Pentagalloyl glucose from Schinus terebinthifolia inhibits growth of carbapenem-resistant Acinetobacter baumannii

Abstract

The rise of antibiotic resistance has necessitated a search for new antimicrobials with potent activity against multidrug-resistant gram-negative pathogens, such as carbapenem-resistant Acinetobacter baumannii (CRAB). In this study, a library of botanical extracts generated from plants used to treat infections in traditional medicine was screened for growth inhibition of CRAB. A crude extract of Schinus terebinthifolia leaves exhibited 80% inhibition at 256 µg/mL and underwent bioassay-guided fractionation, leading to the isolation of pentagalloyl glucose (PGG), a bioactive gallotannin. PGG inhibited growth of both CRAB and susceptible A. baumannii (MIC 64-256 µg/mL), and also exhibited activity against Pseudomonas aeruginosa (MIC 16 µg/mL) and Staphylococcus aureus (MIC 64 µg/mL). A mammalian cytotoxicity assay with human keratinocytes (HaCaTs) yielded an IC₅₀ for PGG of 256 µg/mL. Mechanistic experiments revealed iron chelation as a possible mode of action for PGG's activity against CRAB. Passaging assays for resistance did not produce any resistant mutants over a period of 21 days. In conclusion, PGG exhibits antimicrobial activity against CRAB, but due to known pharmacological restrictions in delivery, translation as a therapeutic may be limited to topical applications such as wound rinses and dressings.

Figure 1

Bioassay-guided fractionation of extract 429 from Schinus terebinthifolia leaves using growth inhibition of CRAB.

Figure 2

Chemical structure of pentagalloyl glucose (PGG).

Figure 3

(A) Growth inhibition of 24 A. baumannii strains by pentagalloyl glucose and (B) comparison of A. baumannii growth inhibition with 429C–F8–PF11–SF4 and commercially-sourced pentagalloyl glucose. Figure made with GraphPad Prism version 8.3.1 for Windows, www.graphpad.com.

Figure 4

Growth inhibition of A. baumannii AR Bank #0035 and human keratinocyte cytotoxicity by PGG and its parent extract 429. Figure made with GraphPad Prism version 8.3.1 for Windows, www.graphpad.com.

Figure 5

Time-kill assay of A. baumannii AB5075 with PGG (256 µg/mL) alone and supplemented with oleic acid, polysorbate 80, and iron (II) sulfate. Meropenem concentration was 64 µg/mL. Figure made with GraphPad Prism version 8.3.1 for Windows, www.graphpad.com.

Figure 6

Daily serial passaging of A. baumannii AB5075 with PGG and tetracycline, showing change in dose–response curves of (A) PGG and (B) tetracycline, with darker lines indicating more recent passages, and (C) change in MIC of PGG and tetracycline. Base MICs are 256 µg/mL for PGG and 4 µg/mL for tetracycline. Figure made with GraphPad Prism version 8.3.1 for Windows, www.graphpad.com.