Abstract Description

A major obstacle facing the biofuels industry is the management of bacterial contamination during yeast fermentations. The contamination of biofuel plants by Gram-negative and Gram-positive bacteria poses a significant threat to the profitability of the industry. Lactic Acid Bacteria (LAB) are among the main contaminants found during yeast fermentations, reducing ethanol yields and thereby decreasing the profitability of biofuel production. The use of antibiotic treatments in industry have contributed to the emergence of drug-resistant bacterial strains, which further underlines the need for novel approaches to control bacterial contamination. Recently, green synthesised silver nanoparticles (AgNPs) have been shown to have antibacterial activities; and we hypothesized that they could have potential applications in controlling biological contaminations in the fermentation industry. Therefore, this study aimed to test the effect of green-synthesized AgNPs using RW extract (RW-AgNPs) as an antibacterial strategy in non-sterile fermentations. The RW-AgNPs were green-synthesized using optimised conditions, and characterised using physicochemical techniques. The antibacterial and yeast toxicity properties of the RW-AgNPs were assessed using microbiological techniques, co-cultures, growth curve analysis and anaerobic non-sterile fermentations. The RW-AgNPs were spherical with a core size of 8.7 ± 1.3 nm, hydrodynamic size of 104.30 ± 5.51 nm, PDI value of 0.343 ± 0.2 and ζ-potential of -11 ± 6.6 mV. Several bacteria associated with contamination in fermentations were selected, and the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values for RW-AgNPs were determined against Lactobacillus. plantarum, and Lactococcus. lactis. The contaminating L. plantarum strain was then co-cultured with S. cerevisiae and treated with the RW-AgNPs. Anaerobic fermentations were also conducted to determine the ethanol yield produced by the S. cerevisiae strains during treatment with RW-AgNPs. The RW-AgNPs showed low minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values against L. plantarum, and L. lactis. The RW-AgNPs suppressed the bacterial growth while remaining minimally toxic to the S. cerevisiae. This study demonstrated that RW-AgNPs have antibacterial effects against bacteria associated with contaminating industrial fermentations. These RW-AgNPs could be useful as an alternative to antibiotics in fermentations. Further studies are underway to elucidate the antibacterial properties of these RW-AgNPs in fermentations. However, further investigations are required to ensure their biocompatibility at concentrations which are minimally toxic to the yeast cells yet still toxic to bacteria at industrial scale. This could potentially be transformed into a novel antimicrobial for use in industrial fermentations.