A fluorescently labelled quaternary ammonium compound (NBD-DDA) to study resistance mechanisms in bacteria

Niclas Nordholt; Kate O'Hara; Ute Resch-Genger; Mark A T Blaskovich; Bastian Rühle; Frank Schreiber

doi:10.3389/fmicb.2022.1023326

A fluorescently labelled quaternary ammonium compound (NBD-DDA) to study resistance mechanisms in bacteria

Front Microbiol. 2022 Nov 24:13:1023326. doi: 10.3389/fmicb.2022.1023326. eCollection 2022.

Authors

Niclas Nordholt¹, Kate O'Hara^{1

2}, Ute Resch-Genger², Mark A T Blaskovich³, Bastian Rühle², Frank Schreiber¹

Affiliations

¹ Division of Biodeterioration and Reference Organisms (4.1), Department of Materials and the Environment, Federal Institute for Materials Research and Testing (BAM), Berlin, Germany.
² Division of Biophotonics (1.2), Department of Analytical Chemistry, Reference Materials, Federal Institute for Materials Research and Testing (BAM), Berlin, Germany.
³ Centre for Superbug Solutions, Institute for Molecular Bioscience, The University of Queensland, Brisbane, QLD, Australia.

Abstract

Quaternary ammonium compounds (QACs) are widely used as active agents in disinfectants, antiseptics, and preservatives. Despite being in use since the 1940s, there remain multiple open questions regarding their detailed mode-of-action and the mechanisms, including phenotypic heterogeneity, that can make bacteria less susceptible to QACs. To facilitate studies on resistance mechanisms towards QACs, we synthesized a fluorescent quaternary ammonium compound, namely N-dodecyl-N,N-dimethyl-[2-[(4-nitro-2,1,3-benzoxadiazol-7-yl)amino]ethyl]azanium-iodide (NBD-DDA). NBD-DDA is readily detected by flow cytometry and fluorescence microscopy with standard GFP/FITC-settings, making it suitable for molecular and single-cell studies. As a proof-of-concept, NBD-DDA was then used to investigate resistance mechanisms which can be heterogeneous among individual bacterial cells. Our results reveal that the antimicrobial activity of NBD-DDA against Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa is comparable to that of benzalkonium chloride (BAC), a widely used QAC, and benzyl-dimethyl-dodecylammonium chloride (BAC₁₂), a mono-constituent BAC with alkyl-chain length of 12 and high structural similarity to NBD-DDA. Characteristic time-kill kinetics and increased tolerance of a BAC tolerant E. coli strain against NBD-DDA suggest that the mode of action of NBD-DDA is similar to that of BAC. As revealed by confocal laser scanning microscopy (CLSM), NBD-DDA is preferentially localized to the cell envelope of E. coli, which is a primary target of BAC and other QACs. Leveraging these findings and NBD-DDA's fluorescent properties, we show that reduced cellular accumulation is responsible for the evolved BAC tolerance in the BAC tolerant E. coli strain and that NBD-DDA is subject to efflux mediated by TolC. Overall, NBD-DDA's antimicrobial activity, its fluorescent properties, and its ease of detection render it a powerful tool to study resistance mechanisms of QACs in bacteria and highlight its potential to gain detailed insights into its mode-of-action.

Keywords: QAC; TolC; antimicrobial resistance; efflux; fluorescence; quaternary ammonium compound.