Vitamin C inhibits staphylococcus aureus growth and enhances the inhibitory effect of quercetin on growth of Escherichia coli in vitro.

Quercetin is a natural flavonoid possessing a number of health beneficial effects. Its bioactivity is restricted by low solubility and sensitivity to oxidative degradation, factors that are often ignored in laboratory studies. We studied the antimicrobial effects of quercetin on Staphylococcus aureus, Escherichia coli and Lactobacillus plantarum at concentrations at which it is soluble and investigated how the antioxidant vitamin C modulates these activities. S. aureus was the most sensitive of the studied bacteria. After 12 hours of culturing, 90 µM quercetin decreased the growth of S. aureus to 75 % of the value for a control culture. 1 mM vitamin C combined with 90 µM quercetin diminished the growth of S. aureus drastically to 3 % of that of the control culture supplemented with vitamin C only. Interestingly, vitamin C by itself inhibited the growth of S. aureus as well, and 5 mM vitamin C inhibited growth completely. The growth inhibition of E. coli was slightly but significantly better in the presence of both quercetin and vitamin C than in the presence of quercetin alone. Probiotic L. plantarum was resistant to quercetin in the presence and absence of vitamin C. Enhancement of quercetin's antimicrobial activity by vitamin C is partly explained by the stabilizing effect of vitamin C on quercetin. Even though the acidity of vitamin C contributes to the inhibition of S. aureus growth, neutralized vitamin C also inhibits the growth efficiently even without quercetin. Our results suggest that vitamin C affects the metabolism of S. aureus and that these changes are likely to result in the observed growth inhibition. Although vitamin C itself is a powerful antioxidant, its aerobic metabolism increases oxidative stress on bacterial cells. Vitamin C may therefore be a safe and natural alternative for restricting the growth of S. aureus when non-toxicity is required.

Disposable electrochemical immunosensor for cortisol determination in human saliva.

Cortisol is a steroidal hormone and an important stress marker. Free serum cortisol concentration has been identified to correlate well with free salivary cortisol. In this present work an electrochemical immunosensor was developed to determine cortisol concentration within the physiological concentration range found in human saliva. The immunosensor is based on a direct competitive enzyme linked immunoassay using a home-made cortisol-alkaline phosphatase (AP) conjugate synthesized in our laboratory with disposable graphite screen-printed electrodes (SPEs). 1-nalphtyl phosphate (1-NP) was used as an enzymatic substrate and a square wave voltammetry (SWV) for electrochemical detection. To study method suitability for use with saliva samples, calibration curves were performed both in buffer and saliva. In buffer standard samples showed a limit of detection (LOD) of 0.6 ng/ml and working range (WR) of 0.2-44.6 ng/ml with good reproducibility (RSD 10%). Saliva matrix effect was removed effectively with Salivette Cortisol collection device (polyethylene) and a calibration curve showed similar characteristics as in buffer with LOD 1.7 ng/ml and WR 0.5-55.1 ng/ml (RSD 8%) demonstrating the possibility to determine human salivary cortisol within the desired human physiological range. Spiked saliva samples were analyzed with the developed immunosensor presenting excellent 92-114% recovery. Comparison to liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) method showed strong 0.90 correlation between methods indicating good accuracy of the developed immunosensor.

Urolithins display both antioxidant and pro-oxidant activities depending on assay system and conditions.

The biological effects of polyphenolic ellagitannins are mediated by their intestinal metabolites, urolithins. This study investigated redox properties of urolithins A and B using ORAC assay, three cell-based assays, copper-initiated pro-oxidant activity (CIPA) assay, and cyclic voltammetry. Urolithins were strong antioxidants in the ORAC assay, but mostly pro-oxidants in cell-based assays, although urolithin A was an antioxidant in cell culture medium. Parent compound ellagic acid was a strong extracellular antioxidant, but showed no response in the intracellular assay. The CIPA assay confirmed the pro-oxidant activity of ellagitannin metabolites. In the cell proliferation assay, urolithins but not ellagic acid decreased growth and metabolism of HepG2 liver cells. In cyclic voltammetry, the oxidation of urolithin A was partly reversible, but that of urolithin B was irreversible. These results illustrate how strongly measured redox properties depend on the employed assay system and conditions and emphasize the importance of studying pro-oxidant and antioxidant activities in parallel.