الفهرس 
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Abstract
Salmonellosis is a prevalent foodborne infection with significant effects on human health and significant financial losses for the poultry industry. The aims of this study were isolation and identification of Salmonella species (Spp.) in chicken flocks of some Egyptian governorates by conventional culture methods, demonstration of the antimicrobial susceptibility of the isolated Salmonellae, and detection of some virulence and antibiotic resistance genes using molecular techniques. In addition, the ameliorating effects of thyme oil (TO), chitosan nanoparticles (CS-NPs), and TO-loaded-CS-NPs on controlling Salmonella typhimurium (S. Typhimurium) infection in broiler chickens were investigated in comparison to the antibiotic ciprofloxacin (Cip).238 tissue samples (81 yolk sac, 79 liver, 20 spleen, 18 heart, 6 cecum, 3 gallbladders, 3 ovary and 28 organs from dead in shell embryos) from 52 flocks of broiler, layer and breeder chickens were processed. With a total prevalence of 56.3% (134/238). The isolation of Salmonella from liver and yolk sac (62%, 58%) respectively, was the highest among tissue samples processed. A total of 32/238 (13.44%) isolates of Salmonellae belongs to 11 serotypes according to the serotyping results obtained. The isolated Salmonellae were serotyped as S. Enteritidis (21.9%), S. Kentucky (15.6%), S. Molade (12.5%) S. Typhimurium (12.5%), S. Takoradi (9.4%), S. Infantis, S. Tsevie, S. Wingrove (6.3% for each) , S. Bargny, S. Shangani, and S. Papuana (3.1% for each ). The majority of the Salmonella strains (31/32; 96.9%) were sensitive to meropenem, while all of them (32/32; 100%) were resistant to streptomycin. The polymerase chain reaction (PCR) amplification of Salmonella isolates’ 16S rRNA gene produced a distinct Salmonella product of about 550 base pair. Multiplex PCR indicated the presence of virulence-associated genes invA (100%), stn (65.6%), and sopB (40.6%), as well as antibiotic resistance genes aadA1 (100%), blaTEM (59.4%), aadB (18.75%), and sul1 (28.1%). Moreover, Two hundred and forty broiler chicks were utilized for experimental infection and divided into 6 equal groups. group (1) was given TO (1ml/ 4L of the drinking water). However, group (2) and group (3) were given CS-NPs and TO-loaded-CS-NPs, respectively in a dose of 2ml/ chick by oral gavage. group (4) was given Cip (0.5 gm/L of the drinking water). Treatments were started at 20th day of age and continued for 5 successive days. Each chicken in groups 1-5 were orally infected with 1ml of 1 × 109 CFU/ml S. Typhimurium at 14-days-old. Chickens in group (6) were kept as blank control negative without treatment or infection. All groups were kept for 4 weeks post-infection for recording clinical signs, performance parameters, shedding rate of S. Typhimurium, and the histopathological parameters of different organs. The results indicated that the challenged untreated group had the lowest body weight, the highest bacterial count, and lesion score in all investigated organs. Treatment with CS-NPs improves performance parameters and the histological image of infected tissues more efficiently than TO. While the best effects were obtained by using TO-loaded-CS-NPs. In conclusion, virulent paratyphoid Salmonella spp. are circulating in Egyptian chicken flocks causing economic loses. Additionally, they became resistant to the most commonly used field antibiotics. As a result, we strongly advise employing thyme oil-chitosan-NPs as an alternative antibacterial agent in treating Salmonella infections.