in mind. These include:

• Lack of access to diagnostic advances such as MALDI-TOF MS, Analytical Profile Index Coryne kits and molecular diagnostics
• Lack of microbiology reference laboratory support
• Cost of selective culture media and confirmation testing
• Many so-called diphtheroids may actually be unrelated skin or environmental bacteria, not true Corynebacteria
• Increased workload in clinical laboratories because of routine work-up of Coryneform bacteria
• Difficulty in ascertaining the clinical significance of Corynebacteria in polymicrobial cultures
• Lack of clinical information accompanying culture requests.

These practical considerations are not insignificant, and any potential change must account for this. It is within this context that a shift in approach is being suggested.

Paradigm shift in the laboratory approach to Corynebacterium diphtheriae

A transition is required in the approach to coryneform bacteria within clinical microbiology laboratories. This shift must also include improved communication between clinicians and laboratory staff. This will ensure that relevant clinical context accompanies specimen submissions and therefore guide appropriate interpretation of findings. The identification of C. diphtheriae can no longer be relegated to incidental findings.⁵ If this remains the case, its epidemiology will remain poorly understood, and the true burden of disease will continue to be underestimated.

To address this emerging threat, the following steps are critical:

1. Microbiology laboratories should have a heightened index of suspicion for nontoxigenic C. diphtheriae.
2. Public health agencies should integrate nontoxigenic C. diphtheriae into antimicrobial resistance surveillance programmes to track trends in antimicrobial resistance and virulence.
3. Investment in cost-effective diagnostic tools for resource-limited settings is essential for improving detection.

The longstanding practice of routinely dismissing coryneform species-level identification must be reconsidered. To ensure accurate identification and reporting, laboratories should consider diagnostic workflows that incorporate selective media, species confirmation using MALDI-TOF MS or Analytical Profile Index, and toxin determination through reference laboratory testing (Table 2).

Conclusion

Routine species-level identification of C. diphtheriae should be prioritised in laboratories, moving beyond classification of diphtheroids as mere contaminants. Diagnostic workflows should incorporate selective media and advanced identification techniques to enhance detection accuracy. National reference laboratories should expand surveillance efforts to track nontoxigenic C. diphtheriae cases, enabling a better understanding of disease burden and informing public health policies. Clinicians must remain aware of C. diphtheriae as a potential pathogen in bloodstream infections, endocarditis, and soft tissue infections to ensure timely diagnosis and appropriate management. Further research should focus on exploring the molecular epidemiology of nontoxigenic C. diphtheriae and standardising detection and treatment protocols.

Acknowledgements

The author expresses sincere gratitude to colleagues at the National Health Laboratory Service, Department of Medical Microbiology, KwaZulu-Natal, South Africa, whose insights, and discussions provided the inspiration for this article.

The author declares that he has no financial or personal relationships that may have inappropriately influenced him in writing this article.

K.R.M. is the sole author of this article.

This article followed all ethical standards for research without direct contact with human or animal subjects.

The author received no financial support for the research, authorship, and/or publication of this article.

The authors confirm that the data supporting the findings of this study are available within the article and its references.

The views and opinions expressed in this article are those of the author and are the product of professional research. The article does not necessarily reflect the official policy or position of any affiliated institution, funder, agency, or that of the publisher. The author is responsible for this article’s results, findings, and content.

References