GRAM-NEGATIVE BACILLI IN BONE AND JOINT INFECTIONS: A FIVE-YEAR RETROSPECTIVE ONE-CENTRE STUDY

Aim Ethiology of bone and joint infections including prosthetic ones is mainly dominated by Gram-positive cocci, particularly Staphylococcus aureus and coagulase negative staphylococci. Gram negative bacilli (GNB) are generally less represented with an isolation rate of about 20%. Antimicrobial resistance in Gram negative bacilli is increasing worldwide making infections caused by these microorganisms difficult to treat. In this study we retrospectively evaluated isolation rate of Gram negative bacilli from bone and joint infections in a single orthopaedic referral centre of Northern Italy during the last five years. Methods A retrospective analysis of microorganisms isolated from patients with diagnosis of bone and joint infections from January 2021 to December 2025 at our division dedicated to management of bone and joint infections was performed. Phenotypic identification was performed by means of a semi-automated instrumentation. Antimicrobial susceptibility was tested by microdilution assays. Results In the studied period, 94 GNB out of 596 isolates (15.8%) were identified from samples collected during 500 surgeries. They were isolated from79 patients as responsible of prosthetic joint infections (33 isolates), osteomyelitis (28 isolates), infections of hardware (12 isolates), spacer infections (5 isolates), septic arthritis (4 isolates), osteitis (1 isolate) and infection of Achille's tendon (1 isolate). In 6 patients polymicrobial infection was caused by 2 GNB. P. aeruginosa was the most frequently isolated rod (n=23) followed by Escherichia coli (n=16), Proteus mirabilis (n=14), Klebsiella spp. (n=13), Enterobacter cloacae (n=8), Citrobacter spp. (n=5), Acinetobacter baumannii complex (n=2), Morganella morganii (n=2), Alcaligenes xilosoxidans (n=1) and Serratia marcescens (n=1). An increase in GNB isolation rate was observed from 2021 to 2024 (from 13 to 20%) but not in 2025. In 8 (4 osteomyelitis, 3 prosthetic joint infections and1 spacer infection) out of 79 cases (10.1%), infection persisted. Resistance to 3rd generation cephalosporins and to carbapenems was observed in 9 and 4 Enterobacterales isolates, respectively. A. baumanni isolates were resistant to all the tested drugs with the exception of colistin and tigecycline. Conclusion Bone and joint infections caused by Gram negative bacilli represent a challenge for orthopaedics, although they are isolated in a small portion of patients. Isolation rate of these microorganisms seems to be increasing over time. Management of these infections could be critical since in 10% of patients treatment failed to eradicate infection and in about one fifth of cases multidrug resistance GNB were isolated.