الفهرس 
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Abstract
This study was carried out to isolate and characterize A.hydrophila from fresh, brackish and marine water fishes (different water salinities). Using traditional methods of isolation and biochemical characterization compared to the recent techniques as polymerase chain reaction. In addition, to compare genetically between the 3 types of A.hydrophila isolates by using RAPD PCR. Moreover, check the antimicrobial sensitivity against A.hydrophila isolated from fishes live in different water salinities. A total number of 170 fishes (100 fresh water, 40 brackish water, and 30 marine water fishes) from different farms in Alexandria, Kafr Elsheikh, and Behira governorates were collected from different water salinities. The prevalence of A.hydrophila was 47% (38% in fresh water fish, 65% brackish water fish, 53.3% in marine water fish). A.hydrophila isolates were isolated on R-S media, SMART media, Aeromonas agar media, TCBS media, Nutrient agar, and tryptic soya agar giving the typical colony characters of the bacteria. A.hydrophila isolates were tested for pathogenicity by inoculation into blood agar and skimmed milk agar 1% for hemolytic and proteolytic activity. All isolates showed β hemolysis and proteolytic activity. A.hydrophila isolates were inoculated into trypticase soya broth with different NaCl concentration (0.5, 1, 2, 3, 4, 5 and 6%) and turbidity was measured by photometer at wave length 610 nm. The higher NaCl concentration, the lower turbidity was found. Moreover, reduction % of total bacterial count of A.hydrophila in relation to different concentration of NaCl in TSB after 10 fold serial dilutions by surface plating technique was calculated. Hemolysin and aerolysin genes were detected by PCR from the isolates from different sources of fishes (fresh, brackish and marine water fishes). RAPD PCR was used to differentiate genetically between the bacteria isolated from fresh, brackish and marine water fishes for the first time and we confirmed that there were genetic variations between the 3 types of isolates upon using 2 RAPD primers. This result explains the reasons why A.hyrophila can infect brackish and marine recently as it was known as a disease of fresh water fish mainly. We tested the tolerance of A.hydrophila isolated from different salinities to different NaCl concentrations on broth and media. On broth, bacterial turbidity measured by colorimetric WL and we found that turbidity reduced with higher NaCl concentrations. On media we used total bacterial count and reduction percent to check the effect of NaCl% on total colony count for different dilutions; we found that isolates from marine water fishes can tolerate excess NaCl than fresh and brackish isolates. A.hydrophila isolates from fresh, brackish and marine fish showed high resistance to Ampicillin ﴾M10﴿, Erythromycin ﴾E15﴿ Nalidixic acid (NA30), and Spectinomycin (SPT10). On the other hand, A.hydrophila isolates from fresh, brackish and marine fish showed high sensitivity to Enrofloxacin (EF10), Ofloxacin ﴾OFX5 ﴿and Gentamicin (CN10﴿. Polymixin ﴾PB300 u ﴿showed activity against A.hydrophila from brackish and marine water fishes only. Doxycycline ﴾DO30﴿ and Nitrofurantoin ﴾F300u) were highly effective against brackish water isolates than others for the field application. Isolation of A.hydrophila from different water salinities raises the public health concern and the importance to find suitable methods to control the infection.