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Escherichia coli (E. coli) is associated with an array of health-threatening contaminations, some of which are related to biofilm states. The pgaABCD-encoded poly-beta-1,6-N-acetyl-D-glucosamine (PGA) polymer plays an important role in biofilm formation. This study was conducted to determine the inhibitory effect of gallic acid (GA) against E. coli biofilm formation. Minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) values of GA against planktonic E. coli were 0.5 and 4 mg/mL, and minimal biofilm inhibitory concentration and minimal biofilm eradication concentration values of GA against E. coli in biofilms were 2 and 8 mg/mL, respectively. Quantitative crystal violet staining of biofilms and ESEM images clearly indicate that GA effectively, dose-dependently inhibited biofilm formation. CFU counting and confocal laser scanning microscopy measurements showed that GA significantly reduced viable bacteria in the biofilm. The contents of polysaccharide slime, protein, and DNA in the E. coli biofilm also decreased. qRT-PCR data showed that at the sub-MIC level of GA (0.25 mg/mL) and expression of pgaABC genes was downregulated, while pgaD gene expression was upregulated. The sub-MBC level of GA (2 mg/mL) significantly suppressed the pgaABCD genes. Our results altogether demonstrate that GA inhibited viable bacteria and E. coli biofilm formation, marking a novel approach to the prevention and treatment of biofilm-related infections in the food industry. [ABSTRACT FROM AUTHOR]

Abstract Peppermint essential oil (PEO) has been reported to prevent microbial growth and prolong the shelf-life of food, but there are few studies about its possible antibacterial mechanism and anti-biofilm activity against Staphylococcus aureus (S. aureus). In this study, the antibacterial mechanism of PEO on planktonic S. aureus was investigated by determining electrical conductivity, cell membrane integrity, cell viability, and bacterial morphology. Moreover, a crystal violet quantification assay and plate colony counts were used to evaluate the effects of PEO on inhibiting and inactivating S. aureus biofilms. The results show that PEO affected the permeability and integrity of the S. aureus cell membrane, as evidenced by the increase in relative electrical conductivity and leakage of nucleic acids, proteins, and ATP. Decreased cell viability and changes in cell morphology further confirmed the cell membrane damage to S. aureus by PEO. In addition, PEO significantly inhibited the formation of biofilm, and inactivated mature biofilm formed by S. aureus. These findings suggest that PEO has the potential to control planktonic S. aureus and its biofilm in food processing environments. Highlights • Peppermint essential oil (PEO) can damage the cell membrane of S. aureus. • PEO had potent inhibitory effect on S. aureus biofilm formation. • PEO inactivated and removed the mature biofilm formed by S. aureus. [ABSTRACT FROM AUTHOR]