Quercus chihuahuensis Leaf Extract and Its Selected Phenolic Constituents Limit Pseudomonas fluorescens Colonization of Stainless Steel

Pseudomonas fluorescens forms biofilms on food-contact surfaces, contributing to persistent contamination in food-processing environments. Plant-derived extracts rich in phenolic compounds offer promising alternatives for controlling bacterial colonization due to their antimicrobial and antibiofilm properties. This study evaluated the antibacterial and antibiofilm activities of Quercus chihuahuensis leaf extract and selected phenolic constituents, alongside effects on quorum-sensing (QS)-associated phenotypes. Previous LC-MS profiling of the leaf extract reported relative abundances of 31.81% for p-coumaric acid, 13.66% for quercetin, and 3.04% for vanillic acid. Minimum inhibitory (MIC), bactericidal (MBC), and biofilm inhibitory (MBIC) concentrations were determined. Bacterial attachment and biofilm development were assessed on AISI 304 stainless steel as a model food-contact surface, while violacein production and swarming motility were evaluated in Chromobacterium violaceum; swarming was also assessed in P. fluorescens. The extract exhibited MIC, MBIC, and MBC values of 5, 7, and 22 mg/mL, respectively. At 5 mg/mL, it reduced attached-cell counts by 2.34 log CFU/cm2; final planktonic counts remained above the initial inoculum reference, with no statistically significant difference detected relative to the untreated control. This concentration was below the MBC but equaled the MIC, so growth-independent activity cannot be inferred. Among the individual phenolics, p-coumaric acid produced the greatest reduction in attachment. The extract and phenolic acids reduced violacein production, while the extract and p-coumaric acid significantly inhibited swarming motility in both bacterial species, with both effects observed at concentrations below their MICs. These findings demonstrate antibiofilm activity against P. fluorescens on stainless steel and alterations in QS-associated phenotypes.