Among coagulase-negative staphylococci (CoNS) species, Staphylococcus pettenkoferi is a relatively recently discussed member. It was first isolated from blood culture in 2002 [1]. Several additional cases have been reported including bacteremia accompanied with tuberculosis and Stevens-Johnson syndrome [2] and osteomyelitis [3]. There were 2 case reports of S. pettenkoferi in Korea [24]. This novel isolate was hardly identified by conventional automated identification methods like Vitek system, thus 16S rRNA sequencing had to be used for confirmation. The matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) was introduced to laboratories recently and the identification of this isolate was confirmed by Brucker Biotyper (Brucker Daltonics, Bremen, Germany), but not Vitek MS (bioMérieux, Marcy-L'Etoile, France) [4]. The present report describes two cases of S. pettenkoferi bacteremia identified by Vitek MS (bioMérieux).

A 38-year-old woman was admitted to our hospital for chemotherapy of gastric cancer after 3 years of the surgery due to peritoneal carcinomatosis. The patient was febrile on the day of admission, but otherwise was asymptomatic. She had mild spiking fever continuously, so blood samples were drawn for culture. They were inoculated into aerobic and anaerobic blood culture bottles and incubated in a BacT/ALERT 3D blood culture instrument (bioMérieux, Marcy-L'Etoile, France). The bottles were scored positive after 22 hours and 40 minutes of incubation at 37℃. The colony was 1.0–2.0 mm sized circular, glistening, and whitish with no hemolysis on blood agar plate. Microscopic examination revealed Gram-positive cocci in clusters and was negative for coagulase and positive for catalase, so the isolates were initially concluded as CoNS. With the Vitek2 system (bioMérieux, Marcy-L'Etoile, France), it was identified as Staphylococcus auricularis or Staphylococcus capitis with low discrimination of 50% each. Therefore, Vitek MS IVD Database Version 3.0 (bioMérieux) was used and identified as S. pettenkoferi (99.9%). As the two results were different, sequencing of the 16S rRNA gene using the MiSeq Microbial Identification System (Macrogen, Seoul, South Korea) was performed. Consensus sequence of 1547bp was obtained and reported the organism to be S. pettenkoferi (99%). Antimicrobial susceptibility test was performed using Vitek2 system. The isolate was resistant to oxacillin (minimal inhibitory concentration (MIC) 4 mg/L), but susceptible to vancomycin (MIC 0.5 mg/L) and linezolid (MIC 2 mg/L) (Table 1). After a week of meropenem therapy, no organisms were grown from blood and she was discharged from the hospital without any symptoms.

The second patient was a 90-year-old woman who had fever of unknown origin for a few days but refused to be treated. She visited emergency department with general weakness, drowsy mentality, and aphasia in addition to high fever for 4 days. She was diagnosed with encephalitis, possible diagnosis of infective endocarditis and bacteremia. Two blood samples from separate venipuncture sites were drawn for culture. After 24 hours of incubation, one aerobic bottle was positive and was subcultured onto blood agar plates. Biochemical tests revealed them as CoNS. With the Vitek2 system (bioMérieux), it was identified as Leuconostoc mesenteroides ssp. cremoris (93%). Susceptibility test using Vitek2 system (bioMérieux) revealed sensitivity to vancomycin (MIC 10 mg/L). As this result conflicted the fact that Leuconostoc spp. is intrinsically resistant to vancomycin, further identification was performed by Vitek MS and concluded as S. pettenkoferi (99.9%). The fever subsided after the patient had received 8 days of naficillin and was transferred to other hospital with no further management because of the patient's refusal.

CoNS are important cause of infections in hospitalized patients, especially in immunocompromised patients as opportunistic pathogens. It is also known that CoNS are associated with infections of indwelling catheters or implanted devices [35]. Staphylococcus epidermidis, Staphylococcus haemolyticus and Staphylococcus saprophyticus are the most common infectious species among CoNS [6]. However, other CoNS still can be the cause of human infections which may be mortal. Therefore, it is important to identify them accurately for treatment with proper antibiotics [7]. In a study of microbiological identification of six S. pettenkoferi isolates [4], all of them were detected by Brucker Biotyper MS (Brucker Daltonics) and 16S rRNA gene sequencing but Vitek MS (bioMérieux) did not identify any of them by Vitek MS IVD Database Version 2 (bioMérieux). By contrast, we had no difficulty to identify S. pettenkoferi with Vitek MS (bioMérieux) in both isolates. This novel isolate may not have been detected even though it existed in the past, because it has not been long since the MALDI-TOF MS was introduced to laboratories. Furthermore, S. pettenkoferi was newly added in the Vitek MS IVD Database Version 3.0 (bioMérieux) which was used in our laboratory. Thus it was identified successfully. The conventional identification may result in misidentification of S. pettenkoferi as Staphylococcus hominis, S. auricularis, S. capitis, Kocuria varians, or even Leuconostoc mesenteroides ssp. cremoris. More cautious approach for accurate identification of CoNS by molecular methods including Vitek MS (bioMérieux) is needed in case of bloodstream infection. Therefore, we recommend to use Vitek MS (bioMérieux) which is rapid and accurate for the identification of the pathogen in bloodstream infection [8] when CoNS is suspected.