First Reported Pediatric Case of Mycobacterium obuense Peritonitis Associated With Continuous Ambulatory Peritoneal Dialysis

To the Editors: Rapidly growing mycobacteria (RGM) are nontuberculous mycobacteria that form colonies within 7 days and are increasingly reported due to improved diagnostics and growing host populations. Mycobacterium obuense is a rare RGM historically considered nonpathogenic, although a few human infections have been described. We present the first case of continuous ambulatory peritoneal dialysis (CAPD)-associated peritonitis caused by M. obuense. A 6-year-old girl undergoing CAPD for congenital nephrotic syndrome due to a WT1 mutation presented with fever and cloudy dialysate. She had recurrent peritonitis and a recent Staphylococcus aureus exit-site infection treated with cefaclor. On admission, her peritoneal fluid cell count was 7380/μL and C-reactive protein (CRP) level was 4.9 mg/dL. Despite intraperitoneal cefepime, her fever persisted and CRP increased to 12 mg/dL by day 4. On day 5, a blood culture bottle inoculated with dialysate tested positive and acid-fast staining was also positive. On day 7, M. obuense was identified using a MALDI Biotyper Sirius System (Bruker Daltonics, Billerica, MA). Empirical sulfamethoxazole-trimethoprim, ciprofloxacin and linezolid were initiated and the catheter was removed on day 8; hemodialysis was initiated. The patient became afebrile by day 11. On day 13, antimicrobial susceptibility testing using the Broth MIC RGM (Kyokuto Pharmaceutical Industrial Co., Ltd, Tokyo, Japan) revealed that M. obuense was susceptible to clarithromycin [minimum inhibitory concentration (MIC) < 0.06 µg/mL], amikacin (MIC < 4 µg/mL), imipenem (MIC < 2 µg/mL), sulfamethoxazole/trimethoprim (MIC = 4.8/0.25 µg/mL), ciprofloxacin (MIC < 0.12 µg/mL) and linezolid (MIC < 1 µg/mL). Once CRP normalized to 0.4 mg/dL (day 35), linezolid was discontinued to minimize the thrombocytopenia risk and sulfamethoxazole-trimethoprim and ciprofloxacin were continued for 12 weeks. A new CAPD catheter was inserted after 3 relapse-free weeks and the patient remains well over a year later without recurrence. M. obuense infection is exceedingly rare, with only 4 reported adult cases: skin infection in an older woman; pneumonia with bacteremia in a 29-year-old man; disseminated infection in an immunocompromised 16-year-old man; and bacteremia in an 84-year-old man (Table 1).1–3 Diagnostic delay is common because RGM grow more slowly than typical bacteria and are often misidentified as Gram-positive rods (eg, Corynebacterium spp.).4 Acid-fast staining and tools such as matrix assisted laser desorption/ionization‒time of flight are therefore essential in suspicious cases. Treatment of RGM infections generally requires combination therapy using agents such as macrolides, amikacin, imipenem, sulfamethoxazole-trimethoprim, quinolones and linezolid. Because resistance varies and some strains show inducible resistance [eg, erm(41)-mediated clarithromycin resistance], susceptibility testing is essential.5 Current guidelines recommend prompt catheter removal and at least 6 weeks of antibiotics,4 although longer durations may be needed. In our case, an extended 12-week treatment was selected to ensure eradication, considering the need for long-term CAPD. The emergence of M. obuense in this patient is unknown but may be related to prior CAPD infections and repeated antibiotic exposure, potentially selecting for atypical organisms. TABLE 1. - Clinical Characteristics, Treatment and Outcomes of Mycobacterium obuense Infections: A Summary of Reported Cases Including the Present Case Age/Sex Underlying Diseases Focus Antibiotics Duration Outcome Not reported/female* Not reported Cutaneous infection CAM → MINO Not reported Not reported 29 yo/male None Pneumonia AMK + CAM + MFLX → AZM + MFLX 2 + 4 wk Cure 16 yo/male Leukemia Disseminated infection AMK + RFP + AZM + MFLX → RFP + AZM + MFLX 4 + 20 wk Cure 84 yo/male Cerebral infarction Bacteremia IPM/CS + AMK + CAM → CAM + LVFX 10 d + 4 wk Cure 6 yo/female (this case) Congenital nephrotic syndrome CAPD peritonitis SMX/TMP + CPFX + LZD → SMX/TMP + CPFX 4 + 8 wk Cure *Described in original report as older woman; exact age not stated.AMK indicates amikacin; AZM, azithromycin; CAM, clarithromycin; CPFX, ciprofloxacin; IPM/CS, imipenem/cilastatin; LVFX, levofloxacin; LZD, linezolid; MFLX, moxifloxacin; MINO, minocycline; RFP, rifampicin; SMX/TMP, sulfamethoxazole-trimethoprim; yo, years old. This is the first pediatric and CAPD-associated case of peritonitis due to M. obuense. Early recognition using acid-fast staining and mass spectrometry, catheter removal and tailored antimicrobial therapy are crucial. In our patient, 12 weeks of antibiotics, combined with catheter removal and temporary hemodialysis, resulted in resolution. Atypical pathogens should be considered in refractory CAPD peritonitis and susceptibility-guided therapy in RGM infection. ACKNOWLEDGMENT We thank Editage (https://www.editage.jp/) for editing a draft of this manuscript.