الفهرس 
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Abstract
The study was carried out during the period extended from September 2020 to February 2021 at Behera Governorate to investigate the role of nanosilver in inhibition of some Gram negative bacteria.
To achieve the aim of the work, the research plan was divided into two sections; the first was isolation and identification of Salmonellae and Shiga toxin producing E. coli from different food sources by bacteriological, serological and molecular methods. The second section was an experiment designed to evaluate the addition effect of different concentration of nanosilver as preservative to control the contamination of minced meat with pathogenic bacteria obtained during the current work.
A total of 125 different food samples were collected from local markets and supermarkets including; minced meat, Burger, Giblets (Liver and Gizzard of chicken), Chicken nuggets and Kariesh cheese (25 of each). Each sample (250 g) was kept in a separate plastic bag and transferred directly with a minimum of delay to the laboratory of Animal Health Research Institute Behera Branch in an insulating refrigerated container under complete aseptic condition.
The obtained results clarified that the rate of isolation of Salmonellae from different food sources was 4, 0.8, 3.2, 0.8 and 3.2 % in the examined samples of minced meat, burger, giblets, nuggets and kariesh cheese, respectively with a total of 15 isolates were recovered from the examined samples.
Serological identification of 15 Salmonellae isolates recovered from food sources cleared the presence of S. Enteritidis (9 isolates), S. Kentucky (2 isolates) and S. Typhimurium (4 isolates).
On the other side, the rate of isolation of E. coli was 3.2, 1.6, 4.8, 1.6 and 5.6 % in the examined samples of minced meat, burger, giblets, nuggets and kariesh cheese, respectively with a total of 21 isolates were recovered from the examined samples.
Multiplex PCR was employed to identify STEC from the recovered 21 isolates of E. coli. It was found that only two isolates were found to be positive for presence of stx1 and stx2 genes represented to STEC.
Antibiogram pattern was performed for selected stains of S. Enteritidis and STEC against 10 antibiotics. The recorded results clarified that S. Enteritidis were highly sensitive to Levofloxacin, Ceftriaxone and Ampicillin while it was moderately sensitive to Imipenem, Amikacin, Norfloxacin and Amoxicillin and it was less sensitive to Gentamycin. On contrary, it was resistant to Vancomycin, Spiramycin and intermediate to amoxicillin.
On the other hand, STEC was found to be highly sensitive to Levofloxacin, Imipenem, Ceftriaxone and Norfloxacin while it was moderately sensitive to Ampicillin and it was less sensitive to Amikacin, Spiramycin Gentamycin and Amoxicillin. On contrary, it was resistant to Vancomycin only and intermediate to amikacin.
The effect of addition of different concentration of nanosilver on S. Enteritidis and STEC count in artificially inoculated minced meat was investigated.
At the 3rd day after inoculation of minced meat with S. Enteritidis, it was recorded that the lowest bacterial count occurred by occurred by using nanosilver 20 % (3×103 cfu/g) followed by using nanosilver 15 % concentration (4×103 cfu/g) and lastly by using nanosilver 10 % concentration (4×104 cfu/g). Also, at 5th day after inoculation, it was recorded that the lowest bacterial count occurred by using nanosilver 20 % (9×106 cfu/g) followed by using nanosilver 15 % concentration (9×105 cfu/g) and lastly by using nanosilver 10 % (6×106 cfu/g).
Concerning STEC, it was recorded that the lowest bacterial count occurred at 3rd day by using nanosilver 20 % (3×10 cfu/g) followed by using nanosilver 15 % concentration (5×102 cfu/g) and lastly by using nanosilver 10 % concentration (6×102 cfu/g). Also, at 5th day after inoculation, it was recorded that the lowest bacterial count occurred by using nanosilver 20 % (9×103 cfu/g) followed by using nanosilver 15 % concentration (9×104 cfu/g) and lastly by using nanosilver 10 % (9×103 cfu/g) so it was concluded that the best day of preservation was the third and the most effective concentration of nanosilver was 20 %.
Finally, it was found that 50 PPM nanosilver was the lowest concentration that inhibited the growth of S. Enteritidis while 12.5 PPM was the lowest concentration that inhibited the growth of STEC by agar plate diffusion method.