Prosthetic infection caused by Ruminococcus gnavus following total knee arthroplasty. Challenges in anaerobic bacteria identification

Background Ruminococcus gnavus is a Gram positive, non-sporulated, obligate anaerobe diplococci. It was first described in 1974 by Moore et al from faeces and contents of the gastrointestinal tract of humans and it is a relatively common member of the human gut microbiota, nevertheless its role as a pathogenic bacteria has not been completely elucidated yet and it seems to depend of numerous factors, including those of the host. Here we present a case of prosthetic joint infection following total knee arthroplasty by R. gnavus. Case presentation A 74 years old patient was admitted to the emergency department (day 0) presenting with acute onset of left knee pain and swallowing 20 days after total left knee arthroplasty due to erythema and oedema without signs of fluctuation or purulent discharge from the surgical wound and elevated inflammatory reactants. Perilesional skin swab and synovial fluid were taken for bacterial culture and antibiotic treatment with ceftazidime and daptomycin was established. Examination of samples revealed abundant polymorphonuclear leucocytes, without visualizing bacteria on both Gram smears. After four days of incubation, the anaerobic culture of the synovial fluid exhibit growth of small, grey, umbilicated colonies in pure culture on Schaedler agar. The microorganism was identified as Ruminococcus gnavus by MALDI-TOF and 16S ribosomal bacterial sequence. The isolated showed susceptibility to all anaerobicidal drugs except for Clindamycin. Wound healing required vacuum assisted therapy and the patient was discharged after six weeks with a 4-month course of oral amoxicillin as consolidation therapy. Conclusions Ruminococcus gnavus have been reported as the causal microorganism in a range of human infections, nevertheless its identification remains challenging. Infection of prosthetic joints by anaerobic microorganisms is uncommon and is not considered in its empirical antibiotic treatment, thus, correct and swift identification of anaerobic bacteria in these cases is paramount.