Table 1.

Classification of natural phenolic compounds.

Polyphenols	General Structure	Representative Compounds	Antibacterial Activity
Phenolic acids		R1=H, R2=H: 4-Hydroxy Benzoic Acid. R1=H, R2=OCH3: Vanillic Acid. R1=OCH3, R2=OCH3: Syringic Acid. R1=H, R2=OH: Protocatechuic Acid.	Antibacterial effect increases significantly with pH values [10].
a. Benzoic acid derivatives
b. Derivates of cinnamic acid		R1=H, R2=H: p-Coumaric Acid. R1=H, R2=OCH3: Ferulic Acid. R1=OCH3, R2=OCH3:Sinapic Acid. R1=H, R2=OH: Caffeic Acid.
Flavonoids		Flavones Flavonols Isoflavones Flavanones Anthocyanidins (flavylium salt) Flavanols	Flavonoids act against bacteria such as S. aureus and P. aeruginosa with a very low minimum inhibitory concentration (MIC) value (0.062 µg/mL) [11].
Lignans		Dibenzylbutanes Dibenzylbutyrolactones, arylnaphthalenes Terahydrofurans Furofurans Dibenzocyclo-octadienes	Due to structural properties, antibacterial activity of lignans is influenced by the stereochemistry of molecules [12].
Stilbenes		R1,R2,R3=OH, R4,R5=H: Resveratrol. R1,R2=OCH3, R3=OH, R4, R5=H: Pterostilbene. R1,R2,R3,R4=OH, R5=H: Piceatannol. R1,R2,R3,R5=OH, R4=H: Oxyresveratrol.	In combination with antibiotics, some stilbenes can be useful in treating infections caused by multidrug-resistance bacteria [13].
Tannins		Gallotannins Ellagitannins Complex tannins (Acutissimin A and Eugenigrandin A) Condensed tannins (Procyanidin B2, Proanthocyanidin A1, Proanthocyanidin A2) Low molecular mass phenolics (gallic acid).	Tannin compounds act against bacteria, causing disintegration of bacterial colonies, by interfering with the bacterial cell wall and inhibiting fatty acid biosynthesis pathways [14].
a. Hydrolysable tannins
b. Nonhydrolysable tannins (Condensed tannins)
c. Pseudotannins