Corynebacterium pseudodiphtheriticum has been described as an emerging respiratory pathogen and forms part of normal upper respiratory tract flora.¹ This organism has been linked to pulmonary disease in patients with underlying lung abnormalities, chronic medical conditions and immunocompromised states, including human immunodeficiency virus (HIV),^1,2,3 although disease is also described in immunocompetent patients. Respiratory tract disease caused by this organism is likely to be underestimated, given its role as a respiratory tract commensal.

We report a case of exudative pharyngitis potentially caused by C. pseudodiphtheriticum in an unvaccinated HIV-positive infant. There have been a handful of cases describing exudative pharyngitis in conjunction with C. pseudodipththeriticum to date; this case provides further evidence of its possible aetiological role and highlights the need for its inclusion in the differential diagnosis of diphtheria. This case is of particular relevance in the current climate of reduced vaccination uptake globally, when clinical acumen surrounding the syndrome of exudative pharyngitis should be maintained.

Throat and nasal swabs were inoculated onto tellurite-containing and routine media and incubated at 35 °C in ambient and 5% CO₂-containing incubators, respectively; colony growth was better in the CO₂-enriched atmosphere. Grey-black colonies suspicious of C. diphtheriae were observed on tellurite-containing media after 48 h incubation, with corresponding pure growth of white, butyrous colonies on tryptose blood agar on the nasal sample only. Gram stain of these colonies revealed non-branching, uniform Gram-positive bacilli.

Neither beta-haemolytic colonies nor yeasts were cultured from any of the samples.

Biochemical identification with the VITEK(R) 2 ANC system (bioMérieux, Marcy l’Etoile, France) confirmed by the BD BBL Crystal™ Gram Positive ID Kit (Becton Dickinson and Company, Franklin Lakes, USA), convincingly identified C. pseudodiphtheriticum. Diphtheria toxin was not detected molecularly or phenotypically (in-house PCR negative at the National Institute for Communicable Diseases, Elek test negative).

Limited susceptibility testing was performed using the gradient-diffusion based E-test method (bioMérieux, Marcy l’Etoile, France). Results were interpreted using Corynebacterium species criteria (Clinical and Laboratory Standards Institute M45 document, 2015) and are summarised in Table 1.

Informed consent was obtained from the child’s mother and ethical approval was granted by Stellenbosch University’s Human Health Research Ethics Committee (Reference number C20/01/002).

Corynebacterium pseudodiphtheriticum is an aerobic non-spore-forming Gram-positive bacillus, forming part of the upper respiratory tract flora.^1,2 Recent studies have investigated the probiotic role of C. pseudodiphtheriticum in reducing nasal colonisation with Staphylococcus aureus,^5,6 Streptococcus pneumoniae and respiratory syncytial virus.⁷

Corynebacterium pseudodiphtheriticum has also been associated with respiratory tract infection, including pneumonia, bronchitis and single cases of necrotising tracheitis and a lung abscess.^3,8,9

These infections have largely been noted in patients who are immunosuppressed, such as HIV-positive patients or transplant recipients; patients with underlying lung disease, such as chronic obstructive airways disease; and patients with underlying medical conditions, such as congestive cardiac failure, ischaemic heart disease and malignancy.^1,2,3,10 Patients with cystic fibrosis may be particularly at risk.¹¹

Pulmonary infection has also been linked to invasive respiratory procedures such as endotracheal intubation, presumably because of direct introduction of this commensal into the lung.¹² Recent data suggest that previous exposure to antimicrobials favours colonisation with this organism.¹ The few cases of lower respiratory tract infection reported in immunocompetent patients suggest that an older age may play a role^3,8 but this has been challenged.¹³

