Caffeic Acid Esters Are Effective Bactericidal Compounds Against Paenibacilluslarvae by Altering Intracellular Oxidant and Antioxidant Levels.

William Collins,Noah Lowen,David J Blake

doi:10.3390/biom9080312

Abstract

American Foulbrood (AFB) is a deadly bacterial disease affecting pupal and larval honey bees. AFB is caused by the endospore-forming bacterium Paenibacillus larvae (PL). Propolis, which contains a variety of organic compounds, is a product of bee foraging and is a resinous substance derived from botanical substances found primarily in trees. Several compounds from the class of caffeic acid esters, which are commonly found in propolis, have been shown to have antibacterial activity against PL. In this study, six different caffeic acid esters were synthesized, purified, spectroscopically analyzed, and tested for their activity against PL to determine the minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs). Caffeic acid isopropenyl ester (CAIE), caffeic acid benzyl ester (CABE), and caffeic acid phenethyl ester (CAPE) were the most effective in inhibiting PL growth and killing PL cell with MICs and MBCs of 125 µg/mL when used individually, and a MIC and MBC of 31.25 µg/mL for each compound alone when CAIE, CABE, and CAPE are used in combination against PL. These compounds inhibited bacterial growth through a bactericidal effect, which revealed cell killing but no lysis of PL cells after 18 h. Incubation with CAIE, CABE, and CAPE at their MICs significantly increased reactive oxygen species levels and significantly changed glutathione levels within PL cells. Caffeic acid esters are potent bactericidal compounds against PL and eliminate bacterial growth through an oxidative stress mechanism.

Highlights

USA and European beekeepers face high honey bee (Apis millifera) colony losses each year
Chemical shifts are reported in ppm, multiplicities are indicated by s, d, t, q, p, h, m, and br
We have shown that a subset of our compound library, caffeic acid phenethyl ester (CAPE), Caffeic acid isopropenyl ester (CAIE), and caffeic acid benzyl ester (CABE)

Summary

Introduction

USA and European beekeepers face high honey bee (Apis millifera) colony losses each year. The BeeInformed Partnership Survey, a national epidemiological analysis of honey bee health, indicated that 40.1% of all USA colonies were lost in 2018 [1]. One of the most dangerous pathogens that can infect honey bee colonies is the endospore-forming bacterium Paenibacillus larvae (PL) [5]. This extremely contagious bacterium, known commonly in the beekeeping community as American foulbrood (AFB), adversely affects honey bees during the larval or pupal stages [5,6]. The endospore, once inside a colony, is typically distributed to larvae during routine feeding by nurse bees, and after ingestion of the spores results in septicemia of the larvae [7]

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