An Unexpected Inverse Relationship Between Biofilm Formation and Antibiotic Resistance in Stenotrophomonas maltophilia

Background/Objectives: Stenotrophomonas maltophilia is an emerging opportunistic pathogen associated with severe infections, particularly in patients with cystic fibrosis (CF). Its intrinsic multidrug resistance and ability to form biofilms significantly complicate treatment. While biofilm growth is widely linked to antimicrobial tolerance, the relationship between biofilm-forming capacity and planktonic antibiotic resistance in S. maltophilia remains unclear. This study aimed to investigate the association between antibiotic resistance profiles and biofilm formation in clinical isolates from CF and non-CF patients. Methods: A total of 86 clinical S. maltophilia isolates (40 from CF airways and 46 from non-CF patients) were analyzed. Antibiotic susceptibility to seven agents was assessed by disk diffusion, with results interpreted according to EUCAST and CLSI criteria. Multidrug resistance phenotypes were defined using standard criteria. Biofilm formation was quantified after 24 h using a crystal violet microtiter plate assay and categorized into five levels of production. Statistical analyses were performed to compare biofilm formation across resistance profiles and clinical origins and to assess correlations between biofilm biomass and multidrug resistance. Results: Overall, high resistance rates were observed, particularly to meropenem (87.2%), ciprofloxacin (80.2%), and rifampicin (72.1%). CF isolates showed significantly higher resistance to piperacillin/tazobactam and a higher prevalence of multidrug resistance. Biofilm production was detected in 94.2% of isolates, with strong and powerful biofilm producers predominating. However, isolates from CF patients formed significantly less biofilm than those from non-CF patients. Notably, resistance to piperacillin/tazobactam and meropenem was associated with significantly reduced biofilm formation, as reflected in both median biomass and the proportion of high biofilm producers. Across the entire collection, the number of antibiotic resistances displayed by an isolate was negatively correlated with biofilm biomass. These trends were maintained after stratification by clinical origin, although some comparisons did not reach statistical significance. Conclusions: These findings demonstrate an unexpected inverse relationship between planktonic antibiotic resistance and biofilm-forming efficiency in S. maltophilia. Enhanced biofilm production may represent an alternative persistence strategy in more antibiotic-susceptible strains, with important implications for infection management and therapeutic failure.