*3.2.2 Efflux pumping*

Active efflux pumping is a mechanism by which a substance that is not needed in the cell is pumped out to prevent the damages that the substance or chemical may bring to the cell, they are used in moving a variety of different toxic compounds out of the cell and in bacteria they use it to pump out antibiotics. It is a major fundamental characteristic in antimicrobial resistance of gram-negative bacteria including *E. coli* [19]. The efflux pump family in enterobacteriaceae called the resistance nodulation division (RND), is the most significant factor behind multidrug resistance and one of the most characterized RND systems in enterobacteriaceae is the AcrAB-TolC efflux systems. Expression of the AcrAB and TolC genes are regulated by the MarA protein in *E. coli* [20].

In the *E. coli* operon, the expression of AcrAB is controlled or mediated by AcrR which is a repressor located at the upstream part of the acrAB operon where the expression can be transcribed or repressed [20, 21]. Studies have shown that mutations are taking place in the acrR which means there is no repression of expression for AcrAB which means the more the expression the higher the rate of pumping out of the drugs [22]. They also have a specific modification of the porin membrane channel proteins which have a specific mediated width that allows the in and outflow of the substance of the cell. The porins for example against vancomycin are modified in such a way that the vancomycin molecules cannot pass through into the cell [23]. The major porins in *E. coli* such as OmpF and OmpC protein were believed to be the drug binding sites, however recent studies show that there have been changes in their structural arrangements making the drugs unable to bind to the proteins. The presence of mutant porins can even cause resistance to carbapenems which are believed to be the most efficient and reliable drugs against *E. coli* and other bacterial species [23, 24].

Most enterobacteriaceae gram-negative bacteria have developed specialized genes which aid in resisting carbapernemdrugs which are called the ndm genes which are often found branch host range conjugative plasmids which work in conjunction with other resistance genes. The ndm genes have a transposon mechanism which means they are found on plasmids as well as the host chromosomes and can move between the two at a much higher frequency thereby enabling the resistance build-up in many cells in a much-minimized time range via transformation mechanisms [25].
