الفهرس 
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Abstract
To investigate the presence of bacterial infection associated with mass mortality in freshwater fish farms a number of 150 mullets (Mugil cephalus) at Tolompate 7 village, Elryad City, Kafrelsheikh. Fish weighing 285 ± 25 g and body length 32 ± 1.5 cm. Sampling procedures were randomly carried out from three sits 1-3. The collected fish was transferred in a septic condition and preserved in ice-tanked till the arrival to the laboratory of Animal Health Research Institute.
Aeromonas hydrophila was isolated by traditional methods on specific media, molecular identification, RNA-sequence. In addition, the presence of virulence genes and antibiotic-resistant genes were determined. Also, the sensitivity test and lethal median dose were detected.
The results revealed the presence of three strains which were Aer. hydrophila and were deposited in NCBI under accession numbers of (OM965641, OM965642, and OM965643).The prevalence rates were 96%, 92%, and 86% in the three sites under investigation.
In the multidrug-resistant, genes were detected in the obtained bacteria. All isolated Aer. hydrophila were harbored resistant genes for tetracycline (TetA(A), sulphonamide (Sul 1), and quinolone (qnr A). More over, virulence genes were detected in the isolated bacteria enterotoxin heat-stable (ast) and cytotoxic enterotoxin (act).
The lethal median dose for the three isolated strain of Aer. hydrophila were determined to be 2.2 × 105, 2.2 × 105, and1.18× 106 CFU/mL, respectively
The isolated bacteria were sensitive to ciprofloxacine,enrofloxacin, levofloxacin and chloramphenicol. And were resistant for pencillins and cephalosprins, while it was intermediate for tetracyclin, oxytetracyclin, and nalidixic acid.
So, it was concluded that the mass mortality of M. cephalus was associated with Aer. hydrophila infection.
Therefore, from previous results, it should monitor for bacterial infection in freshwater fish farms and rapid interference with suitable antibiotic after sensitivity test consequentely, To overcome the bacterial resistance I investigate the impact of titanium dioxide nanoparticles on the very virulent clinically and MDR resistant strain of A.hydrophila, detected the minimum inhibitory concentration 20 µg/ml and its effect on the LD50( in vivo). Evaluated the TiO2 NPs effect by the use of scanning electron microscopy (SEM) which showed different ratio of TiO2 NPs and microorganisms as well as different contact times ended finally with a morphological alteration of the bacteria and pit in the cell wall and/or ruptures of some bacterial cells.
It is concluded that mass mortality of M. cephalus was associated with Aer. hydrophila infection also,the isolated strains of Aer. hydrophila were deposited at NCBI (OM965641, OM965642, and OM965643), strains were isolated at a rate of 96%, 92%, and 86%, also it was proved the pathogenecityof the isolated strains have cytotonic enterotoxins heat-stable (ast) and cytotoxic enterotoxin (act) virulence genes in addition to some resistant genes TetA(A), Sul1, qnrsA,( results of LD50).