Imipenem-resistant ceftazidime-susceptible Pseudomonas aeruginosa isolated from prosthetic joint infection in China

Sir, We read with interest two recent studies in the present journal on the treatment of streptococcal prosthetic joint infections and the virulence genes among Pseudomonas aeruginosa isolates from diabetic foot infections [1,2]. Hip and knee replacements are among the most common orthopaedic surgeries worldwide. Nevertheless, these procedures are accompanied by some complications, the most challenging of which is prosthetic joint infection [3]. Staphylococci account for more than one-half of cases, but also Gram-negative bacteria are responsible for a substantial proportion of prosthetic joint infections, ranging from 5% to 23% [4]. P. aeruginosa isolates are among the most common antibiotic-resistant pathogens and their presence has been associated with a substantial morbidity for prosthetic joint infections [5]. Outcomes of antibiotic treatment of prosthetic joint infections may depend not only on the antimicrobial resistance of the pathogens, but also on the isolates expressing virulence factors. Here, we report an imipenem-resistant ceftazidimesusceptible P. aeruginosa isolated from prosthetic joint infection of knee. A 72-year-old diabetic woman had a supracondylar knee fracture after a fall. The patient underwent open reduction and internal fixation. Two years later, the patient presented to the authors’ orthopaedic department with failure of the fixation and pseudarthrosis of the distal femoral fracture associated with pain and varus deformity. The condition was treated surgically using revision knee prosthesis. During the early postoperative period, less than 2 months after the surgery, the patient presented with signs of knee infection including knee pain, redness, oedema, effusion, and increased local temperature. An arthrocentesis of the left knee joint was performed. Synovial fluid was purulent. Cultures revealed a multidrug-resistant P. aeruginosa (PA0318) that was sensitive to ceftazidime but resistant to imipenem. The antimicrobial resistance profile of PA0318 was further tested by the microdilution method following Clinical and Laboratory Standards Institute (CLSI) guideline. Eight antimicrobial agents were tested: imipenem, meropenem, ceftazidime, cefepime, piperacillin-tazobactam, ciprofloxacin, amikacin, and colistin. Susceptibility was interpreted according to CLSI breakpoints. Escherichia coli ATCC 25922 and P. aeruginosa ATCC 27853 served as quality control strains. For genotyping, multilocus sequence typing (MLST) was performed on PA0318 as described previously [6]. PA0318 were screened by PCR for carbapenemase encoding genes including blaIMP, blaVIM, blaSPM, blaGIM, blaSIM, blaNDM, and blaKPC. mRNA transcription levels of oprD, ampC, efflux pump genes (mexB, mexD, mexF, and mexY) were determined by quantitative reverse transcription PCR (qRT-PCR) as described elsewhere [7,8]. The expression of the housekeeping proC gene was used as the internal control for relative quantification. It was considered to be MexAB-OprM, MexCD-OprJ, MexEF-OprN, and MexXYOprM hyperproducers when the levels of expression of mexB, mexD, mexF, and mexY were at least fivefold higher than that of the reference strain P. aeruginosa PAO1, respectively. Simultaneously, we determined the following virulence genes: lasI, lasR, lasA, lasB, rhll, rhlR, toxA, aprA, rhlAB, plcH, fliC, plcN, exoS, exoY, exoT, exoU, phzII, phzM, phzS, phzI, phzH, pvdA, pilA, and pilB as described previously [2].