Abstract
The high antimicrobial activity of epigallocatechin gallate (EGCG), the most bioactive component of tea polyphenol with a number of health benefits, is well-known. However, little is known about the mechanism involved. Here, we discovered the relationship between reactive oxygen species (ROS), the Cpx system, and EGCG-mediated cell death. We first found an increase in ampicillin resistance as well as the transcription level of a LD-transpeptidase (LD-TPase) involved in cell wall synthesis; ycbB transcription was upregulated whereas that of another LD-TPase, ynhG, appeared to be constant after a short exposure of Escherichia coli to sub-inhibitory doses of EGCG. Additionally, the transcription level of cpxP, a downstream gene belonging to the Cpx regulon, was positively correlated with the concentration of EGCG, and significant upregulation was detected when cells were treated with high doses of EGCG. Through analysis of a cpxR deletion strain (ΔcpxR), we identified a constant ROS level and a notable increase in the survival rate of ΔcpxR, while the ROS level increased and the survival rate decreased remarkably in the wild-type strain. Furthermore, thiourea, which is an antioxidant, reduced the ROS level and antimicrobial activity of EGCG. Taken together, these results suggest that EGCG induces ROS formation by activating the Cpx system and mediates cell death.
Highlights
In Escherichia coli, a typical Gram-negative bacterium, there are at least five envelope stress response systems (ESRSs) including Cpx, σE, BaeSR, Psp, and Rcs (Bury-Mone et al, 2009)
Cells showed increased resistance to ampicillin but not to spectinomycin (Figure 1). These results indicated that epigallocatechin gallate (EGCG) might have an effect on the bacterial cell wall
After treatment with 400 μg/ml EGCG, ycbB was upregulated while the transcription level of ynhG remained unchanged (Figure 2), which may indicate YcbB is relatively more important than YnhG when exposed to EGCG. ycbB is reported to be a Measurement of reactive oxygen species (ROS)
Summary
In Escherichia coli, a typical Gram-negative bacterium, there are at least five envelope stress response systems (ESRSs) including Cpx, σE, BaeSR, Psp, and Rcs (Bury-Mone et al, 2009). Antimicrobial agents, including antibiotics and some natural products, have been used for many decades in healthcare and cleaning products Most of their antimicrobial mechanisms have been well-studied, especially for antibiotics (Kohanski et al, 2010). Quinolone-type antibiotics can inhibit DNA replication by targeting DNA topoisomerase IV/DNA gyrase–DNA complexes (Dwyer et al, 2007) Other antibiotics, such as rifamycins and macrolides, inhibit RNA transcription and protein translation, respectively (Kohanski et al, 2010). It has been clarified that aminoglycosides can bind to the 30S ribosomal subunit and interrupt bacterial protein synthesis, which could further trigger the two-component Cpx system and induce reactive oxygen species (ROS) formation (Kohanski et al, 2008)
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