Abstract
<title>Abstract</title> <p> Plant-derived polyphenol-rich extracts are of interest as resistance-modifying agents because they may affect bacterial redox balance, membrane-associated bioenergetics, and antibiotic susceptibility. In this study, the antibiotic-modulatory potential of <italic>Ocimum basilicum</italic> var. <italic>purpureum</italic> ethanol extract (OBEE) was investigated against <italic>Escherichia coli</italic> NM111, a <italic>katG::lacZ</italic> reporter strain derived from <italic>E. coli</italic> K-12. The phytochemical composition by LC-Q-Orbitrap HRMS analysis tentatively annotated 102 phytochemical constituents in OBEE. Hydroxycinnamic acids and their derivatives formed the most numerous group (32 compounds), including a series of caffeic-acid oligomers and salvianolic-type depsides, followed by flavonoids (29 compounds). The extract was dominated by caffeic-acid–derived metabolites, principally rosmarinic acid, chicoric acid, sagerinic acid, salvianolic acid K and caftaric acid, together with quercetin, luteolin and cyanidin derivatives. OBEE showed high total phenolic content (317.75 ± 4.11 µg GAE mg⁻¹) and strong DPPH radical-scavenging activity, with an IC₅₀ of 19.37 ± 0.38 µg mL⁻¹, close to that of catechin, with rosmarinic acid identified as the dominant antioxidant compound. Although OBEE did not exhibit direct antibacterial activity against <italic>E. coli</italic> NM111 up to 2 mg mL⁻¹, it potentiated ampicillin at non-inhibitory concentrations, reducing the ampicillin MIC two-fold (modulation factor ≥ 2) and showing synergistic interaction (FICI < 0.5). No synergy was observed with kanamycin. The OBEE–ampicillin combination reduced colony-forming ability by ~ 65%, decreased the specific growth rate ~ 2-fold, prolonged the generation time, and markedly suppressed proton flux (~ 2.1-fold) and total ATPase activity (~ 4.2-fold). OBEE also increased SOD (~ 12%) and catalase (~ 30%) activities and induced <italic>katG::lacZ</italic> expression (1.27-fold basal, 1.44-fold after H₂O₂ challenge). These findings indicate that OBEE acts mainly as an antibiotic-modulatory agent rather than a direct antibacterial agent, with the bioenergetic axis (proton flux and ATPase) appearing as the principal mechanistic candidate and oxidative-stress responses representing a parallel adaptive signature. </p>