Case Report: Rhodococcus erythropolis Osteomyelitis in the Toe
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- Roy, M., Sidhom, S., Kerr, K.G. et al. Clin Orthop Relat Res (2009) 467: 3029. doi:10.1007/s11999-009-0901-z
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The Rhodococcus species rarely cause musculoskeletal infections, with only two cases reported in the literature. We report the case of a 53-year-old woman who had an infection develop after first metatarsophalangeal joint fusion. A year after surgery, she continued to have pain and swelling with nonunion. She underwent revision of the arthrodesis and tissue samples from surgery revealed Rhodococcus erythropolis. The patient’s symptoms improved with oral antibiotics. One year after the revision surgery, the fusion had united. We believe this is the first report of a case of a musculoskeletal infection caused by Rhodococcus erythropolis.
Rhodococcus species are gram-positive bacteria that are distributed widely in the environment and have excited considerable interest because of their potential ability to render environmental pollutants harmless (bioremediation) . Rhodococcus equi is well recognized as a cause of infection in humans and animals and is associated with a range of clinical manifestations, especially cavitating pneumonia in immunocompromised individuals, such as recipients of solid organ transplants and those with AIDS , although infection in apparently immunocompetent persons also has been reported . Osteomyelitis associated with the bacterium appears to be very uncommon in humans [3, 6]. Human infection associated with species of Rhodococcus other than R equi is infrequently reported. We report the first case of an orthopaedic infection in humans caused by R erythropolis.
The genus Rhodococcus consists of 42 species. It belongs to the Nocardioform group of the aerobic actinomycetes, which also includes genera such as Nocardia and Mycobacterium . Recently Rhodococcus erythropolis has been identified as part of the regional microflora of individuals with normal and dry eyes . The importance of this finding is uncertain, although there is a case report of Rhodococcus erythropolis endophthalmitis after lens implantation . There is only one other case report of human infection with this bacterium cultured from an aspirate from an HIV-positive individual who presented with multiple subcutaneous nodules . Because the bacterium is environmental in origin, its isolation from clinical material must be assessed carefully, as growth from a microbiologic specimen could merely indicate contamination during specimen collection or subsequent processing in the microbiology laboratory rather than true infection. Marked colonial variation exhibited by the bacterium (Fig. 3) also may lead to erroneous discarding of cultures as contaminated. However, in this case, the specimen was taken under sterile conditions; the bacterium was seen on a gram-stained preparation of the specimen obtained at surgery and also grew in pure culture, which thus would make contamination an extremely unlikely explanation. Our patient’s good clinical response to an antimicrobial to which the isolate was susceptible also argues against contamination. As with many other bacteria of environmental origin, Rhodococcus erythropolis likely has limited pathogenicity, which may explain why removal of the foreign body and source of infection may have yielded a good clinical response to treatment with a short course of antibiotics. Our patient had no obvious risk factors for infection with the Rhodococcus species and was not immunocompromised. Unlike other Rhodococcus species, Rhodococcus erythropolis is not well recognized as a cause of nonhuman infection. There is only one report of infection in this context relating to systemic infection in farmed salmon . Although our patient gave a history of bathing in the sea, it remains unclear regarding how she might have acquired the infection. Our case report could represent a single incidental finding; however, orthopaedic surgeons and microbiologists should be aware this bacterium is capable of causing infection, especially as the colonial morphology of the bacterium (Fig. 3) may result in its misidentification as a diphtheroid and thus it may be dismissed as a contaminant in clinical specimens.
We thank the Microbiology Department at the Harrogate District Hospital.