الفهرس 
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Abstract
Despite the presence of variety of modern antibacterial drugs, bacterial infections are remaining a major threatening problem due to the enormous increase in Multi- Drug resistant bacteria. Nanoparticles have become extensively used as an applicable and safe alternative to antibiotics. We aimed to explore the inhibitory effect of Silver nanoparticles on Extended Spectrum Beta Lactamase (ESBL) producing E. coli and Klebsiella spp. invitro as well as their effect on the expression of antibiotic resistance genes. Different samples (Wound swabs, Fecal swabs and urine samples) were collected from dogs and cats. Both phenotypic and molecular identification, antibiotic susceptibility testing, Double Disk Synergy test were carried out for identification of ESBL producing E. coli and Klebsiella spp. Silver nanoparticles were tested for their invitro antibacterial potential and the Minimum bactericidal concentration (MIC) and Minimum bactericidal concentration (MBC) were evaluated. Moreover, the effect of silver nanoparticles on the expression of antibiotic resistance genes (blaTEM, blaSHV and blaCTX) was assessed as well as their effect on the structural integrity of the bacterial cells using Scanning Electron Microscope (SEM). Results revealed that 23 isolates (19.16%) (E. coli =17, Klebsiella spp.=6) were confirmed as ESBL producing. Silver nanoparticles showed a promising antibacterial effect where MIC of AgNPs for ESBL producing E. coli was 0.31 mg/ml, and 0.62 mg/ml for ESBL-producing Klebsiella spp., while MBC of ESBL-producing E. coli and Klebsiella spp. was 0.15 mg/ml and 0.3 mg/ml respectively. Consequently, the expression of antibiotic resistance genes was down regulated in both bacteria species and there was a noticeable toxic effect of AgNPs on E. coli and Klebsiella spp. cells which was investigated using SEM. It can be concluded that silver nanoparticles have a promising antibacterial activity and could be considered an applicable alternative for control of ESBL producing bacteria