Postoperative Hardware-Related Infection from Kytococcus schroeteri: Its Association with Prosthetic Material and Hematological Malignancies—A Report of a Case and Review of Existing Literature

Introduction Kytococcus schroeteri is an infrequently isolated Gram-positive coccus often encountered as a commensal bacterium. Only eighteen cases of human infection associated with this organism have been previously reported. Most of these cases involved patients with implanted prosthetic materials or patients with immunosuppressive conditions. It has been described in prosthetic valve endocarditis and in select patients with hematologic diseases but only one prior report as being involved in osteoarticular infections. Case Presentation We describe a case of postsurgical osteoarticular hardware-related infection by K. schroeteri and discuss a possible association with implanted prosthetic material. Conclusion Other clinical presentations of K. schroeteri, including reported infection syndromes, antimicrobial susceptibility profiles, and treatment outcomes, are also reviewed.


Introduction
Kytococcus schroeteri is a Gram-positive coccus that was first identified by 16S rDNA analysis in 2002 [1]. It is a yellowpigmented, aerobically growing, nonencapsulated, nonmotile bacterium. e natural habitat of K. schroeteri is not well defined, but other members of the genus Kytococcus (K. sedentarius) have been isolated from human skin. Human infection caused by this commensal bacterium is uncommon. e first case of K. schroeteri infection was described in 2002, in a patient with prosthetic valve endocarditis [1]. Over the subsequent sixteen years, 18 cases have been reported. Of those, only one prior case has been reported of K. schroeteri causing an osteoarticular infection [2]. We discuss a case of K. schroeteri associated with a postsurgical hardware-related infection and review its known role in other select infection syndromes.

Case Report
Our patient was an 80-year-old female with a history of chronic adrenal insufficiency on oral prednisone. She suffered a left-sided intertrochanteric hip fracture and underwent a surgical implantation of a cephalomedullary nail to stabilize the femoral neck. Over the next two weeks, she developed continuous drainage from the surgical incision. On presentation to the hospital, she had ecchymoses on her left flank and serosanguinous drainage from her left hip incision. She was afebrile on admission but had an elevated white blood cell count of 29 × 10 9 /L. An ultrasound of the hip and groin region showed a hematoma and a large left groin pseudoaneurysm from the profunda femoral artery, which was confirmed by a CT angiogram. e patient underwent coil embolization of the pseudoaneurysm and surgical wound debridement.

Investigations and Diagnosis.
ere were multiple positive culture results for K. schroeteri on hip tissue/perijoint tissue sent intraoperatively; and the treating infectious disease team with orthopedic infectious disease speciality focus felt this was real and constituted a prosthetic joint infection, warranting full treatment and suppression. is strain was resistant to penicillin but susceptible to clindamycin and vancomycin by Mueller-Hinton agar dilution.

Treatment.
e patient was discharged to a care facility and received four weeks of daptomycin. is medication was chosen for out-of-hospital convenience of administration, owing to the once-a-day dosing. She recovered complete mobility of the joint and had no further complications in her course.

Discussion
e increasing number of case reports, including our own, suggests that K. schroeteri can be a formidable pathogen in the appropriate host. e significance of Kytococcus as a human pathogen may not have been fully recognized in years past or previously misidentified as Micrococcus sp. Recent case reports have been able to identify this organism by molecular sequencing or using matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF MS) technology, as in our case. e organism grew on a blood agar plate (a trypticase soy agar plate with 5% sheep's blood) at 35°Celsius in a CO 2 -enriched environment. Colony growth showed muddy yellow-pigmented bacteria. In this report, we identify a case where Kytococcus was isolated from bacterial cultures and was implicated in a postsurgical hardware-related infection. To our knowledge, this is the second known report of an osteoarticular infection caused by K. schroeteri. e first known case of orthopedic infection caused by this organism was described in a case of spondylodiscitis after surgery [2]. Our patient was moderately immunocompromised as she was taking prednisone 10 mg daily for several years for her chronic adrenal insufficiency. Our review of the literature (Table 1) identified 13 other cases (Table 1) with Kytococcus infection that were associated with implanted foreign material: eight with prosthetic valve endocarditis, two with VP shunt infections, one with prosthetic discitis, one with silicon tendon graft infection, and one with medullary nail infection (our patient). ese reports underscore the importance of evaluating for foreign body infections in patients who have a positive blood culture with K. schroeteri.
Our literature review also identified 6 reported cases of Kytococcus pneumonia. Five of these were in conjunction with hematological malignancy (4 AML and 1 hairy cell leukemia) [11][12][13]17]. e sixth patient had no hematologic condition but was moderately immunocompromised by taking prednisone 20 mg daily for 2 years for management of refractory asthma [3]. Indeed, at our institution, we have also encountered a separate and additional patient with acute myelogenous leukemia who developed fever, hypoxic respiratory failure, and consolidated infiltrates on chest imaging. K. schroeteri was identified in multiple cultures from respiratory samples (unpublished).
K. schroeteri is frequently misidentified as Micrococcus sp. because of similar morphological features. However, a distinction between the two is important since Micrococcus sp. is inherently susceptible to penicillin and oxacillin, whereas Kytococcus sp. is not. Antimicrobial susceptibility testing (AST) of K. schroeteri from both our patients demonstrated in vitro resistance to penicillin and susceptibility to vancomycin and clindamycin. We also reviewed the antimicrobial susceptibility testing (AST) profile from all prior cases associated with this organism (Table 1), and the characteristic resistance to penicillin and susceptibility to vancomycin were uniform.
An important point to highlight is the fact that there are no formal established MIC breakpoints by the Clinical Laboratory Standards Institute (CLSI); therefore, the laboratory interpretation of MIC breakpoints reflecting drug susceptibility vs. resistance are implied and must be interpreted with caution. In 2015, CLSI did publish MIC breakpoints for Micrococcus sp. Several genera, including Kytococcus sp., have been included under the Micrococcus group for AST interpretation. While CLSI defines clinical "susceptible" vs. "resistant" Micrococcus MIC breakpoints for penicillin, there is no formal "resistant" MIC breakpoint defined for vancomycin. Rather, only an MIC breakpoint below which vancomycin susceptibility is reported. erefore, vancomycin resistance is implied when reported at or above the MIC breakpoint. Whether there are clinically relevant MIC breakpoint differences between Micrococcus and Kytococcus has not been adequately determined at this time. An AST database of Kytococcus and Micrococcus isolates by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) has been developed at our center to augment the currently available CLSI reporting standards.
In summary, we describe the second known case of an osteoarticular infection from K. schroeteri and reviewed the published reports of K. schroeteri disease. While often considered part of the natural skin bacterial flora, human infections with K. schroeteri do occur and have presented in patients with infected implanted prosthetic material and as respiratory infections in patients with significant immunosuppressive conditions, including select hematologic malignancies. Kytococcus appears to be susceptible to vancomycin and resistant to penicillin when grouped with Micrococcus by CLSI, although species-specific MIC breakpoints have not yet been formally established. e advancements made in laboratory diagnostic techniques along with rising applications of prosthetic materials may collectively enable an increased awareness of Kytococcus sp. as a potentially formidable human pathogen. Early identification and appropriate treatment will be critical toward successful outcomes.

Consent
Appropriate consent has been obtained from the patient.

Conflicts of Interest
e authors declare that they have no conflicts of interest.