Antimicrobial Tendency of Bagasse Lignin Extracts by Raman Peak Intensity

Abstract

In this study, Raman peak ratio was used as a screening parameter to evaluate antimicrobial tendency before conducting a susceptibility test between lignin extracts and targeted pathogenic bacteria. Methoxyl group in lignin basic units, syringyl (S) and guaiacyl (G) units, was reported to be responsible for antimicrobial characteristic in lignin extracts. By using Raman spectroscopy to quantify relative amount of methoxyl group based on peak of lignin basic units in lignin extracts, quick evaluation of antimicrobial tendency can be identified. Due to an overlapping peak, only syringyl was used in calculation. Raman peak ratio of lignin basic unit was calculated by dividing represented peak height over peak height of lignin peak signature from Raman wave number at 1600 cm−1. A 4-hydroxyl phenyl (H-unit), a lignin basic unit without methoxyl group, was used to construct normalized S/H ratio. Peak height of S-unit at Raman wave number 1331 cm−1 and peak height of H-unit at Raman wave number 1213 cm−1 were used in evaluation. Bagasse lignin extracts from two different conditions (NaOH 1% and NaOH 4%) and commercial alkaline lignin extracts were used. Peak ratios were calculated and compared with minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) against two gram-positive pathogenic bacteria: Staphylococcus epidermidis and Staphylococcus aureus and two gram-negative pathogenic bacteria: Pseudomonas aeruginosa and Escherichia coli. Different in antimicrobial tendency, decreased in MIC and MBC, was observed when S/H ratio increased from 1.2 to 2.0 for gram-positive bacteria. For samples with similar S/H ratio, lignin-free basis value is required to identify the antimicrobial tendency.