Abstract
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 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, 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. 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 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.

Key words: A.hydrophila, Fresh, Brackish, Marine fishes, PCR