Septic arthritis of native hip joint by Staphylococcus lugdunensis : a case report

Staphylococcus lugdunensis is a rare virulent coagulase-negative staphylococcus (CoNS) that behaves similarly to Staphylococcus aureus in causing deep abscesses, skin and soft tissue infections, and central nervous system infections. Additionally, there can be certain blood stream infections including sepsis, septic shock, toxic shock syndrome, and endocarditis complicated by embolic events. Reports of septic arthritis of a native joint associated with this organism have been infrequent, justifying the presentation and discussion of this case.


INTRODUCTION
Staphylococcus lugdunensis is a virulent coagulasenegative staphylococcus (CoNS) that behaves similarly to Staphylococcus aureus, and it has been associated with rare cases of osteomyelitis of the vertebra, prosthetic implants, and endocarditis.Routine microbiological laboratory analyses may misidentify S. lugdunensis, since the colony morphology often resembles that of S. aureus, which leads to the underreporting of this infection.More recent and improved identification methods have determined an incidence of 53 infections per 100,000 inhabitants per year, in contrast to previous estimates of 5 infections per 100,000 inhabitants per year 1 .S. lugdunensis can be aggressive and virulent, highlighting the importance for the correct identification of this organism.

CASE REPORT
A 62-year-old African-American male on a methadone program for ten years, with a medical history of prior intravenous (IV) heroin use, hepatitis C treated five years ago with sustained viral response, hypertension, and latent tuberculosis (status posttreatment after 14 years) came to the emergency department (ED) with complaints of worsening right hip pain over the past eight weeks.Pain was 9/10 in intensity, dull to sharp in nature, localized on the anterior aspect of the right hip, radiating to the lower leg, and worsening with minimal leg flexion and internal rotation.Pain was associated with subjective chills and weight loss.Six weeks ago, the patient was admitted to another facility for similar complaints, was found to have methicillin resistant Staphylococcus aureus (MRSA) bacteremia with septic arthritis of the same hip, diagnosed after right hip joint aspiration and cultures.Patient had then received treatment with IV vancomycin for six weeks.
On arrival to the ED, the patient was hemodynamically stable.The patient was in mild distress due to the pain.There was tenderness to palpation on the right hip with no visible erythema, and significant limited range of motion for the right leg.Laboratory findings on admission are provided in Table 1.Other laboratory studies and serological markers for septic arthritis were unremarkable.
A computer tomography (CT) scan of the pelvis without contrast showed erosive changes within the acetabulum with poor cortical definition superiorly as well as erosive changes within the right femoral head and neck.There was fluid within the joint space, and thickening of the obturator internus muscle and soft tissues of the right hip.Magnetic resonance imaging with contrast showed findings suggestive of infectious arthritis (Figure 1).Initially, the patient received one dose of empiric IV vancomycin and piperacillin/tazobactam.
The patient was evaluated by the orthopedics team and underwent joint aspiration by Interventional Radiology.During the procedure, the joint space and anterior soft tissues were irrigated with 5mL of normal saline, and the irrigated fluid was sent for culture and sensitivity.Empiric broad-spectrum antibiotics were restarted, and then later narrowed to IV nafcillin after Staphylococcus lugdunensis was identified in the aerobic cultures from the joint aspirate.Subsequently, blood cultures  throughout the course remained negative.Bone biopsy showed a fragment of viable mature bone with fi brotic intertrabecular tissue with acute and chronic inflammation and a focal osteolytic interface.The patient was discharged to a short-term rehabilitation facility for six weeks of IV antibiotic therapy.

DISCUSSION
The identifi cation of S. lugdunensis is challenging; however, it can be differentiated from other CoNS by biochemical markers, specifi cally using the L-pyrrolidonyl arylamidase enzyme test and ornithine decarboxylase positivity test 1,2 .Most contemporary automated identifi cation systems are able to correctly identify S. lugdunensis from other Staphylococcus species.However, S. lugdunensis is also able to protect itself from the immune system via the production of biofi lm 2 .Even though organisms are susceptible to an array of antimicrobials, infections can be very diffi cult to treat if they form and proliferate within a biofi lm [3][4][5] .
Most isolates of S. lugdunensis are sensitive to an array of antimicrobial therapy.However, there have been isolated case reports of resistance to commonly used empiric antibiotic therapy 5 .No cases of vancomycin resistance have been mentioned so far.It remains unclear whether our patient developed vancomycin resistance or later had a co-infection/ relapse with S. lugdunensis.It is noteworthy that susceptibility testing of a joint aspiration in our laboratory showed vancomycin sensitivity with a minimum inhibitory concentration of 1µg/mL.
Staphylococcus lugdunensis is an unusual cause of bone and joint infection.However the total number of patients with infection due to this organism may remain under-diagnosed because some clinical laboratories do not separate all isolates of CoNS in laboratory cultures and therefore may not fully identify isolates of S. lugdunensis 6,7 .Moreover, even physicians who are considering CoNS as a possible pathogen may not ask for further identification of a particular isolate of CoNS, and may therefore be unaware that the patient has a virulent S. lugdunensis as the true pathogen in a clinical infection, thereby undertreating the patient [8][9][10][11][12] .
In conclusion, the importance of correctly identifying S. lugdunensis is underscored.Isolation of S. lugdunensis is signifi cant, and the organism should not be discarded as a contaminant without careful consideration.If the clinical situation does not correlate with CoNS, prompt identifi cation to the species level should be performed.

FIGURE 1 :
FIGURE 1: Prominent bony destruction involving the right femoral head and acetabulum with extensive synovitis.There is also severe periarticular soft tissue edema and ill-defi ned fl uid.

TABLE 1 :
Pertinent fi ndings and other studies*.
MRSA Negative throughout the course WBC: white blood cells; ESR: erythrocyte sedimentation rate; CRP: C-Reactive protein; TTE: transthoracic echocardiography; TEE: transesophageal echocardiography; HCV: Hepatitis C Virus ; MRSA: methicillin resistant Staphylococcus aureus; S.: Staphylococcus.*Information on laboratory and imagining studies during both hospital course.