Listeria monocytogenes and Staphylococcus aureus coinfection of a prosthetic joint.

1. Introduction
Listeria is an unusual cause of infection in prosthetic joints. We report a case of a prosthetic joint coinfected with Listeria monocytogenes and Staphylococcus aureus.

2. Case report
A 73-year-old man was admitted to hospital with an 8-month history of pain in his left hip. The patient was systemically well. He was known to have suffered from osteoarthritis for which he was treated with diclofenac for 9 months but due to the progressive nature of the illness, he had subsequently undergone a left total hip replacement 3 years earlier. He did not have rheumatoid arthritis (RA) and was not on steroids. On examination, the operative wound on the left side had two broken areas neither of which was inflamed. The left leg was 1 cm shorter than the right with a fixed flexion deformity of 10° and a positive Trendelenburg test.

Radiograph of the left hip showed marked osteolysis around the polythene acetabular cup and the proximal end of the femoral component. Two-stage hip revision surgery was planned. In the first stage, all cement and both components were removed and samples were taken for culture. Three out of the five intra-operative samples grew S. aureus susceptible to flucloxacillin, rifampicin, vancomycin, and teicoplanin while one sample grew L. monocytogenes susceptible to amoxicillin, rifampicin, vancomycin, and teicoplanin. The patient was treated with flucloxacillin for the first 10 days but due to its poor efficacy against listeria, therapy was switched to intravenous teicoplanin plus oral rifampicin. Antibiotics were administered for 6 weeks. A second stage operation was successfully performed 3 months after the initial surgery. Aspirates from the joint taken at the second operation were sterile.

3. Discussion
Listeria is a non-motile, non-sporulating gram-positive rod. Impaired cell-mediated immunity is a risk factor for invasive listeriosis. It is a rare cause of joint infections. It has been reported to cause prosthetic joint infections in patients with RA [1]. Polyarticular involvement has also been reported [2]. Cases of listeria bacteraemia have been described following therapy with infliximab and etanercept for RA and Crohn's disease [3]. Wilson et al. reported a case of knee joint infection with listeria following intra-articular injection of yttrium-90 [4]. Septic arthritis with this organism has been described in a renal transplant recipient [5]. In contrast to listeria, S. aureus is a common cause of infection in both native and prosthetic joints either through direct inoculation or following bacteraemia [6].

Isolation of bacteria from multiple samples increases the likelihood of infection [7]. In this report, listeria was isolated from one sample only. However, listeria is a fastidious organism and it could be easily masked on bacteriology plates by the rapidly growing S. aureus. Moreover, listeria is not a recognized laboratory or hospital contaminant. Hence, it was likely to be a pathogen in this case. This distinction is clinically relevant because standard treatment for S. aureus with penicillinase-resistant penicillins is inadequate for listeria [8]. This is the first report that describes S. aureus and L. monocytogenes coinfection of a prosthetic joint.