a 94-year - old woman was referred to the general internal medicine service for functional decline secondary to longstanding knee pain . her medical history included osteoarthritis and her surgical history was significant for a right total knee arthroplasty in 1996 . on examination , her vital signs were normal ; she was afebrile , but her right knee was red , warm and swollen . her initial blood work showed an erythrocyte sedimentation rate of 54 mm / h ( normal 0 mm / h to 27 mm / h ) , a c - reactive protein level 52.6 mg / l ( normal 0 mg / l to 1 mg / l ) and a white blood cell count of 6.210/l ( normal 4.010/l to 10.510/l ) . a plain radiograph of the knee demonstrated loosening of the tibial component , with a large joint effusion believed to show metallosis  a deposition of metallic debris in the periprosthetic soft tissues from abrasion of metallic components ( 1 ) . an aspirate of the knee revealed calcium pyrophosphate crystals , with a total nucleated cell count of 11.310/l and differential of 97% neutrophils . later , an enrichment broth subculture of the aspirate demonstrated growth of a gram - positive bacillus , which was identified as a lactobacillus species based on the following results : unidentifiable profile on rapid ana ii panel ( oxoid canada ) , catalase - negative , pyr - negative , lap - negative and vancomycin resistance . she presented six weeks later with escherichia coli bacteremia arising from an acute urinary tract infection . a reassessment of the right knee showed a marked increase in swelling with persistent pain and tenderness . infectious diseases was subsequently consulted to comment on the significance of the lactobacillus species that had grown on culture from the previous admission . a second aspirate was requested and grew the same organism , which was identified as a lactobacillus species . of note , there were no calcium pyrophosphate crystals apparent in this aspirate and the total nucleated cell count and differential were 3.110/l and 96% neutrophils , respectively . the isolate was sent to the ontario public health laboratory ( toronto , ontario ) for identification using 16s ribosomal rna sequencing ( 2 ) . 16s ribosomal rna sequencing revealed that the organisms from both knee aspirate cultures were identical and were subsequently identified as weissella confusa . susceptibility testing of the organism from both isolates was performed using the agar dilution method and minimum inhibitory concentrations for various antibiotics are summarized in table 1 . weissella was first described as a new genus in 1993 and was named after norbert weiss , a german microbiologist known for his contributions to lactic acid bacteriology . it is identified using 16s ribosomal rna sequencing to differentiate it from other organisms such as lactobacilli and other members of the family leuconostocaceae ( 3,4 ) . usual phenotypic identification methods have been shown to be ineffective and often lead to misidentified organisms ( 4 ) . fourteen species in total have been identified , of which two  w confusa and weissella cibaria  are clinically important because of their ability to infect human hosts and intrinsic resistance to vancomycin . weissella species are alpha - hemolytic , gram - positive coccobacilli that typically grow in chains and are catalase negative and bile esculin positive . what differentiates w confusa and w cibaria are the acidification of different sugars ( 5 ) . it was named confusa because it was often mistaken for members of the leuconostoc , pediococcus and lactobacillus genera ( 3 ) . w confusa has a widespread environmental distribution and has been found in a variety of foods including sugar cane , carrot juice , milk , fermented meats , garlic mix and banana leaves . congruently , it can be part of the normal microflora of the human intestine and can be found in human stool ( 4 ) . infection  and  immunocompromised  , revealing a total of 19 cases in the english literature : 15 with bacteremia , two with endocarditis , one with osteomyelitis and one with a thumb abscess ( table 2 ) ( 412 ) . to our knowledge , we present the first case of a prosthetic joint infection caused by w confusa . in the largest case series , lee et al ( 10 ) reported 10 patients with bacteremia in a tertiary care centre in taiwan . risk factors for invasive infection in this series included an immunocompromised host , central line catheter insertion and concurrent polymicrobial bacteremia . these findings were similar to case reports by harlan et al ( 8) and salimnia et al ( 12 ) , who identified bacteremia in patients with hepatocellular carcinoma post - liver transplant and acute lymphocytic leukemia undergoing autologous stem cell transplantion , respectively . other case reports have included risk factors for immunocompromised states , which consisted of chronic alcoholism , previous long - term steroid use and severe burn patients ( 7,12 ) . similarly , gastrointestinal manipulation via endoscopy or surgery may be one of the routes of translocation into the bloodstream because six of the reported 19 cases had documented medical procedures within three months of infection ( 811 ) . w confusa is intrinsically resistant to vancomycin and exhibits high minimum inhibitory concentrations ( 412 ) . this is important to recognize because clinicians often use vancomycin empirically as a treatment option in the immunocompromised patient when cultures initially reveal a gram - positive organism in the blood . this can be fatal in cases in which recognition of the organism is not identified in a timely fashion . studies have reported high levels of resistance to trimethoprim / sulfamethoxazole , metronidazole , teicoplanin , ceftazidime and ceftriaxone . in general , penicillin , clindamycin , erythromycin , daptomycin and the fluoroquinolones are effective agents for treating w confusa infections ( 10 ) . what was unique to our case was the lack of predisposing factors in our patient . while the patient did have evidence of a concomitant e coli bacteremia at her second presentation , she had already grown w confusa from the first aspirate of her knee six weeks previously . she received a one - week course of levofloxacin for her e coli bacteremia , which unfortunately , provided minimal clinical benefit to her prosthetic knee infection . she was discharged from hospital and returned to her long - term care facility in stable condition . given her premorbid state and nonambulatory status , it was decided in discussion with the patient and her family to not pursue further medical or surgical therapy for her prosthetic joint infection . w confusa is a rare , but well documented , cause of invasive infection in humans . w confusa infection can be treated with a variety of antimicrobials such as penicillins and fluoroquinolones ; however , it exhibits intrinsic resistance to vancomycin .
this article describes a case involving a 94-year - old woman with an infection of her knee , which had previously undergone total knee arthroplasty . 
 following culture of aspirate from the knee , the causative organism was tentatively identified as a lactobacillus species . 
 the infection was later determined to be caused by weissella confusa . 
 the authors discuss the difficulties in identifying w confusa and present a literature review of infections caused by the species .