الفهرس 
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Abstract
Shrimp covers a major part of the aquaculture industry and as demanding seafood; it comprises 15-20% of total fishery products worldwide. Seafood act as a vehicle for all important species of foodborne pathogens to human and cause serious human disturbances which sometimes leads to death. Therefore, this study aimed to demonstrate the drug-resistant pathogenic bacteria that affected the cultured shrimp, and considered as important human disease factors associated with seafood consumption and try to find an antibacterial alternative to face the worsening problem of antibiotic resistance.
A total number of 250 cultured shrimp samples were collected from different private shrimp farms in Alexandria Governorate, Borg Al-Arab city and Kafr El-Sheikh during the period from May (2018) till June (2019).
A total of 62 bacterial isolates were recovered from cultured shrimp samples including 53 isolates (85.5%) of Gram-negative bacteria and 9 isolates (14.5%) of Gram-positive bacteria. After the biochemical characterization, using conventional phenotypic characteristics and automated VITEK 2 system and then confirmation by MALDI-TOF MS, the results identified the isolated bacteria as following, the most common isolated bacteria among shrimp samples were V. alginolyticus constituting 15 isolates (24.2%) followed by P. aeruginosa and A. hydrophila, 13 isolates (21%) and 12 isolates (19.4%); respectively. Then Shewanella algae and Shewanella putrefaciens showed moderate to low prevalence constituting 9 isolates (14.5 %) and 4 isolates (6.4%); respectively. On the other hand, Staphylococcus aureus was the only isolated Gram-positive bacterial species and it was 9 isolates with a percentage of (14.5%).
On 5% sheep blood agar medium most of the isolated bacteria gave beta hemolysis and some of them gave alpha hemolysis, but few isolates didn’t show any hemolytic activity.
Antibiogram sensitivity test and multi-drug resistant bacteria detection showed that some of the tested isolates had a strong resistance to the antibiotic as; P. aeruginosa (2 isolates), Staph. aureus (2 isolates), S. algae (one isolate) S. putrefaciens (one isolate) and A. hydrophila (one isolate) and some of them showed moderate resistance to the antibiotic as; S. algae (2 isolates), S. putrefaciens (2 isolates), V. alginolyticus (2 isolates), A. hydrophila (one isolate), P. aeruginosa (one isolate) and Staph. aureus (one isolate). I tried to find out the mechanism of this resistance by uncovering the biofilm formation ability of these MDR bacteria. Biofilm formation was observed in all tested isolates but with different degrees. I found that; Out of 24 isolates, 16 isolates (66.6%) were strong to moderate biofilm-forming 7 isolates and 9 isolates; respectively. But the other 8 isolates (33.3%) had weak biofilm formation. Also, I found a strong relationship between MDR ability and biofilm formation ability.
I tried to apply alternative anti-bacterial components to counteract antibiotic resistance and biofilm formation. The antibacterial effects of pure chitosan against sensitive, intermediate and strong antibiotic-resistant isolated bacteria were tested, also, I incorporated silver NPS and zinc oxide NPs into chitosan polymer and test the antibacterial effects of them. Blank chitosan beads at 2% concentration showed acceptable inhibitory effects against all the drug-sensitive bacteria and moderately drug-resistant bacteria, but they didn’t show any antibacterial effect against the MDR, strong biofilm-forming bacteria. Also, ZnO NPs- CS beads in their low concentration (10- 20 ppm) had a very good antibacterial effect on sensitive and moderately resistant isolates but the antibacterial effect on MDR isolates is low, at high concentrations (50-100 ppm) ZnO NPs- CS beads showed a significant antibacterial effect on all isolated bacteria. Referring to Ag NPs- CS beads, they showed magnificent antibacterial effects on all isolated bacteria at all their tested concentrations.
from the aforementioned results, I found that many antibiotic resistant bacteria were present and were isolated from cultured shrimp. Bacterial diseases affecting shrimp cause significant economic losses in farms, and the presence of antibiotic resistance reduce the efficacy of antibiotic treatment for their infections. Also, there is a strong relationship between MDR ability and biofilm formation ability, as well as increasing antibiotic use and its concentrations to overcome antibiotic resistance and biofilm, makes the condition get worse. So trying to find alternatives to antibiotics has become an urgent necessity. After my experiments on the antibacterial activity of chitosan hydrogel beads and on ZnO NPs and Ag NPs incorporated into chitosan beads, I recommend chitosan as a natural and prophylactic antibacterial alternative to reduce the rate of antibiotic use, I also recommend ZnO NPs (50 ppm) and Ag NPs (10 ppm) which were incorporated into chitosan beads in case of recurrent diseases precedence due to MDR bacteria.