A 71-year old immunocompetent female, with past medical history of osteoarthrosis, chronic obstructive lung disease, chronic kidney disease, hypertension, and nephrolithiasis, presented with postoperative left arthroplasty knee pain. The patient was 40 days post-operative from an uncomplicated left total knee arthroplasty.
Approximately, one week prior to admission, she was treated outpatient with oral sulfamethoxazole/trimethoprim DS as a superficial incisional stitch abscess, which involved the proximal aspect of the linear arthroplasty incision. Clinical follow-up demonstrated persistence and increasing peri-incisional soft tissues edema and erythema, progressively extending along the incisional line, and peri-incisional soft tissues. This, in spite of the oral antimicrobials, raised concern for an associated septic bursitis. Upon hospital admission, empiric intravenous vancomycin and ampicillin-sulbactam were initiated for broad antibacterial coverage of gram negative and positive bacteria, including potential methicillin sensitive and methicillin resistant pathogens, targeting a pre-patellar septic bursal infection.
Laboratory analysis revealed normocytic normochromic anemia (Hemoglobin 10.2 g/dL); normal White count (10,000 per cubic millimeter); thrombocytosis (716,000 per cubic millimeter); elevated C-reactive protein (7.5 mg/dL); elevated Westergren sedimentation rate (>140 mm/h); negative HIV I & II; negative QuantiFERON-TB Gold; non-reactive treponemal and hepatitis C antibody. She presented with acute renal failure (Creatinine 2.3 mg/dL: her baseline is 1.73 mg/dL), BUN 28.2 mg/dL. Gram stain revealed many polymorphonuclear leukocytes and few Gram-positive bacilli.
Bedside tissue debridement was performed without findings of an associated joint effusion. Routine tissue bacterial cultures demonstrating an unidentifiable, slow-growing, aerobic Gram-positive bacilli. A Peripherally Inserted Central Catheter (PICC) line was placed, and she was discharged home targeting a four week therapeutic course of empiric intravenous vancomycin. Unfortunately, local peri-patellar swelling persisted, prompting a repeat outpatient clinic arthrocentesis. Routine bacterial cultures, again, revealed the slow growing, aerobic Gram-positive bacilli.
The repeated joint and tissue recovery of an unidentifiable, slow growing, aerobic Gram-positive bacilli, raised the possibility of an atypical microbial pathogen. A therapeutic decision was made to electively perform intraoperative debridement of the joint and bursa, and possible explantation of the arthroplasty hardware, and to include repeat cultures to rule out typical and/or atypical microbial infection, including AFB and fungi. Successful resection of the knee arthroplasty was completed, with the insertion of antimicrobial impregnated (vancomycin) cement. She was discharged home on oral levofloxacin and intravenous ampicillin-sulbactam for a planned six weeks antimicrobial course awaiting final culture results.
Multiple cultures of the operative specimen were sent to specialty lab for Acid-Fast bacterial smear and culture, and ultimately Mycobacterium smegmatis group was identified from several tissue cultures sent. The specific species within the group was not identified. Antimicrobial susceptibility is listed in Table 1. Antimicrobials were adjusted to include oral doxycycline and levofloxacin targeting a 6-12 months course of preoperative antimicrobial therapy (Two step arthroplasty revision). Patient completed a 10 months antimicrobial course, noting that antimicrobial therapy was complicated by a transient doxycycline photosensitivity reaction.
Several weeks' preoperative, repeat joint fluid analysis was negative for microbial growth, including AFB. A successful revision knee arthroplasty was completed; and operative cultures and joint fluid analysis, remained negative for ongoing infection. She was maintained on the same antimicrobial combination of oral doxycycline and levofloxacin for 30 days post-operative. Patient tolerated the therapy well; with a 10-month post-operative follow-up, there has been no infection relapse.