Characterization of the virulence factors of the microorganisms identified in the surgical field during reverse total shoulder arthroplasty

Introduction: Reverse total shoulder arthroplasty (RTSA) has been increasingly used for conditions such as glenohumeral osteoarthritis, irreparable rotator cuff tears and proximal humerus fractures. Despite offering pain relief and functional recovery, its complication rate ranges from 10% to 47%. Potential complications include scapular notching, rotator cuff injuries, periprosthetic fractures, and periprosthetic infection. Despite preventive strategies, infection rates after RTSA are reported approximately to 4%, often correlated to patient's endogenous flora. The implant design and lack of surrounding rotator cuff muscles enhance the susceptibility to bacterial colonization. Common causative pathogens include anaerobic bacteria, with C. acnes being the most frequent, coagulase-negative staphylococci, followed by Staphylococcus aureus, Gram-negative bacilli and Enterococcus spp. Antimicrobial resistance and biofilm production further complicate PJI management. More than half of PJI cases involve antibiotic resistance, with increasing resistance rates observed especially for Clindamycin, Macrolides and Methicillin. Biofilms enhance bacterial persistence by protecting against host immune responses and antimicrobial agents. To address PJI risk, intraoperative lavage with diluted betadine (0.35%) has emerged as a strategy to reduce acute postoperative deep infections in TJA. Methods: this study included 65 RTSA patients, each contributing 6 samples, with 3 taken before and 3 after intraoperative betadine irrigation. Identical sampling was performed before and after lavage, consisting of 2 swabs from prosthetic components and 1 periprosthetic tissue sample. The collected specimens were analysed for microbial load and species identification, antimicrobial susceptibility and biofilm production. Lastly, the potential correlation between intraoperative lavage and the isolation of less virulent microorganisms was investigated. Results: the most commonly identified anaerobic microorganism was C. acnes (30.77%). Among aerobic bacteria, CoNS were the most prevalent (16.41%), while S. aureus (0.26%) and E.coli (0.51%) were also detected. Among the CoNS isolates, the most frequently encountered species were S. epidermidis. Although most isolates revealed susceptible to the tested antimicrobial drugs, 3.33% out of C. acnes isolates were found to be Clindamycin-resistant and 46.87% out of CoNS isolates displayed some levels of antibiotic resistance. The highest resistance rates among CoNS isolates were observed for Erythromycin, Macrolides, Tetracyclines and Fluoroquinolones. Biofilm production was detected in 24.17% of C. acnes isolates and 38.46% of staphylococcal strains. However, no significant correlation was found between antimicrobial resistance and biofilm production (p 0.89). Overall, the frequency of antibiotic resistance among aerobic bacteria remained unchanged before and after betadine irrigation (45%). Although the proportion of antibiotic-resistant C. acnes strains was lower post-lavage (4% vs. 2%), also in this case the relation was not statistically significant. Similarly, no significant association was found between intraoperative irrigation and a reduced biofilm production frequency among staphylococcal strains, despite the proportion of staphylococcal strains classified as biofilm producers being lower post-lavage (41% vs. 27%). However, notably a significant reduction in biofilm-producing C. acnes strains was observed after lavage (31% vs. 12%), showing a statistically significant correlation (p 0.02). Conclusion: this study confirmed that RTSA surgical field can be frquently colonised by C. acnes, followed by straphylococci. Some of these bacterial strains display worrisome virulence factors, particularly some levels of antibiotic resistance and biofilm production. Most importantly, intraoperative betadine irrigation has been proven effective in reducing the frequency of C. acnes biofilm-producing isolates.