Effects of certain physical stresses on the composition of the membrane of bacteria implicated in food and environmental contamination.

Bacterial membranes are implicated in the adaptation process of bacteria to numerous environmental conditions. In this context, our aim was to explain the consequences of a few physical stressor factors, like UV radiations and magnetic fields underlying the structural adaptation of cellular membranes to physical factors. The goal was also to review the state of the art about the link between membrane composition and bacterial resistance. According to comparative studies between ionizing γ-radiation, non-ionizing UVc radiations and Static Magnetic Field SMF, the response of some Gram negative bacteria appears to be generalized and was manifested by a membrane unsaturation, because of a production peak of unsaturated fatty acids. However, disturbances found inside the membrane, after UVB and Pulsed Electric Field (PEF) exposure were marked by a lower unsaturated fatty acids rate. This result is not concordant to disturbance seen after UVC treatment, even if the treatment is by UV radiation.

Effect of static magnetic field (200 mT) on biofilm formation in Pseudomonas aeruginosa.

Several studies have investigated the effects of ionizing and non-ionizing radiations on microorganisms. However, the interaction between the magnetic field radiations and bacteria is less studied. The aim of our study was to study the effect of static magnetic field on the biofilm formation in Pseudomonas aeruginosa and its isogenic sod mutants. Our results revealed that the exposure to the static magnetic field (200 mT) increases significantly the swarming in the wild strain. The fliC gene expression did not show significant difference after 6 h exposure of the wild-type strain. The release of some compounds of the biofilm matrix such as rhamnolipids has been considerably enhanced after 6 h of exposure in the wild type. On the other hand, the pyocyanin and biofilm production was increased significantly in all strains compared to controls. Furthermore, our results revealed that the biofilm formation was confirmed by the pslA and ppyR gene expressions.

Antioxidant enzymes expression in Pseudomonas aeruginosa exposed to UV-C radiation.

It was well known that, UV-C irradiation increase considerably the reactive oxygen species (ROS) levels in eukaryotic and prokaryotic organisms. In the enzymatic ROS-scavenging pathways, superoxide dismutase (SOD), Catalase (CAT), and peroxidase (POX) were developed to deal with oxidative stress. In this study, we investigated the effects of UV-C radiations on antioxidant enzymes (catalase, superoxide dismutase, and peroxidases) expression in Pseudomonas aeruginosa. Catalase, superoxide dismutase, and peroxidases activities were determined spectrophotometrically. Isozymes of superoxide dismutase were revealed by native gel activity staining method. Lipid peroxidation was determined by measuring malondialdehyde formation. Our results showed that superoxide dismutase, catalase and peroxidase activities exhibited a gradual increase during the exposure time (30 min). However, the superoxide dismutase activity was maximized at 15 min. Native gel activity staining assays showed the presence of three superoxide dismutase isozymes. The iron-cofactored isoform activity was altered after exposure to UV-C stress. These finding suggest that catalase and peroxidase enzymes have the same importance toward UV-C rays at shorter and longer exposure times and this may confer additional protection to superoxide dismutase from damage caused by lipid peroxidation. Moreover, our data demonstrate the significant role of the antioxidant system in the resistance of this important human pathogen.

Fatty Acids Composition and Biofilm Production of Attenuated Salmonella typhimurium dam and seqA Mutants After Exposure to UV-C.

The goal of this work was the investigation of correlation between some peculiarities of membrane fatty acids composition, biofilm formation, and motility of dam and/or seqA mutants in Salmonella typhimurium bacterial cells and UV-C radiations. The exposure changed the fatty acids composition of dam and seqA/dam strains. Significant increase of unsaturated fatty acids was observed. Swarming and swimming were enhanced only in dam mutant and biofilm formation increased significantly in all tested strains after UV-C exposure. These results suggest that increased sensitivity toward UV-C rays in dam strains might be due to fatty acid alteration.

Tertiary nitrification using moving-bed biofilm reactor: a case study in Tunisia.

In this study, the effect of operational conditions on biofilm development and nitrification in moving-bed biofilm reactor (MBBR) was investigated. The reactor was operated in a continuously fed regime during 170 days and with theoretical hydraulic retention time of 7 h, respectively. The presence of chemical oxygen demand (COD) increased the time required to form stable nitrifying. Subsequent stepwise increase of influent COD caused an increment in total polysaccharide (PS) and protein (PN) content, which was accompanied by an attachment of the biofilm, as shown by atomic force microscope (AFM). PS and PN concentrations proved to be good indicators of biomass development and attachment in MBBR system. Reactor was operated and water quality was characterized before and after treatment. Parameters including pH, 5-day biochemical oxygen demand (BOD5), total suspended solids (TSS) (COD), PN, PS, and fecal bacteria in both raw and treated wastewater were monitored during the treatment. The removal rates of ammonium-nitrogen (NH4 (+)-N), BOD5, COD, and TSS are 95, 67.5, 69.2, and 73.33 %, respectively. The average bacterial reduction between the inlet and the outlet was of the order of 5 ± 1 logarithmic units for fecal coliforms. AFM showed that distinct biofilm and extracellular polymeric substances were formed in biofilm was thicker in the 70 days than in the 30 days. These results showed that the consumption rate for each substrate increased parabolically with biofilm thickness due to the increased amount of biomass Thus, MBBR can serve as a promising technology for wastewater treatment and can be scaled up for small communities in the developing countries.