Among children in developing countries, despite the existence of effective treatment regimens, shigellosis continues to be the main public-health concern with an annual estimate of 163.2 million new cases and 1.1 million deaths . In Iran, there are no specific guidelines to employ antibiotic therapy, and in most cases, physicians prescribe antibiotics without any stool cultures . Therefore, performing epidemiological studies and gathering information regarding phenotypic and molecular mechanisms of antibiotic resistance to control infections and the development of local treatment guidelines are necessary.
In the present study, we surveyed the prevalence and molecular characterization of Shigella species harboring ESBL genes in patients with diarrhea from the northwest of Iran. This study showed a prevalence of 3.8% (49/1280) of shigellosis that was lower than the rates found in previously published studies conducted by Farajzadeh Sheikh et al. , Soltan Dallal et al. , Abbasi et al. , and Ranjbar et al. . All of these studies were performed in Iran, and it was reported that the prevalence of Shigella spp. was 7.2%, 7.9%, 8.2%, and 9.4%, respectively.
However, the results of our study are in agreement with those of previous studies from Dhital et al. in Nepal , Aggarwal et al. in India , Jomehzadeh et al. from Iran , and Bakhshi et al. in Iran . These studies found that the frequency of Shigella species in the patients with diarrhea was 2.1%, 1.9%, 5.9%, and 1.3%, respectively.
The detected variation in the prevalence of Shigella spp. could be due to the difference in the sample collection seasons, geography, specimen size, diversity of specimen type, study population, and applied detecting methods . The result of the present study indicates that shigellosis is the main public health problem in the northwest of Iran. Therefore, it is critical to take several measures such as promoting awareness about the safety of water and food and improving health conditions.
Our finding revealed that S. sonnei had the highest frequency among Shigella species. In contrast, S. boydii was not detected in fecal samples. This result was in line with previous reports from Farajzadeh Sheikh et al. in Iran , Zhang et al. in China , Abbasi et al. in Iran , Bakhshi et al. in Iran , Ranjbar et al. in Iran , and Tajbakhsh et al. in Iran .
According to previous studies, it seems that in the southern regions of Iran, in the cities such as Shiraz, Ahvaz, and Kerman, S. flexneri is the dominant strain, while in northern regions of Iran, S. sonnei strain is prevalent [27, 32,33,34,35,36]. In contrast with our results, several different studies in various countries including Nepal , Ethiopia , Bulgaria , India , Iran , and Peru  have reported the S. flexneri as the leading cause of shigellosis.
In general, Shigella species can cause several diseases such as diarrheal in both developed and developing countries . It has been reported that S. flexneri is the predominant serogroup in developing countries and is responsible for 44.5–80% of all Shigella infections such as diarrhea, while S. sonnei is a major cause of diarrheal in industrialized and developed countries .
In the present study, 10.2% were ESBL-producing strains by the double-disk method. Most ESBL-producing species were detected in S. sonnei which was following studies reported by Ranjbar . In contrast, in a study performed by Aminshahidi et al., 54.5% of S. flexneri strain was ESBL positive .
Several different studies have surveyed the frequency of ESBL-producing Shigella strains. Results of a study performed by Li et al. from China revealed that ESBLs were produced by 18.1% of Shigella isolates, and most of the ESBL-producing species belonged to the S. flexneri isolates (19.5 %) . In another study, Zhang et al. in China revealed that 10 Shigella isolates produced ESBLs including 8 S. sonnei isolates and 2 S. flexneri isolates .
Moreover, the prevalence of ESBL-producing Shigella isolates in the studies carried out by Aminshahidi et al. in Iran , Tau et al. in South Africa , Zamanlou et al. in Iran , Farajzadeh Sheikh et al. in Iran , Abbasi et al. in Iran , and Dhital et al. in Nepal  was > 50%, 0.3%, 54.2%, 43%, 52.6%, and 6.7%, respectively. In most studies, the highest prevalence of S. sonnei strains was ESBL producing.
The detected variation in the frequency of ESBL-producing Shigella species could be due to the differences in the applied detecting methods, variations in geographical location, and differences in sample size, sample type, and study participants . Mobile genetic elements such as integrons, plasmids, and transposons can transmit the drug resistance genes among different bacteria and are responsible for antibiotic resistance in Shigella spp. . In general, the main aim of antibiotic prescription in children with bloody and chronic diarrhea is the reduction in the duration of the disease. Because most Shigella infections are contagious and severe, appropriate antibiotic prescription and suitable treatment are essential .
Recently, the emergence of multidrug-resistant (MDR) Shigella species is increasing. The treatment of MDR strains is very difficult and is considered an alarming public health concern, worldwide .
In the present study, the blaCTX-M with a frequency of 65.3% was the most predominant ESBL encoding gene followed by the blaTEM gene with a frequency of 61.2% which was in line with previous reports by Andres et al. from Argentina , Vasilev et al. from Israel , Abbasi et al. from Iran , Farajzadeh Sheikh et al. from Iran , and Liu et al. from China .
β-Lactamases are the enzymes encoded by several genes and were considered the main mechanisms of resistance to β-lactam antibiotics such as cephalosporins (especially third-generation cephalosporins) among gram-negative bacteria. The high prevalence of ESBL-producing and β-lactamase genes leads to complicated treatment [33, 45].
Identification of ESBL among Shigella strains is an undeniable concern because it reduces antibiotic treatment options, and the spread of mobile resistance determinants will be a great threat to the treatment of invasive diseases in the future.
The present study has several limitations including the following:  the antibiotic susceptibility of Shigella species against different classes of antibiotics such as carbapenems was not determined,  the frequency of virulence genes was not determined,  the prevalence of other antibiotic resistance encoding genes such as genes encoding AmpC β-lactamase was not examined, and  the patients’ demographic data such as age, underlying disease, duration of hospitalization, and the outcome of treatment are not accessible; therefore, we were unable to perform deep analyses.