Influence of the uronic acid composition on the gastroprotective activity of alginates from three different genus of Tunisian brown algae.

Alginates from three genus of Tunisian brown algae were isolated and characterized by size exclusion chromatography and Solid-state NMR spectroscopy. Alginate from Padina pavonica (APP) had the highest molecular weight (Mw) with 147,000g/mol while it was 85,000g/mol for alginate from Cystoseira compressa (ACC) and 58,000g/mol for alginate from Dictyopteris membranaceae (ADM). The mannuronate (M) to guluronate (G) ratios were estimated from spectral deconvolution of the 13C CP/MAS spectra and the results has shown that all the extracts are mannuronic acid-rich alginates with M/G ratio increased in the order ADM - ACC - APP. An interesting gastroprotective effect was observed for the extracts; ADM and ACC exhibited the highest inhibition of gastric lesions, at 50mg/kg, with 83.41% and 75.39% respectively. Otherwise, it has been shown that the gastroprotective effect of alginates depends mainly on their uronic acid composition.

Influence of freezing stress on morphological alteration and biofilm formation by Listeria monocytogenes: relationship with cell surface hydrophobicity and membrane fluidity.

The morphological changes and adhesive property of three Listeria monocytogenes strains submitted to freezing stress (-20 °C) were studied. The atomic force micrographs showed a reduction in the cell size and an evolution to coccoid shape. The phenotypic slime production of L. monocytogenes and the expression of the adhesive gene were investigated before and after 10 months of incubation in salmon at -20°. Our results showed that after ten months, stressed stains become more adherent and able to produce slime. In addition, we noted that this pathogen presents same physiological changes to adapt to starvation conditions. The cellular fatty acids composition of adhered and floating cells of three L. monocytogenes strains was taken into consideration. The stressed strains presented different chain lengths and therefore an increase in the hydrophobicity level. Moreover, we noted that the adhesive property of L. monocytogenes strains affects the Benzalkonium chloride bacterial sensitivity which increased after biofilm formation.