*2.1.1 Penicillin and cephalosporin* β*-lactam mode of action*

The gram-negative bacterial cell wall is made up of a complex structure which is made of a thinner peptidoglycan layer with a structure of crosslinking peptidoglycan precursors made by adjoining N-acetyl glucosamine and the N-acetyl muramic acid proteins which are then cross-linked to form several layers of peptidoglycan catalyzed by Transpeptidase and de-alanyl carboxypeptidase. The penicillinbinding proteins form the D-ala D-ala cross-linkages of the peptidoglycan wall in cell wall synthesis. The β-lactam ring in penicillin and cephalosporin will bind to the enzymes (Trans-peptidase and D-ala- carboxyl peptidase) thereby preventing bacterial cell synthesis leading to bacterial cell wall damage that will cause bursting after being subjected to the low osmotic pressure of the surrounding environment. The antibiotic penicillin-binding complex will stimulate the release of autolytic compounds that are capable of digesting the cell wall [7].

#### *2.1.2 Resistance mechanisms*

Quite several researches have outlined that multidrug-resistant species which include *E. coli* have been a long-term migraine problem with the drastic increase in resistance as of date, with special attention on the development of the extendedspectrum β-lactamase (ESBL). The genes which are encoded by the ESBLs are located in the plasmid of the bacterium cell and most cases, they are transferred through horizontal transfer to other cells. *E. coli* has acquired resistance to β-lactam antibiotics through the production of the β-lactamase enzyme which is used to break down the β-lactam ring of most penicillin derivatives [8]. β-lactamase enzymes are the biggest and greatest reason why penicillin drugs are failing to eradicate infections behind *E. coli* bacteria.

With this problem being pointed out by scientists, new-generation drugs of the cephalosporin class were invented which were believed to defy the stability of many bacterial β-lactamases on the drug, thereby allowing the drug to temper with the bacterial structure and eliminate them. With persistent use and exposure to the third-generation drugs which include; cedox, cefixime, cefotsxime and avycaz which have been successfully superior to older penicillin drugs in terms of

#### *Mechanisms of Antimicrobial Resistance of* E. coli *DOI: http://dx.doi.org/10.5772/intechopen.101671*

effectiveness on treatment assays. In the early 1980s, in response to the increasing prevalence and spread of β-lactamase, third-generation cephalosporins or oxyimino groups were introduced into clinical practice. Resistance to these broad-spectrum cephalosporins quickly emerged. As early as 1983, Germany published the first report on the SHV2 enzyme that can hydrolyze these antibiotics [8]. The continuous use of these third-generation cephalosporins has brought along dynamic inducement on the production of many mutated lactamases in many bacteria, allowing survival and denying drug effects. The β-lactamase in ESBLs contains serine chemicals at their active site which hydrolyzes the spectrum of cephalosporins using an oxyminoside chain [9].

The TEM1, TEM2 are genes that aid in coding for the ESBLs through mutation to alter the amino acid configuration of the β-lactamases, thereby extending the degree of affinity and complementarity for the spectrum of the β-lactam antibiotics to be susceptible for hydrolysis. There are several groups of ESBLs with similar behaviors but different evolutionary histories. The largest population is TEM and sulfhydryl reagent variable (SHV) β-lactamase mutants, with members exceeding 150 [10]. Mutations affecting a small number of key amino acids expand the active site of the enzyme, allowing it to bypass the oxyimino substitution that normally protects the β-lactam ring. Therefore, although the classic TEM and SHV enzymes cannot significantly hydrolyze the oxyiminocephalosporin, the mutant can do so, thereby conferring resistance to its host strain [10].

The CTX-M enzyme is another type of ESBLs. Based on sequence homology, they are divided into five subgroups. Most of these subgroups have evolved due to the leak of the chromosomal β-lactamase gene of Kluvera spp., which is a less clinically significant Enterobacter spp.). After migrating to mobile DNA, CTX-M β-lactamase can further evolve. *E. coli* isolates that produces CTX-M enzyme have been identified as the cause of urinary tract infections. Some reports indicate that the CTX-M ESBL may now be the most common ESBL type in the world [10, 11].

**Figure 1**; Showing the mechanisms in which gram-negative bacteria can be resistant to penicillin and third-generation cephalosporin drugs. The penicillinbinding protein is being modified in such a way to prevent complementary pairing

**Figure 1.** *B1-Metallo-*β*-Lactamases: Where do we stand? Adapted from Mojica et al. [12].*

with the drug (C) that is the modification of the drug target. The B-lactamase enzyme (D) cleaving the B lactam structure of the drug defying its susceptibility and action. At (E) showing efflux pumping of the drugs from the cell [12].

## **3. Resistance on fluoroquinolones**

Quinolones are the most frequently used drugs against *E. coli* infection because they are highly bioavailable meaning they have a good tissue distribution once administrated in the body orally [13]. This fact has caused several doctors to prefer referencing quinolones once an *E. coli* infection is detected. However, the major factor behind *E. coli* resistance in Fluoroquinolones is through mutations in the genome of the bacteria that is DNA gyrase [14].

#### **3.1 Fluoroquinolone mode of action**

The mode of action of fluoroquinolones is by making complexes with DNA Gyrase and topoisomerase IV on the DNA chromosome thereby allowing for the disruption of the DNA sequence of *E.coli*. Fluoroquinolone antibiotics have a chemical structure that allows them to interrupt *E. coli* activity through the alteration of the DNA Gyrase and the Topoisomerase IV protein structure, thereby preventing any form of replication and translation processes for protein synthesis bringing for the destruction of *E. coli* microorganisms. The interruption with DNA Gyrase affects the conversion of the relaxed double-stranded DNA into a negatively super twisted form that allows the replication to commence, this diminishes relegation through entrapping of the enzymes changing their protein arrangement in their active site preventing complementarity with the DNA strand. The replication fork is held steady by Topoisomerase IV and the interruption of its structure affects the replication fork formation, therefore, prohibiting replication to proceed [14].

#### **3.2 Mechanisms of resistance**

#### *3.2.1 DNA Gyrase and Topoisomerase Base substitution*

The mutation in the genome results in amino acid-base substitution in the Gyrase A Gene (GyrA) and topoisomerase IV proteins [14]. Changes on those two genomic structures have been termed the quinolone resistance determining regions. The research conducted by Friedman [15], outlined that the amino acid substitution happens between 67 and 106 bases specifically at bases 83 and 87, therefore altering the drug targets. Further researches proved that there are other sites found on the nalidixic acid-resistant mutant that was not thermo-tolerant had a 5′ base change of guanine to thiamine, in the codon 87 which is expected to reduce the susceptibility of quinolone to nalidixic acid due to the substitution of tyrosine for aspartic acid [15].

However, some investigations are taking place with the intention to defend and uplift the bactericidal status and this includes recent studies done on nybomycin, where investigations on the susceptibility of fluoroquinolone-sensitive and fluoroquinolone-resistant strains were conducted and discovered that nybomycin was successively efficient in destroying the bacterial species [16].

It is important to determine whether the *E. coli* mutants are thermotolerant or non-tolerant because this aids in determining how they can be susceptible to drugs and how temperatures can affect the genomic structures [17]. *E. coli* strains have adaptation characteristics such as physiological, metabolic and proton consuming

acid-resistant mechanisms that allow their survival and perpetuation in acid environments below pH 2. They reduce the effects of acid damage by modifying the membrane, altering membrane porins to reduce proton influx and periplasmic chaperons [18].
