Contaminated poultry and eggs with non-typhoid Salmonella, especially S. Enteritidis, are the major sources of food-borne diseases in humans [6, 8]. Therefore, continuous monitoring of contamination in these animal-origin foods with salmonellae is necessary. Since the invA gene, which encodes the inner membrane protein, is only present and conserved in the genus Salmonella , we attempted to amplify this gold international marker to make a definite and rapid diagnosis of salmonellae in the samples . Using this method, 8.7% of poultry samples and 6.3% of poultry egg samples, sent to the administrative centers of veterinary microbiology laboratories in six provinces of Iran, were found to be contaminated with Salmonella; this prevalence rate can be of major public health and economic importance for the country.
The rate of contamination of poultry samples with Salmonella was 3–66% in various epidemiological studies from different countries . In this study, the rate of contamination of poultry egg samples with Salmonella was higher than the rates reported in some other studies, such as 0% in Cairo, Egypt , 0.3% in Dhaka, Bangladesh, 2.9% in Eastern Ethiopia, 3% in Belgium , 3.3% in North India , 3.8% in Tehran, Iran , and 5.40% in Guangdong, China . However, the prevalence of Salmonella contamination of poultry eggs in the present study was lower than the rates reported in South India (7.7%), Nigeria (24.17%) , and Spain (34%) . These differences in the rate of Salmonella contamination in poultry samples and egg samples can be related to differences in the hygienic control and management programs of different countries.
Considering the presence of discriminative Salmonella difference fragments (sdf) in chromosomes of S. enterica serovars , 26.7% of Salmonella isolates from poultry samples and 20.0% of Salmonella isolates from poultry egg samples were confirmed as S. Enteritidis. The results of other epidemiological studies conducted in 37 countries also revealed the importance of S. Enteritidis as the most prevalent serovar in contaminated poultry. For example, the prevalence of S. Enteritidis contamination in poultry samples was 19.2–49% in Africa and 5–93.7% in Asia and Europe .
Salmonellae have various virulence factors that contribute to their pathogenicity and increase the risk of serious infections in humans. The prevalence of spvC gene (50.8%) in the studied S. Enteritidis isolates was lower than that of chromosomally encoded virulence genes. This result was consistent with the findings of a study by Gritli et al., which reported a prevalence of 45.8% for spvC gene in S. Enteritidis isolates from chicken consumed in Tunisian military cantines . However, this result contradicted the findings of a study that reported the higher prevalence of spvC gene (80%) in S. Enteritidis isolates  and the study indicating the lower prevalence of spvC gene (25.9%) in S. Enteritidis isolates .
In the present study, the invA and msgA genes were detected in all S. Enteritidis isolates and showed the highest prevalence among eight studied virulence genes. These results were consistent with the findings of other studies, which reported a prevalence of 100% for invA gene [4, 18,19,20,21,22] and msgA gene [19,20,21] in Salmonella isolates. The cdtB gene had the lowest prevalence as compared to other studied virulence genes, which is consistent with previous studies, reporting the low prevalence of this toxin-encoding gene [19,20,21]. Inequality of the virulence genes prevalence in Salmonella isolates of various studies can be due to genetic diversity and differences in pathogenicity of various Salmonella strains in different geographical regions.
The World Health Organization (WHO) surveillance programs indicate the S. Enteritidis as a principal foodborne pathogen in many countries . In the past decades, the prevalence of resistant and MDR S. Enteritidis has increased globally, and poultry and poultry products are considered as a source of MDR S. Enteritidis in humans. Correspondingly, in our study, 28.8% of S. Enteritidis isolates from poultry samples and 18.2% of S. Enteritidis isolates from poultry egg samples were found to be MDR. This problem could limit the therapeutic options for infections, caused by antibiotic-resistant S. Enteritidis strains [6, 20, 24].
In the present study, the highest prevalence of antibiotic resistance (87.3%) in S. Enteritidis isolates was found against nalidixic acid. Also, the most common resistance profile (60.3%) in all tested S. Enteritidis isolates was resistance to nalidixic acid alone. The high prevalence of resistance to nalidixic acid was also reported in studies by Khaltabadi Farahani et al. (94.1%), En-Nassiri et al. (82%), and Ziyate et al. (61%) [8, 18, 22]. Conversely, in a study by Mezal et al., all S. Enteritidis isolates from poultry were sensitive to nalidixic acid. Besides, in a study by Han et al., resistance to nalidixic acid was only detected in 7.4% of S. Enteritidis isolates, and in a study by Gritli et al., resistance to nalidixic acid was seen in 16.66% of Salmonella isolates [5, 19, 20]. Since nalidixic acid is one of the recommended antibiotics for the treatment of Salmonella infections in humans, the high rates of nalidixic acid-resistant S. Enteritidis strains in poultry and poultry products are of great public health importance [25, 26]. The significance of this finding is related to the potential risk of transmission of these resistant strains to humans via consumption of poultry products, including poultry eggs .
On the other hand, fortunately in the present study, resistance to ceftazidime (11.1%), followed by cephalothin (19.0%), showed the lowest prevalence. Although these prevalence rates are not very low, the results are somewhat promising, as β-lactam antibiotics are suggested as the last option for the treatment of severe salmonellosis . The low prevalence of resistance to β-lactam antibiotics (0–7%) has been also found in S. Enteritidis isolates in previous studies [5, 8, 27]. Conversely, Ghazaey and Mirmomeni reported that 90% of S. Enteritidis isolates from poultry samples were resistant to cephalothin . Disparity in the prevalence and patterns of antibiotic resistance in Salmonella isolates of various studies may be due to difference in the amount and types of the prescribed antibiotics for prophylactic and therapeutic purposes and therefore different selection pressure in Salmonella strains of various geographical regions.