Co-infection with recognised respiratory tract pathogens has been reported^2,3,11,12,13 and a co-pathogenic role theorised.¹⁰ Although carriage of these organisms can result in sample contamination, microbiological methods to determine significance include moderate-to-substantial palisading Gram-positive bacilli¹² or diphtheroids within polymorphonuclear leucocytes on Gram stain,³ and substantial growth of diphtheroids on culture,¹⁰ in conjunction with clinical presentation and underlying risk factors.²

Other infectious processes associated with C. pseudodiphtheriticum include infective endocarditis,^14,15 keratitis,¹ endophthalmitis,¹ skin infection,¹⁶ urinary tract infection,¹³ osteitis and septic arthritis.^13,17,18 To our knowledge, three cases of exudative pharyngitis because of C. pseudodiphtheriticum have been reported to date, as summarised in Table 2.

Despite the fact that C. pseudodiphtheriticum has traditionally been described as non-toxigenic,²² we believe confirmation of this is advisable in view of the mechanism of transmission of diphtheria toxin. Diphtheria toxin is carried on a β-phage that infects bacterial cells belonging to the species, C. diphtheriae, C. ulcerans and C. pseudotuberculosis. Clinical experience suggests that this toxin does not affect other Corynebacterium species, although this is not impossible.

The identification of Corynebacterium species can prove complex, adding to the laboratory’s diagnostic dilemma.²³ The gold standard for laboratory identification is sequencing of the hypervariable rpoB gene, which is not routinely available. Sequencing of the 16S ribosomal RNA gene is more widely available but may be inferior to partial rpoB sequencing,²⁴ although database updates may have addressed this.

Common, presently available biochemical/proteomic methods to identify C. pseudodiphtheriticum in South Africa include:

Corynebacterium pseudodiphtheriticum is usually susceptible to penicillin, vancomycin, rifampicin and the aminoglycosides,^2,29 but geographical variability may be marked. A recently published study from Canada reported C. pseudodiphtheriticum/C. propinquum as displaying the lowest minimum inhibitory concentration (MIC) to penicillin.²⁹ In contrast, penicillin resistance was reported in 38.9% – 44.2% of C. pseudodiphtheriticum strains from a variety of anatomical sites in Brazil in 2009; 15.0% of these strains were multidrug-resistant (resistant to more than 10 antimicrobial agents).¹³ In general, Corynebacterium species other than C. diphtheriae have become less susceptible to the beta-lactams in the last two decades.²⁹ In this patient, de-escalation from ceftriaxone to penicillin was not possible because of a shortage of penicillin.

Macrolide and lincosamide resistance occurs consistently,^{1,2,10,12,29,30} which is concerning as erythromycin and azithromycin are recognised alternative options in the management of the clinical syndrome of diphtheria.³¹ Quinolone and trimethoprim-sulphamethoxazole resistance is variable.^3,8,13,29 Susceptibility rates to tetracycine, linezolid and daptomycin are high.²⁹ In this case, the low azithromycin MIC was reassuring for the prophylactic use of this agent in the carriers identified, although the clinical significance of carriage in this context is uncertain.

The limitations of this case study include the isolation of this organism from only a single admission nasal swab with a suggestive but unproven causative link and the absence of comprehensive antibiotic susceptibility testing. We did not assess strain relatedness between isolates, although the absence of this organism from any other contacts screened increases the likelihood of these being genetically related. The production of a novel toxin producing similar effects to diphtheria toxin was not explored and could be researched in future studies.

This case adds evidence to the likely role of C. pseudodiphtheriticum as an emerging pathogen and highlights diagnostic dilemmas faced both clinically and microbiologically. The direct causal link was not present, but the absence of an alternative explanation for the clinical syndrome lends more weight to the possible aetiological role of this organism in exudative pharyngitis. Local susceptibility data are needed, as are further reports exploring the significance of isolation of this organism in a South African setting with our high prevalence of HIV infection. It is important that this organism is considered in the workup of exudative pharyngitis although, until its role is clarified, emphasis should remain on excluding diphtheria in the current climate of suboptimal vaccination coverage and anti-vaccination sentiment.