Inhibitory effects of flavonoids on biofilm formation by Staphylococcus aureus that overexpresses efflux protein genes.

This study evaluated the efficacy of glycone (myricitrin, hesperidin and phloridzin) and aglycone flavonoids (myricetin, hesperetin and phloretin) in inhibiting biofilm formation by Staphylococcus aureus RN4220 and S. aureus SA1199B that overexpress the msrA and norA efflux protein genes, respectively. The minimum inhibitory concentration (MIC) and minimum biofilm inhibitory concentration (MBIC50 - defined as the lowest concentration that resulted in ≥50% inhibition of biofilm formation) of flavonoids were determined using microdilution in broth procedures. The flavonoids showed MIC >1024 µg/mL against S. aureus RN4220 and S. aureus SA1199B; however, these compounds at lower concentrations (1-256 µg/mL) showed inhibitory effects on biofilm formation by these strains. Aglycone flavonoids showed lower MBIC50 values than their respective glycone forms. The lowest MBIC50 values (1 and 4 µg/mL) were observed against S. aureus RN4220. Myricetin, hesperetin and phloretin exhibited biofilm formation inhibition >70% for S. aureus RN4220, and lower biofilm formation inhibition against S. aureus SA1199B. These results indicate that sub-MICs of the tested flavonoids inhibit biofilm formation by S. aureus strains that overexpress efflux protein genes. These effects are more strongly established by aglycone flavonoids.

Prunin- and hesperetin glucoside-alkyl (C4-C18) esters interaction with Jurkat cells plasma membrane: consequences on membrane physical properties and antioxidant capacity.

Prunin (P)- and hesperetin glucoside (HG)-alkyl esters are lipid-soluble compounds with antimicrobial and antioxidant capacities in vitro. The effects of P- and HG-alkyl (C4-C18) esters (0.1-100µM) on human leukemia T (Jurkat) cells viability and plasma membrane fluidity were evaluated. After 1h of exposure, cell viability was not affected in the range 0.1-10µM. The decrease of cell viability found at 100µM concentration depended on the length of the alkyl chain and reached a maximum with C6-C12 derivatives. At this concentration, cell hyperpolarization and shrinkage were also observed. Cell plasma membrane fluidity was not affected, regardless the depths of the membrane level evaluated, but mild changes in plasma membrane hydration were found. Esterification did not affect the antioxidant capacity of P and HG (0.1-10µM) against 1mM H2O2. When exposed to 1mM AAPH, P-alkyl esters retained P antioxidant capacity, but HG-derivatives acted as pro-oxidants. Together, present experimental evidences suggest that short term exposures to 0.1-10µM concentrations of P- and HG-alkyl (C4-C18) esters can be considered safe for cultured human cells, and further studies are required to investigate their long term effects, as well their safety for human consumption.

Antibacterial activity of naringin derivatives against pathogenic strains.

AIMS: To study the antimicrobial activity of naringin (NAR), a flavonoid extracted from citrus industry waste, and NAR derivatives [naringenin (NGE), prunin and alkyl prunin esters] against pathogenic bacteria such as L. monocytogenes, E. coli O157:H7 and S. aureus. The relationship between the structure of the chemical compounds and their antagonistic effect was also analysed. METHODS AND RESULTS: The agar dilution technique and direct contact assaying were applied. NGE, prunin and NAR showed no antimicrobial activity at a concentration of 0.25 mmol l(-1). Similarly, fatty acids with a chain length between C2 and C18 showed no antimicrobial activity at the same concentration. However, prunin-6″-O-acyl esters presented high antibacterial activity, mainly against Gram-positive strains. This activity increased with increasing chain length (up to 10-12 carbon atoms). Alkyl prunin esters with 10-12 carbon atoms diminished viability of L. monocytogenes by about 3 log orders and S. aureus by 6 log orders after 2 h of contact at 37°C and at a concentration of 0.25 mmol l(-1). The compounds examined were not effective against any of the Gram-negative strains assayed, even at the highest concentration. CONCLUSIONS: Addition of sugars to the aglycone did not enhance its antimicrobial activity. Attachment of a saturated aliphatic chain with 10-12 carbon atoms to the A ring of the flavonoid (or to sugars attached to this ring), seems to be the most promising modification. In conclusion, alkyl prunin esters with a chain length of C10-C12 have promising features as antimicrobial agents because of their high antilisterial and antistaphylococcal activity. SIGNIFICANCE AND IMPACT OF THE STUDY: This study shows that it is possible to obtain NAR derivatives with important antimicrobial activity, especially against Gram-positive pathogenic bacteria. It also provides guidelines on the structural modifications in similar molecules to enhance the antimicrobial activity.

Antimicrobial properties of prunin, a citric flavanone glucoside, and its prunin 6″-O-lauroyl ester.

AIMS: To determine the antimicrobial potential of prunin (P), a flavanone glucoside resulting from the hydrolysis of naringin present in grapefruit, and of its prunin 6″-O-lauroyl ester (PL), synthesized by enzymatic catalysis. METHODS AND RESULTS: P and its lauroyl ester were tested against Gram-negative and Gram-positive bacteria, yeasts and moulds. P showed no inhibitory effect against the micro-organisms assayed, but stimulated growth of Pseudomonas aeruginosa and different Bacilllus sp. However, 150 µg ml(-1) of PL inhibited Escherichia coli, Salmonella enterica serovar Enteritidis, Salmonella enterica serovar Typhimurium, many Bacillus sp., Staphylococcus aureus ATCC29213, Enterococcus avium DSMZ17511, and different Listeria monocytogenes strains. In the last case, L. monocytogenes, sensitive or bacteriocin-resistant cells, lost nearly 4-log reductions after 30 min of contact. A bactericidal mode of action was determined using both scanning and transmission electronic microscopies. CONCLUSIONS: PL could be used as a food additive, because at low concentration (150 µg ml(-1)) it exhibited antimicrobial activity against important food-borne pathogens. A bactericidal effect was also determined on L. monocytogenes sensitive and bacteriocin-resistant mutant strains. P did not show any antimicrobial property at all. SIGNIFICANCE AND IMPACT OF THE STUDY: PL is a potential antimicrobial compound with a high anti-Listeria property.