A case of cutaneous Mycobacterium chelonae infection requiring a differential diagnosis of Mycobacterium stephanolepidis infection

Abstract

A case of cutaneous Mycobacterium chelonae infection requiring a differential diagnosis of Mycobacterium stephanolepidis infectionAn 87-year-old Japanese man was referred to our hospital with a complaint of violaceous plaques progressively growing for over a year on his left forearm. The patient's medical history comprised contact dermatitis (treated with oral prednisolone; 7.5 mg/day), chronic kidney disease, myocardial infarction, diabetes mellitus, and abdominal aortic aneurysm post artificial blood vessel replacement. Physical examination revealed continuous, violaceous plaques with papules and nodules (Figure 1A,B). Histopathological analysis revealed a granulomatous lesion with neutrophil infiltration in the dermis (Figure 1C,D), and Ziehl-Neelsen staining revealed the presence of numerous acid-fast bacilli (Figure 1E). Plaque culture revealed rapidly growing mycobacteria. Matrix-assisted laser desorption ionization time-offlight mass spectrometry (MALDI-TOF MS) with MBT Mycobacteria Library v6.0 (Bruker Daltonik) revealed that the isolate was most probably Mycobacterium stephanolepidis (Score Value: 1.85) (the second possibility being Mycobacterium chelonae; Score Value: 1.83). late were ≥99% identical to those of M. chelonae subsp. gwanakae MOTT36W (1537/1537 bp, 485/485 bp, 747/752 bp, 485/489 bp, respectively), 1,2 but they were less homologous to those of M. stephanolepidis (1530/1537 bp, 428/439 bp, 737/752 bp, 482/489 bp, respectively). 3 This resulted in a definitive diagnosis of cutaneous M. chelonae infection.M. stephanolepidis was first identified as a pathogenic agent in thread-sail filefish (Stephanolepis cirrhifer) and black scraper (Thamnaconus modestus) by Fukano et al. in 2017. 3,4 It is a rapidly growing mycobacterium, closely related to M. chelonae, and only two reports have described its identification and genome sequence till date. 3,4 In contrast, M. chelonae is ubiquitous in various environments and is commonly associated with skin and soft tissue infections in humans.MALDI-TOF MS is a rapid and cost-effective method that precisely identifies mycobacterial species 5 ; however, absolute discrimination may not always be feasible. In our case, cutaneous infection with either M. stephanolepidis or M. chelonae was indicated by MALDI-TOF MS with very close score values, probably as the two strains were closely related. Subsequently, WGS conclusively confirmed M. chelonae as the infective agent. Given that M. stephanolepidis is closely related to M. chelonae, it may be potentially infectious to humans. Though there have been no reports of M. stephanolepidis in humans till date, it is conceivable that a proportion of the previously reported M. chelonae infections may have been caused by M. stephanolepidis.