**3.2 Plasmids**

Plasmids are circular, self-replicating extra-chromosomal DNA elements. Besides the genetic information required for the autonomous multiplications, it has extra genetic information needed for suppression of antibiotic actions. It also encodes genes for virulency, involves in the removal of hazardous material, or is required for regulation of other metabolic functions [41]. Plasmids are commonly used cloning vectors and are categorized into different incompatibility (Inc) groups. Inc. groups are designated on basis of the incapability of two plasmids to co-exist together [42]. Same Inc. group of the Inc. plasmids have the same type of replication region and thus have incompatible replication, it cannot co-exist. Plasmids belonging to the IncX family encode different resistance genes, mostly circulated among *Enterobacteriaceae* [43]. For example, an IncX plasmid, which is responsible for encoding *bla-SHV-12* resistant gene was reported in *E. coli*. The *bla-IMP-2* gene, encoding an imipenem-hydrolyzing β-lactamase, is carried by pRJ-18, an IncFIB plasmid [44]. In Europe, ESBL-encoding plasmids belonging to the Inc. F, A/C, N, H12, 11, and K type have been reported. Another important ESBL genes, *CTX-M-1*, is reported in Inc1 or IncN plasmids. For example, *CTX-M-1* ß-lactamase was derived from an animal source disseminated through Inc-1 ST3 plasmid [45]. Similarly, F plasmid, has been reported in *Enterobacteriaceae* [34]. F-like plasmids are also reported in nonpathogenic as well as in pathogenic *E. coli* strains. The whole genome sequence of *E. coli* ST-131 showed the *CTX-M* resistance gene dissemination

### *Antimicrobial Resistance in* Escherichia coli *DOI: http://dx.doi.org/10.5772/intechopen.101583*

and mainly conjugative F plasmid was involved [46]. *Mcr-1* gene conferring resistance to colistin is also spread with help of F Plasmid. Furthermore, this *Mcr-1* gene was found to be carried by 13 various plasmid incompatibility groups and these are Incl-2, Inc-X4, and Inc-HI2 [47]. Some studies have reported transposons involvement in the dissemination of *Mcr-1*. Other *Mcr* genes comprising *Mcr-2*, *Mcr-3*, *Mcr-4*, *Mcr-5* have been seen in a plasmid [48]. Recently in Denmark, a strain identified as *E. coli* ST-410 has been reported harboring resistance toward fluoroquinolones, 3rd-generation, carbapenems, and cephalosporins. Other variants like Inc-X3 plasmid carrying *blaOXA-181* resistant gene and Inc-FII plasmid carrying *blaNDM-5* resistant gene [49]. Plasmids can transfer between bacteria through the conjugation process, that is transfer of genetic material between recipient and donor cell. Conjugative plasmids can transmit transposons or integrons, and such genetic information can be further disseminated horizontally by the conjugation process [50, 51]. For example, *E. coli* is isolated from pig have a conjugative plasmid with *cfr* gene, which conferred resistance to lincosamides, phenicol, pleuromutilin, oxazolidinones, streptogramin [52]. Another important plasmid, a ColV (pCERC3) from a commensal *E. coli* ST95 strains have been reported and revealed resistance against sulfonamide encoded by *sul-3* associated with a class 1 integrons [53] The pE80 plasmid from a foodborne *E. coli* strain encodes multiple resistance determinants *oqx-AB*, *fos-A3*, *blaCTX-M-55*, and *blaTEM-1* and therefore confers resistance to streptomycin, tetracycline, kanamycin and olaquindox/quinolone [54]. In addition to antibiotic resistance genes, plasmids are involved in the transfer of virulence-associated genes. In Germany, outbreaks of enteroaggregative *E. coli* (EAEC)-enterohemorrhagic *E. coli* (EHEC)-O104:H4 strain was reported harboring have three-different plasmids: p-AA (7.4 kb), p-ESBL (89 kb), and p-G (1.5 kb) [55, 56]. p-AA plasmid harboring information for different virulence factors like fimbriae for adherence, diffusion in surface protein, protease, and the virulence regulator A and R [57]. Moreover, an EHEC O104:H7 strain, being isolated from animal's waste, possessed Inc-B/O/K/Z and IncFIB plasmids. It encodes genes responsible for the expression of main virulence genes, including, entero hemolysin and auto transporter [58]. Another important *E. coli* serotype is the O103 serotype, the 2nd most common serogroup main causative agent of human foodborne disease. It has pO157 plasmid encoding different virulence factors including entero hemolysin and type II secretion protein [59].
