Biofilm-associated reduced antimicrobial susceptibility in Pseudomonas aeruginosa isolates from cystic fibrosis patients: MIC-MBEC discordance.

Chronic Pseudomonas aeruginosa infections in cystic fibrosis are characterized by biofilm formation, which substantially alters antimicrobial susceptibility profiles. In this study, planktonic minimum inhibitory concentration (MIC) and minimum biofilm eradication concentration (MBEC) values were compared for P. aeruginosa isolates obtained exclusively from patients with cystic fibrosis using a peg-based biofilm model. All tested antimicrobials showed significantly reduced susceptibility under biofilm conditions compared with planktonic conditions (p < 0.0001), and no isolate exhibited an MBEC value lower than its corresponding MIC. While ciprofloxacin, amikacin, tobramycin, and azithromycin retained relatively more preserved activity in the biofilm state, beta-lactams, levofloxacin, and colistin showed pronounced loss of efficacy. These findings highlight the biological and clinical relevance of biofilm-associated antimicrobial tolerance in cystic fibrosis and support the potential role of biofilm-based susceptibility testing as a complementary tool to conventional MIC testing in selected clinical contexts.