Chlorination of quorum sensing molecules: Kinetics and transformation pathways

The impact of chlorination on quorum sensing molecules (QSMs) is not often addressed in disinfection research. Yet pathogenicity and biofilm formation are controlled by quorum sensing (QS) in many bacteria. Chemical transformation of the compounds could have an impact on all of these processes. For this reason, our study elucidated the reaction kinetics and transformation pathways of several N-acyl homoserine lactones (AHLs) and 2-heptyl-4-quinolone (HHQ) in contact with free available chlorine (FAC), a potent QS inhibitor. Both AHLs and HHQ, are known as QSMs for Gram-negative bacteria. Using FAC, a complete degradation of the target compound was observed for p-coumaroyl AHL (pC-AHL), C_14:1-AHL, HHQ and 3-Oxo-C₁₄-AHL. The reaction order for FAC varied between 1.19 (±0.07) (pC-AHL) to 1.62 (±0.13) (HHQ). This means that different reactive species (e.g. hypochlorous acid and dichlorine monoxide) are likely to be involved in the reaction mechanism. The first-order rate constants were strongly pH-dependent. For C_14:1-AHL and HHQ, the first-order rate constants decreased from pH 6.0 to pH 8.5. A maximum was observed for pC-AHL at pH 8.5 ranging from pH 6.0 to 10. In addition to the distribution of the reactive species, the phenol/phenolate ratio strongly influenced the first-order rate constants for pC-AHL. In total, at pH 7 (phosphate buffered) 29 transformation products were identified and the related transformation pathways were proposed via non-target and suspect screening using high-resolution mass spectrometry. The observed reaction mechanisms can be transferred to structurally similar QSMs to further understand QS-controlled processes during chlorination. We assumed that the transformation of the QSMs affects QS of the bacteria, thereby blocking QS-controlled processes such as biofilm formation.