Biofouling is a process of surface colonization by microorganisms through cell adhesion and production of extracellular polymers (polysaccharides and proteins). It often causes serious problems in the chemical, medical and pharmaceutical industries. Recently, it was demonstrated that some natural phenolic compounds found in plants and vegetables have an antibiofouling effect, reducing formation of biofilm by Gram-negative bacteria. In this study, Streptococcus mutans, a Gram-positive bacterium was investigated for the antibiofouling effect of polyphenols. It was hypothesized that the two enzymes, glucosyltransferase and fructosyltransferase, produced by S. mutans, would be inhibited by the natural phenolic compounds. When these two enzymes were inhibited, less (or no) biofilms were formed. Enzymes were separated from a S. mutans culture medium, and their activities were measured with five different polyphenols using microtiter-plates and high-performance liquid chromatography. The results of minimum inhibitory concentration (MIC) were used to determine the enzyme inhibition effect of polyphenols on biofilm formation without killing the cells. Most of the polyphenols used showed considerable reduction of biofilm formation. Gallic acid and tannic acid showed significant enzyme inhibition effects below their MICs.

The retention of a surrogate pathogenic bacterium, Escherichia coli(T) , in Pseudomonas aeruginosa biofilms (with various EPS excreting capacities) was investigated using a laboratory flow cell system. The structural characteristics of the biofilm, as well as the quantity of E. coli(T) retained in the biofilm, were assessed using confocal laser scanning microscopy coupled with image analysis. In addition, the total interaction energy between E. coli(T) and the P. aeruginosa biofilm was computed with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which provided an additional context to explain the pathogen interaction in aquatic biofilms. The correlations between the quantity of detained E. coli(T) cells and the structural characteristics of the biofilm were analysed and the results indicated that the heterogeneity of the biofilm could create a quiescent zone leading to temporary retention of E. coli(T) within the biofilm. Overall, this study provided insights toward understanding the retention of pathogenic bacteria in environmental biofilms.

Natural polyphenols, gallic acid, tannic acid, quercetin and salicylic acid, were investigated for their antimicrobial and antioxidant activities against Streptococcus mutans. Ascorbic acid, well known for its antimicrobial and antioxidant activities, was used as a criterion for the polyphenols. The antimicrobial effect was assessed using the plate dilution assay and the minimum inhibitory concentration (MIC) of each polyphenol was then determined from the antimicrobial activity results. Salicylic acid was the weakest antimicrobial with the highest MIC (3.8 mg/mL), and tannic acid was the strongest antimicrobial with the lowest MIC of 0.4 mg/mL. Antioxidant capacities were evaluated using the DMPD and ABTS decolorizing assays. These polyphenols show high antimicrobial activity and inoxidizability. Antioxidant activity for quercetin according to the DMPD method was inconclusive because it had color interference with the DMPD radicals. Although some conflicting results were observed between the DMPD and ABTS methods, the polyphenols with high antioxidant capacities still showed high antimicrobial activities, which suggest that the antioxidant capacity attributes to the antimicrobial effects.