**3.4 Genomic islands (GIs)**

Genomic islands (GIs) comprise of more than 10 kb DNA in length, exchanged frequently among bacterial isolates. GIs encode proteins for transfer, restriction/ alteration, or other proprieties and recombination, for example, gene groups for metabolic adaptation, virulence, and or bacterial resistance [67]. GIs that are involved in the expression of virulence factors is called pathogenicity islands (PAIs) [68]. It encodes VFs comprising of adhesins, invasions, capsule formation, toxins, uptake system of iron, distinct secretion systems. Their GC contents vary in comparison to the genome. Their integration site is situated on the tRNA genes and repeated sequences, which is comprising at least one MGEs containing plasmids remnants, integrons, insertion sequences, and related gene cassettes. For the integration of foreign DNA, tRNA-encoding genes are considered as the hot spot. By site-specific recombination, some PAIs can be edited from bacterial chromosome [69]. Primarily, PAIs have been described in the uropathogenic *E. coli* genome and later cases were reported in other pathogenic bacteria [70]. Currently, PAIs are spread between plants and animals associated with bacterial pathogens, have a great influence on the rapid evolution of virulent and resistant strains. In *E. coli*, the locus of enterocyte effacement (LEE) is best example of PAIs, and its size is about 35 kb. It has a main role in bacterial adherence to the epithelial cells of the intestine [71]. High-pathogenicity Island (HPI) was found in enteroaggregative, enteropathogenic, entero-invasive, and enterotoxigenic *E. coli* [72].
