الفهرس 
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Abstract
Nile Tilapia Oreochromis niloticus considered as a principle source of white protein for people in Egypt and it is the most famous cultured fish in various Egyptian Governorates. So, infection of aquaculture with bacterial diseases leading to high mortality rate, decreasing fish production and sever economic loss. P. fluorescens affect fresh and marine water leading to scale detachment on fish body with congestion and red spots on skin with congested internal organs. Bacteriological examination of 400 fish samples from apparently healthy and diseased Nile tilapia (Oreochromis niloticus) of various sizes showed a high prevalence of 32% (128\400). Regarding the occurrence of Pseudomonas fluorescens in organs and tissues of diseased fish (gills, liver, kidney and intestine) with prevalence 22.75% (364\1600), the highest prevalence was observed in gills of fish 31.3% (114/364) followed by intestine 26.9% (98/364), liver 24.2% (88\364), and kidney 17.6%(64\364). All strains tested for biochemical reactions. The results showed that the isolates were positive for catalase, oxidase, urease, triple sugar iron, methyl red test, citrate utilization .On the other hand, the isolates were negative biochemical reactions for vogues-proskauer, and hydrogen sulfide production tests. Morphological examination of P. fluorescens colonies on Bacto Pseudomonas Agar F aerobically at 25ºC for 24 h showed fluorescent colonies with fluorescent yellow color surrounded with yellow pigment all over the media. PCR technique for 16SrDNA gene was applied on 50 representatives P. fluorescens strains isolated from different organs (liver, gills, kidney, and intestine) of diseased Nile for confirmation. The PCR results for 16SrDNA gene of P. fluorescens showed the presence of 16SrDNA gene in 30% (15\50) of examined isolates. The 16SrDNA gene was amplified giving product of 850 bp. Sequencing of 16SrDNA gene from three purified PCR product of three P. fluorescens strain isolated from Nile Tilapia fish O. niloticus achieved. This P. fluorescens strains BMS1, BMS2 and BMS3 were identical to diverse strains of P. fluorescens subsp. with 98.7–99.8% similarity. The sequence was recorded in GenBank under the accession numbers MT752964, MT752968 and MT753009. Furthermore, the phylogenetic tree based on nucleotide sequence analysis of the 16SrDNA genes signifying genetic relationship among many P. fluorescens strains found in different countries. The phenotypic resistance of P. fluorescens isolates using the disk diffusion assay displayed a high resistance to Peperacillin (100 %), Ceftazidime (29.7%) and Cefepime On the other hand, the highest susceptibility rate to Cefotaxime (74.2%), Ceftriaxone (70.3%), Ofloxacin (70.3%). Moderate sensitive to Tobramycin and Gentamycin (100 %), Doxycycline (92.3%), Amoxicillin\Clavulanic Acid (89%). Furthermore, multidrug resistant (MDR) to more than two antimicrobial classes was discovered in 108 out of 364 (29.7 %) isolates showing seven resistance patterns. Among these resistance isolates, the most common pattern was FEP/ CAZ/ E/ PRP represented by 30 strains followed by FEP/ CAZ/ TPZ/ PRP displayed by 16 strains, FEP/ CAZ/ MEM/ PRP displayed by 15 strains, FEP/ CAZ/ CTX/ TPZ/ IPM/ E/ PRP represented by 14 stains, CAZ/ TOB/ AMC/ PRP represented by 12 strains, CRO/ FEP/ CAZ/ Do/ GN/ PRP represented by 11 strains, and CTX/ FEP/ CAZ/ TPZ/ PRP displayed by 10 strains.