Skin infection by Mycobacterium farcinogenes–senegalense group in an immunocompetent patient: a case report

Mycobacterium farcinogenes–senegalense group is rapidly growing, non-tuberculous mycobacterium that causes bovine farcy. Mostly documented in sub-Saharan Africa, Mycobacterium farcinogenes and Mycobacterium senegalense result in chronic suppurative granulomas of skin and lymphatics in cattle. They are closely related to Mycobacterium fortuitum and Mycobacterium houstonense. In fact, Mycobacterium farcinogenes–senegalense group rarely affects human beings. [1]

A 66-year-old Taiwanese woman, who was free from immunocompromised conditions including diabetes mellitus or acquired immunodeficiency syndrome, developed enlarging tender nodules on her left dorsal foot for 2 months. Two months prior to this presentation, her left dorsal foot was traumatized by the spring of a trashed mattress. Physical exam showed reddened, indurated, and confluent nodules with pus discharge (Fig. 1). She was afebrile and she denied other constitutional symptoms. She initially tried acupuncture on the left lower leg for but failed. 10 days of empirical oral amoxicillin clavulanate (Augmentin^® 1250 mg/day) were administered after the sampling of the pus. Though the culture turned out to be negative, the lesion deteriorated. To exclude atypical infection, we performed skin biopsy for pathology exam, as well as fungal and mycobacterial culture.

Microscopically, skin specimen demonstrated dermal suppurative granulomas composed of histiocytes and multinucleated giant cells. A few acid-fast bacilli were identified by Ziehl–Neelsen stain. 14 days after the culture, wheat-colored colonies developed (Fig. 2). Using Matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS; Bruker’s MALDI Biotyper, Bruker Libraries/Mycobacteria Library V2.0) to compare the extracted proteins from the colony to the reference, we successfully identified the pathogen to be the Mycobacterium farcinogenes–senegalense group (MS score 1.94; MS score 1.800–1.999 for species level for mycobacteria). After confirming the pathogen, the antibacterial regimens, featuring the combination of oral clarithromycin (1000 mg/day) and sulfamethoxazole/trimethoprim (Baktar^®, 1960 mg/day), were administered for 2 months. Nevertheless, sulfamethoxazole/trimethoprim was held 2 weeks later due to hyperkalemia. The skin lesion resolved gradually thereafter and the residual lesion was eventually cured by surgical removal (Fig. 1d).

MALDI-TOF MS detects the bacterial protein profile and identifies organisms based on database. It provides simple, fast, and reliable identification in most Mycobacterium species and is a potential alternative laboratory method in routine clinical care [2, 3].

We report a human skin infection by Mycobacterium farcinogenes–senegalense, which usually causes bovine farcy, in an immunocompetent woman after traumatic injury by rusty springs. The first human victim of Mycobacterium senegalense was a 49-year-old woman with non-Hodgkin’s lymphoma in Korea due to catheter-related bloodstream infection [4]. An immunocompetent 67-year-old man in Hong Kong had prosthetic infection of Mycobacterium farcinogenes after receiving total hip arthroplasty [5]. An immunocompetent 3-year-old North American girl got soft tissue infection of Mycobacterium senegalense after traumatic injury [6]. A North American 55-year-old immunocompetent male developed chronic osteomyelitis of Mycobacterium senegalense after traumatic ankle fracture [7]. Some following cases of osteomyelitis related to Mycobacterium farcinogenes were reported. [8, 9]. We summarized the above-mentioned cases in Table 1. We reported an immunocompetent case of Mycobacterium farcinogenes–senegalense group soft tissue infection in Asia, indicating that the infection is worldwide and not limited to immunocompromised patients. Due to its rarity, the consensus of the standard treatment is lacking. Treatment may include amikacin, cefoxitin, clarithromycin, ciprofloxacin, doxycycline, erythromycin, imipenem, trimethoprim/sulfamethoxazole, which is similar to the regimens for rapidly growing mycobacterium. Drug sensitivity test could be beneficial. Surgical excision could be considered in localized lesion in immunocompetent hosts.

The conventional method in isolating bacterial strains from the tissue allows in vitro drug susceptibility test which is useful for clinical decision making. Using the MALDI-TOF method, we could only identify the strain as Mycobacterium farcinogenes–senegalense group. However, due to the institutional biosafety regulation, the sample was disposed once the lab data of MALDI-TOF was obtained. Therefore, we could not perform the drug susceptibility test at this point. Furthermore, the current database does not allow us to distinguish the species of these two closely related Mycobacterium farcinogenes and Mycobacterium senegalense.

In conclusion, we reported one rare Mycobacterium farcinogenes–senegalense group cutaneous infection in an immunocompetent patient in Taiwan. The lesion was partially resolved after 2 months of clarithromycin and finally cured by the surgical removal of the remaining. Mycobacterium farcinogenes–senegalense group infection should be considered as potential pathogen of skin infection.