Elsevier

Tetrahedron

Volume 72, Issue 25, 23 June 2016, Pages 3559-3566
Tetrahedron

From antimicrobial activity to mechanism of resistance: the multifaceted role of simple quaternary ammonium compounds in bacterial eradication

https://doi.org/10.1016/j.tet.2016.01.014Get rights and content

Abstract

Quaternary ammonium compounds (QACs) are a prominent class of antibacterial agents. Drawing inspiration from commercial disinfectants and antimicrobial natural products, we have derivatized structurally diverse tertiary amines to generate several different classes of QACs. We have synthesized over 200 QACs, many of which exhibit potent antibacterial and antibiofilm activity. Analysis of the structure–activity relationship of our compounds have led to the facile production of inexpensive QACs that display ∼1 μM MIC against a suite of bacteria, and furthermore do not appear to trigger bacterial resistance systems in methicillin-resistant Staphylococcus aureus (MRSA).

Introduction

Pathogenic bacteria have long plagued humans and are consistently among the foremost health crises worldwide. While we are accustomed to repelling the invasion of such pathogens with both innate defenses as well as medical protections, many bacteria that are encountered evade our immunities and are resistant to multiple antibiotics of both natural and synthetic origin.1, 2 Bacterial infections lead to hundreds of thousands of deaths and billions of dollars in healthcare costs annually, necessitating a constant push to discover and improve strategies to counter these threats.3, 4 Furthermore, bacterial biofilms that exist outside of the body on abiotic surfaces pose severe consequences, including biocorrosion and biofouling, across a range of industries.5 Such detrimental biofilm processes can contribute toward the contamination of drinking water and industrial products, increased maintenance costs, and destruction of essential equipment.

One prominent and critical class of antibacterials is the quaternary ammonium compounds (QACs).6 QACs with permanent positive charges are found in antimicrobial natural products such as berberine,7 whereas tertiary nitrogens in polyamines such as norspermidine provide examples of pH-dependent QACs (Fig. 1).8 Synthetic amphiphilic QACs are among the most heavily produced and utilized commercial products and have found use as surfactants, phase transfer catalysts, and disinfectants1, 9—the latter of which will be the focus of this report. Though initial reports date back to the early 1900s,10 QACs first gained traction in the disinfectant market in the 1930s and 1940s with the approval of benzalkonium chloride as an antimicrobial agent.11 Since the dawn of this class of antimicrobials, several QACs have been marketed as antibacterials for use in several arenas including hospitals and healthcare facilities, various industries, and commercial and residential settings. A sampling of QACs currently on the market are depicted in Fig. 1 and include benzalkonium chloride (BAC), cetylpyridinium chloride (CPC), didecyldimethylammonium chloride (DDAC), and chlorhexidine (CHX). In examining the structures of commercial QACs, it is apparent that most are monocationic derivatives with little structural variability, thus making this an area ripe with opportunity for expansion and investigation.

Of significant concern, reports of bacteria resistant to a battery of commercial QAC disinfectants have been rampant in recent years.6 This defense mechanism, which was originally attributed to the efflux of antimicrobial natural products such as berberine, has further been characterized through QAC-related dyes. Resistance occurs via QAC-specific efflux pumps such as QacA, the transcription of which is regulated by DNA-binding regulator QacR.6 Herein we report the culmination of over five years of research that started as a straightforward structure–activity relationship study of QAC scaffolds as antimicrobials and has since shifted toward using this library to elucidate the mechanism of QAC-resistance.

Section snippets

BisQACs and MultiQACs

The impetus of this research program began with our vision to explore the utility of QACs that bore more than one positive charge, in contrast to the commercially available agents. In addition, we wanted to focus on maintaining flexible synthetic routes that would allow for significant structural variation. Multi-cationic QACs were viewed as synthetically accessible targets that would possess a fundamentally different interaction with bacterial cell membranes, the long-assumed target of

Conclusion

Through our studies on the antibacterial activity of various classes of QACs, several key points have become apparent. Amphiphilic structures bearing significant alkyl chains are adept at penetrating membranes of both Gram-positive and Gram-negative bacteria. We have shown that aryl, linear alkyl, cyclic alkyl, and natural product-derived QACs can be prepared in a straightforward manner from economic starting materials, yielding a variety of potent antimicrobial QAC scaffolds. The evolution of

Experimental section

All reactions were carried out under ambient atmosphere with reagent grade solvents and magnetic stirring. All reagents were purchased from commercial vendors. Yields refer to spectroscopically pure compounds. 1H NMR spectra were recorded on a Varian Mercury 300 spectrometer. Chemical shifts are reported in δ values relative to tetramethylsilane. The reader is referred to the experimental detail for approximately 200 compounds, provided in references 13, 15, 17, 19, 20, 28, 29, 31, and 32.

Acknowledgements

This work was funded by a University City Science Center QED Proof of Concept Grant (Grant S1403 to WMW), a Research Corporation Grant Multi-Investigator Cottrell College Science Award (MICCSA 10709 to KPCM, KS, and KLC), the National Science Foundation (NSF-REU CHE-1062629 and CHE-0754521), as well as by Temple, Villanova, and James Madison Universities. MCJ acknowledges a National Science Foundation Pre-Doctoral Fellowship (DGE1144462).

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