Karima Bekir

Adhesive ability and biofilm metabolic activity of Listeria monocytogenes strains before and after cold stress

Listeria monocytogenes is an important pathogen responsible for major outbreaks associated with food products. Adhesion to surfaces leads to significant modifications in cell physiology. In this work, the ability of L. monocytogenes to produce biofilm and its ability to adhere to abiotic surfaces under cold stress were evaluated. Metabolic activity of biofilm formed by L. monocytogenes before and after cold stress was measured in vitro using the colorimetric method based on the reduction of the tetrazolium salt 2,3-bis(2methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT). The ability to adhere to abiotic surfaces was determined by the ability of the cells to metabolically reduce bromure de 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium (MTT) to a formazan dye. Our results show that L. monocytogenes strains were able to adhere to abiotic materials with different degrees. In fact, cold stressed strains (-20°C) were more adhesive to polyethylene, glass, polyvinyl chloride and stainless style surfaces than non-stressed cells. Our observations show that the hydrophily varied with cold stress period. At freezing temperature, L. monocytogenes was strongly hydrophobic. Genetic studies of adhesive genes of L. monocytogenes will be required to fully understand the importance of this observation. Keywords: Listeria monocytogenes , slime production, cold stress, abiotic-surfaces, biofilm formation

Survival and retention of the probiotic properties of Bacillus sp. strains under marine stress starvation conditions and their potential use as a probiotic in Artemia culture

The probiotic properties of Bacillus strains isolated from Artemia culture and the effect of marine stress on viability and survival were investigated, as well as the changes occurring in their properties. Analyses showed that these bacteria corresponded to the genus Bacillus sp. Antagonism and adherence assays revealed that Bacillus strains have an inhibitory effect against tested pathogenic bacteria and are fairly adherent. Normal and starved cells showed different enzymatic profiles. Challenge tests performed with Artemia larvae provided evidence that the tested Bacillus strains were neither pathogenic nor toxic to the host and conferred protection for Artemia culture against pathogens. The tested strains maintained their viability and their probiotic properties during the period of study. The results suggest that the tested strains have suffered changes allowing them to survive in seawater in the absence of nutrients and outside their natural host, identifying them as potential probiotic candidates for Artemia culture.

Biofilm formation, cell surface hydrophobicity, and fatty acids analysis of starved Salmonella enterica serovar Typhimurium in seawater.

Salmonella is an international foodborne pathogen widely disseminated in seawater that regularly causes large outbreaks of food poisoning. In this study, we have investigated the effect of starvation on the ability of Salmonella enterica serovar Typhimurium cells to adhere to polystyrene microplate and Hep2 cells in seawater microcosms after incubation for 3 years. Cell surface hydrophobicity was evaluated. Effect of stress on fatty acids composition was also established. Our results showed that after incubation in seawater, the ability of starved cells to adhere to polystyrene microplate was decreased significantly. However, the adhesion values to Hep2 cells have increased. In addition, cells surface hydrophobicity was decreased. The fatty acids composition of starved cells was modified.