Chemicals.

Ciprofloxacin, tobramycin, ceftazidime, colistin, and all chemicals used for C11 and C₄-HSL synthesis were obtained from Sigma-Aldrich (Saint-Quentin-Fallavier, France).

C11 [N-(2-pyrimidyl)butanamide] was prepared by condensation of butyryl chloride with 2-aminopyrimidine (in pyridine). The reaction mixture was stirred under reflux for 30 min. After the mixture was cooled at room temperature, it was filtered and the resulting solution was evaporated under reduced pressure. The crude product was washed with water and recrystallized from ethanol to obtain colorless needles with a melting point of 103°C at 60% yield. The compound was characterized by its ¹H nuclear magnetic resonance (NMR) and infrared (IR) spectra.

N-(2-Pyrimidyl)butanamide: molecular weight, 165 g/mol; melting point, 103°C; ¹H NMR (CDCl₃), δ (parts per million) 8.63, 2H, d (J = 4.9 Hz) H_4′, H_6′; 8.40, 1H br, NH; 7.01 1H, t (J = 4.85 Hz) H_5′; 2.75, 2H, t (J = 7.40 Hz), CH₂—CO; 1.81, 2H, m, CH₂; 1.05, 3H, t, CH₃; IR (KBr) ν (centimeters⁻¹): 3,400, 3,218, 3,074, 3,001, 2,961, 2,902, 2,873, 1,685, 1,583, 1,525, 1,446.

C₄-HSL was synthesized following the procedure previously described (26) and was characterized by its ¹H NMR and IR spectra.

All compounds tested were dissolved at the appropriate concentrations in sterile distilled water.