Corynebacterium species except Corynebacterium diphtheriae are part of the normal flora of the skin and mucous membrane and occur as common contaminants in diagnostic specimens. Therefore, physicians often tend to consider these organisms as contaminants. However, these microorganisms have increasingly been recognized as pathogens in immunocompromised patients [1]. Corynebacterium amycolatum, in particular, is a common clinical isolate that often shows multidrug resistance to commonly used antibiotics such as β-lactams, macrolides, and fluoroquinolones [2, 3]. In this report, we present a case of bacteremia caused by C. amycolatum possessing a novel mutation in the gyrA gene, which confers high-level quinolone resistance to the organism.

An 88-yr-old man with diabetes mellitus presented to our emergency department with chest pain. There was no definite evidence of acute coronary syndrome, but laboratory examination findings indicated impending end-stage renal disease. Hemodialysis was performed through a subclavian double-lumen catheter on alternate days. After 3 sessions of hemodialysis, on the 11th hospital day, the patient developed fever (temperature, 38.0℃), and the blood profile showed a total white blood cell count of 8,800/µL with 74% neutrophils and an elevated C-reactive protein level of 134.2 mg/L. Hence, samples for blood culture were drawn in triplicate, and empiric antibiotic therapy was initiated with 200 mg of ciprofloxacin administered intravenously every 12 hr. On the 14th hospital day, after 72 hr of incubation, "diphtheroid-like organisms" were detected in all 3 aerobic blood-culture bottles. Although the patient had fever and elevated C-reactive protein level, which do indicate bacteremia, the positive blood culture was considered to show merely contaminants, and therefore, additional blood cultures were performed to identify the causative agent. The repeated blood cultures yielded the same results. Therefore, the antibiotic therapy was changed to vancomycin (1 g/day), and the indwelling double-lumen catheter was removed because of suspected catheter-related infection. Follow-up blood cultures were negative, and the patient recovered fully.

Although its pathogenic role was apparent only in few cases, C. amycolatum, like Corynebacterium striatum, can cause endocarditis, bacteremia, and catheter exit-site infection. In cases wherein clinically significant infection by any of the coryneform bacteria is suspected, species identification and antibiotic susceptibility determination are important to decide and administer the optimal antibiotic and treatment regimens. However, possibilities like misidentification of species or non-availability of adequate data for using automated kits with identification cards should be considered. Genotypic assays such as 16s rRNA sequencing may be required to confirm the identity of species. In this present case, the clinical isolate showed a high level of resistance to ciprofloxacin (MIC value of >32 µg/mL), probably due to the novel mutation in the gyrA gene, wherein the codon for Ser-87 mutated to form an Ile codon and the codon for Asp-91 mutated to a Gly codon [6]. Therefore, in cases of suspected Corynebacterium spp. infection, the physician and the microbiologists in laboratories should confirm the antibiotic susceptibility patterns of the clinical isolates to help decide the optimal antibiotic and treatment regimens.