Abstract
The current study was carried out to investigate the role of nanosilver in inhibition of growth of some Gram negative bacteria isolated from foods. To achieve this aim, the research plan was divided into two sections; the first was isolation of Salmonellae and Shiga toxin producing E. coli (STEC) from different foods. The second section was experimental synthesize of colloidal silver solution by reducing silver nitrate using sodium borohydroxide and starch as a stabilizer and characterize its optical properties by UV- Vis spectroscopy and evaluation of its addition effect by different concentrations of nanosilver to control the contamination of minced meat with isolated bacteria. A total of 125 different food samples were collected from local markets, street vendors and supermarkets including; minced meat, Burger, Giblets, Chicken nuggets and Kariesh cheese (25 of each). The rate of isolation of Salmonellae from different foods was 4, 0.8, 3.2, 0.8 and 3.2 % in minced meat, burger, giblets, nuggets and kariesh cheese, respectively. Serological investigation of 15 Salmonellae isolates cleared the presence of S. Enteritidis (n= 9), S. Kentucky (n=2) and S. Typhimurium (n =4). The rate of isolation of E. coli was 3.2, 1.6, 4.8, 1.6 and 5.6 % in the examined samples, respectively. A multiplex PCR showed that only two E. coli isolates were positive for presence of stx1 and stx2 genes characteristic to STEC. Antibiogram pattern was performed for selected strains of S. Enteritidis and STEC against 10 antibiotics was performed. The effect of addition of different concentration of nanosilver on S. Enteritidis and STEC count in artificially inoculated minced meat showed that the best results of inhibition of bacterial growth of both S. Enteritidis and STEC occurred during the third day post inoculation and the most effective concentration of nanosilver was 20% compared to 10 and 15%. Finally by using well diffusion and micro-dilution tests to evaluate the bactericidal activity of nanoparticles, 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.

Key words: Nano silver, Salmonella spp., E. coli, Food