Attributed to a Gram-negative bacterium, identified as Erwinia amylovora, the fire blight disease, recently detected in Tunisia, has become a real threat to our economy and our arboriculture. There is no effective curative treatment to eliminate the installed bacterium. It is therefore essential to prevent attacks and to limit the spread of the bacteria if it is already present. To do so we have chosen the biocontrol by bacteriophages. In this work we have isolated new strains of Erwinia amylovora (10 strains) from plant tissues where fire blight symptoms are persistent. These strains have been the subject of a molecular study. The isolated phages (30 isolates) were studied for their infectivity on Erwinia amylovora isolates and showed a surprising effect. Our phage isolates formed plaque of different sizes, with a diameter ranging between 0.8 and 7 mm on the soft agar layer containing the test bacterium. The phages thermostability showed that all these viruses resist at 80 ° C for 45 min.
The rapid development of aquaculture, its intensification, and the occurrence of fish health problems on farms push to develop alternative methods to antibiotics and chemotherapy for controlling fish diseases. Probiotics may provide a potential alternative method to protect fish from opportunistic and pathologic bacteria and promote a balanced environment. In this work, we have assessed the in vitro probiotic properties of twenty one bacteria from aquatic and fish origin, for their application in aquaculture. Selection was based on their antimicrobial activity (Bacteriocin) against fish pathogens and their in vitro safety assessment. This includes the evaluation of their haemolytic, proteolytic and mucinolytic activities, bile salt deconjugation ability and antibiotic susceptibility. Twelve of the twenty one bacteria isolated from several showed strong antibacterial activity against several pathogenic species such as Lactococcus garvieae, Vibrio anguillarum, Vibrio harveyi, Aeromonas hydrophila and Aeromonas salmonicida, and were taxonomically identified by partial 16S rDNA gene sequencing. The cell-free culture supernatants from cultures of these twelve strains were treated with proteinase K (10 mg/ml; 37ºC, 1h) and submitted to heat treatment (100ºC, 10 min), which showed that eleven strains exert extracellular antimicrobial activity against fish pathogens due to the production and secretion of thermo-stable antimicrobial peptides (i.e., Bacteriocins). The tested strains showed a great heterogeneity respect to their safety and antibiotic susceptibility.
A number of newly isolated halophilic microorganisms were screened for protease production. A bacterium designated as strain SBJ9 showed an important enzyme production at high salt concentrations and was then retained. The 16S DNA identification put this strain in the genus of Salicola with two reference species only. Protease production was higher at salinities ranging from 150 to 200 g/l (3.2 M) NaCl, when monitored at 35 °C and pH 7. The protease activity was optimal at 2.5 M NaCl, 40°C and pH 8, with high stability at wide ranges of salinity (1-5 M NaCl), temperatures (20-70 °C) and pH values (5-11). It was slightly improved by 5 mM CaCl 2 and totally inhibited by PMSF which indicated the dominance of serine proteases. Besides, it was perfectly stable in the presence of many detergent additives and organic solvents at high concentrations. These important features make Salicola sp. strain SBJ9 protease activity a good candidate for many industrial applications such as detergency and organic synthesis.
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