Clinical and Microbiological Profile of Gram-Negative Bacteria Causing Orthopedic Implant Surgical Site Infections

Background: Orthopedic implant–associated infections (OIAIs) are difficult to eradicate due to biofilm formation and rising antimicrobial resistance. While Gram-positive cocci are classically implicated, Gram-negative bacilli (GNB) are increasingly reported in surgical site infections (SSIs) involving implants. Methods: This prospective observational study (January–December 2024) was conducted in the Departments of Microbiology of a tertiary hospital. Of 511 postoperative specimens from patients with implantrelated SSI, 305 were culture-positive and analyzed. Isolates were identified by standard microbiological methods. Antimicrobial susceptibility testing followed CLSI 2024 (Kirby–Bauer); ESBL was confirmed by combined disc testing. Colistin MICs were determined by broth microdilution, with selected isolates tested on VITEK-2. Of 305 culture-positive infections, 186 (6 Results: 1.0%) were GNB. Most patients were >50 years (64.9%), with males predominating (60.2%). Specimens were pus (52.2%) and wound swabs (47.8%). The leading GNB were Klebsiella pneumoniae (34.4%), Pseudomonas aeruginosa (21.0%), Escherichia coli (17.7%) and Acinetobacter baumannii (13.4%), with Proteus mirabilis, Enterobacter spp. and Citrobacter spp. less frequent. The hip (26.3%), knee (18.8%) and spine (16.1%) were the most common infection sites. Carbapenems showed the best activity (e.g., meropenem/imipenem: E. coli 81.8%/69.7%, Enterobacter 90%/90%), though resistance was notable in K. pneumoniae (~50%) and A. baumannii (56–60%). P. aeruginosa was highly susceptible to netilmicin (94.9%). Doxycycline showed good activity against Klebsiella (60.9%), E. coli (63.6%) and Acinetobacter (88.0%). Cephalosporins and fluoroquinolones performed poorly, reflecting ESBL prevalence. GNB form the majority of OIAIs, led by Conclusion: K. pneumoniae, P. aeruginosa and E. coli, with significant multidrug resistance. Empiric cephalosporins or fluoroquinolones are unreliable; carbapenems remain essential but require careful stewardship. Targeted agents such as netilmicin and minocycline may be valuable adjuncts.