**1. Introduction**

Antimicrobial resistance (AMR) has become a public health problem, reaching alarming levels in many parts of the world [1,2]. In recent years, resistances in the Enterobacteriaceae family have increased significantly because of the extensive use of antibiotics in human treatment, veterinary, and agriculture, leading to the selection and global spread of resistant clones [3,4]. In particular, the dissemination of extended-spectrum β-lactamases (ESBLs) have increased dramatically in the recent years, becoming a serious global threat [5,6].

Several genetic mechanisms have been involved in the acquisition and dispersion of antimicrobial resistances. The commonly called "mobilome" [7,8] is composed of a variety of mobile genetic elements (MGEs), including plasmids, transposons (Tn), insertion sequences (IS), integrons (*intI*), and introns. Conjugation, transformation, and transduction are the main mechanisms for the horizontal transfer of MGEs [9,10].

Integrons are DNA elements capable of capturing gene cassettes (including antimicrobial resistance genes) and disseminating them using an MGE [11]. Integrons are usually composed of two conserved segments (termed 5 -conserved region (5 -CS) and 3 -conserved region (3 -CS)) separated by a variable region which contains the gene cassettes. The 5 -CS end includes (i) the *int* gene coding for an integrase, that belongs to a distinct family of the tyrosine-recombinase; (ii) a primary recombination site (*attI*); and (iii) a promoter (Pc), which ensures the transcription of the cassette genes. On the other hand, the 3 -CS region is formed by (i) a truncated gene of resistance to quaternary ammonium compounds (*qacE*Δ*1*); (ii) a sulfonamide resistance gene (*sul1*); and (iii) an unknown function sequence (orf5) [12]. Class 1 (*intI1*) and class 2 (*intI2*) integrons are the most commonly involved in antibiotic resistances [13–17], while limited work has shown the presence of class 3 (*intI3*) in Enterobacteriaceae. The gene *intI3* was reported for the first time in a carbapenem-resistant *Serratia marcescens* strain [18] and has been also detected in *Klebsiella pneumoniae* isolates [19] and other Enterobacteriaceae [20]. In addition, *bla* ESBL genes have been associated with insertion sequences. These IS are the smallest transposable elements (<2.5 kb), and are classified into families according to different characteristics, with transposases (enzymes that catalyze the IS movement) being the main classification system used [21,22]. It has been well documented that IS*26*, IS*Ecp1*, IS*CR1*, and IS*903*, in association with class 1 integrons, are the most involved elements in the antimicrobial resistance to β-lactamics [23–27].

Therefore, the investigation of these elements might be critical, in order to predict the potential spread of ESBL-producing strains. In this context, the aim of this study was to evaluate the presence of different types of integrons (*intI1*, *intI2*, and *intI3*) and insertion sequences (IS*Ecp1*, IS*26*, IS*CR1*, and IS*903*) in a collection of 150 ESBL-producing *E. coli* isolated from different sources in Navarra, Spain.
