Study of antioxidant activity of fodder grasses using microbial test systems.

AIM: To measure the biological activities of extracts of fodder grasses Onobrýchis arenária, Galéga orientális and Rhaponticum carthamoides that are commonly planted in Europe, Middle East and eastern Africa. METHODS AND RESULTS: Microbial test-systems based on Escherichia coli BW25113 that allow measurement of gene expression, growth and survival, biofilm formation (BF) in combination with the standard chemical procedures were used. The extracts studied had radical scavenging and metal-chelating activities and induced expression of antioxidant genes via generation of hydrogen peroxide. However, the extracts did not affect bacterial growth in planktonic cultures but dose-dependently inhibited BF. CONCLUSIONS: The most remarkable effects were observed in G. orientalis, a high-yielding crop, rich in crude protein and fibres. SIGNIFICANCE AND IMPACT OF THE STUDY: Taking into account the antibiofilm activities of the extracts, a perspective for decreasing colonization of ruminants' gut with pathogenic bacteria might be suggested in case of feeding with all the grasses studied.

Effect of resveratrol and quercetin on the susceptibility of Escherichia coli to antibiotics.

Activities of plant polyphenols (PPs), resveratrol and quercetin, alone or in combination with four conventional antibiotics against Escherichia coli have been investigated. In medium without antibiotics, both polyphenols caused a dose-dependent growth inhibition. However, pretreatment with resveratrol (40 and 100 µg ml-1) and quercetin (40 µg ml-1) reduced the bacteriostatic effect of kanamycin, streptomycin, cefotaxime and partially of ciprofloxacin. With few exceptions, both PPs also reduced the bactericidal effect of tested antibiotics. Paradoxically, low doses of PPs enhanced the bactericidal effect of kanamycin and partially ciprofloxacin. Compared to quercetin, resveratrol showed a weaker effect on the induction of antioxidant genes and the resistance of E. coli to the oxidative stress generated by hydrogen peroxide treatment. Both polyphenols at high doses reduced membrane potential. Altogether, these findings suggest that the decrease in the bactericidal effect of antibiotics by high doses of polyphenols is mostly due to bacteriostatic action of the latter. In the case of quercetin, the contribution of its antioxidant activity for antibiotic protection may be significant. There is a growing interest in the use of plant-derived compounds to enhance the toxicity of traditional antibiotics. This and other studies show that, under certain conditions, the use of polyphenols as adjuvants may not exert the expected therapeutic effect, but rather to decrease antimicrobial activity of antibiotics.