Thesis (Ph.D.) - Cairo University - Faculty of Agriculture - Department of Microbiology

In recent decades, many studies evaluated the use of probiotic bacteria in the field of bacterial disease control and prevention instead of using antibiotics which lead to arising antibiotic-resistant bacteria and contaminating the aquatic ecosystem. El-Manzala Lake is the largest coastal lake in Egypt for fish aquaculture, and it is known as one of the most significant sources of inexpensive different fish species for humanconsumption.The current study targeted probiotics from the intestine of Nile Tilapia fish (Oreochromis niloticus) collected from El-Manzala Lake to characterize them partially. Moreover, the effect of probiotics on cyanobacteria, isolated from El-Manzala Lake, was evaluated. In this study, fifty-two potential probiotic bacteria were isolated from intestine of Nile Tilapia fish from El Manzala Lake during the period from October 2015 to September 2016. Only, three isolates were characterized as G+, catalase negative and Þ hemolytic. The 16s rRNA gene sequence confirmed that one isolate was Lactobacillus sakei, while the other two isolates were Staphylococcus hominis. The cell-free supernatant (CFS) of the three isolates has good inhibitory effect against Staphylococcus aureus and Vibrio cholerae isolated from fish. The largest inhibition zone was recorded with CFS of Lb. sakei as the inhibition zone ranged from 28 {u2013} 31.4 and 18.6 {u2013} 28 mm against Staphylococcus aureus and Vibrio cholerae, respectively. In acidic conditions, the survival rate around of or above 90% was observed during the growth of Lb. sakei for 2 h at pH 3, 4 and 5. For all isolates, the relatively strong bile salt resistance, represented by above 50%, was recorded with concentrations of 0.1 and 0.2 with different incubation periods

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