**3. Antimicrobial therapy**

to spread laterally through the cell side walls to adjacent cells (via actin). This damage results in dysentery (watery diarrhea with pus, mucus, and blood). The EIEC bacteria do not produce toxins, but participate in direct damage and induce production of IL-1 and IL-8. The pathogenic mechanisms and disease symptoms associated with EIEC are so similar to *Shigella* spp. that differential diagnosis can be difficult. Diagnosis is usually based on physiological and biochemical characteristics that can be detected in the clinical

84 *Escherichia coli* Escherichia coli - Recent Advances on Physiology, Pathogenesis and Biotechnological Applications - Recent Advances on Physiology, Pathogenesis and Biotechnological Applications

The EPEC strains do not directly invade enterocytes. Instead, these bacteria adhere to the microvilli and inject effector proteins into the cell via a type III secretion system (T3SS). One of these effectors is the translocated intimin receptor (Tir), which initiates recruitment of the host cell actin to form a pedestal under the bacteria. The recruitment of actin and formation of the pedestals result in destruction of the rest of the microvilli and also

of water into the intestinal lumen. Another effector, the *E. coli* secreted proteins (Esps), interacts with the host cell cytoskeleton and results in disruption of the cell tight junctions

The ETEC strains are a common cause of acute travelers' diarrhea. These strains usually colonize the proximal small intestine, adhering to the microvilli via various colonization factors including fimbrial, nonfimbrial, helical, and fibrillar types. The EPEC strains secrete two types of toxin: a heat-labile toxin (LT) and heat-stable toxins (STs). The LT is an AB toxin, and the B subunits bind to the monosialoganglioside GM1, which induces the cell to take in the toxin. The LT toxin activates adenylyl cyclase, which increases the cAMP in the cell, resulting in hypersecretion of water and electrolytes into the intestinal lumen. The STs bind to guanylyl cyclase receptors on the microvilli, which stimulate guanylate cyclase and activate the cystic fibrosis transmembrane receptor (CFTR). This results in an increase in cGMP in the cell and

The UPEC strains are responsible for most uncomplicated urinary tract infections (UTIs). These strains possess a capsule and bind to uroepithelial cells via fimbriae. The interaction of the bacteria with the host cell induces the internalization of the bacteria where the bacteria multiply rapidly and form biofilm-like intercellular bacterial communities (IBCs). The bacteria are shed intermittently from the uroendothelial cells into the lumen of the bladder. UPEC strains produce several types of toxins including hemolysin A (HlyA) which has pore-forming capability and two cytotoxins, cytotoxic necrotizing factor (CNF-1) and secreted auto-transporter toxin (Sat)

in the cell, which results in the subsequent exodus

, which causes hypersecretion of water into the intestinal lumen

laboratory [19, 22, 23].

inhibit the transport of Na+

impaired absorption of Na+

[19, 24, 27].

[19, 28, 29].

[24–26].

**2.5. Enteropathogenic** *Escherichia coli* **(EPEC)**

**2.6. Enterotoxigenic** *Escherichia coli* **(ETEC)**

**2.7. Uropathogenic** *Escherichia coli* **(UPEC)**

and Cl-

As mentioned above, antimicrobial drugs are often classified in groups based on their mechanism of antimicrobial action. **Table 1** displays those groups along with examples of the antimicrobial drugs included in each group. The β-lactam drugs, which were among the first antimicrobials to be discovered, target the bacterial cell wall (via peptidoglycan synthesis) and are most useful against gram-positive bacteria (having little effect on gram-negative bacteria because of the lipopolysaccharide cell envelope that protects the thin peptidoglycan cell wall in these bacteria). Over the years, because the β-lactam drugs were readily available and inexpensive and caused few side effects, physicians commonly treated their patients initially with these drugs.


**Table 1.** Antimicrobial groups based on mechanism of action.

When bacterial resistance to the β-lactam drugs became an issue (very early on), scientists developed synthetic versions of penicillin such as ampicillin, amoxicillin, and methicillin. In addition, scientists discovered the natural cephalosporin β-lactam drugs. The initial cephalosporins (first generation) were most useful against gram-positive cocci, with some activity against a few gram-negative bacilli. Further development of these drugs has produced second generation (less effective against gram-positive cocci, more effective against gram-negative bacilli); third generation (generally with a broad spectrum of activity against gram-negative organisms); fourth generation (extended-spectrum activity against gram-positive cocci and gram-negative bacilli); and recently, fifth generation (hopefully effective against various multidrug-resistant organisms), with more still in development. Other β-lactam drugs developed during this time frame were the carbapenems (broad-spectrum activity) and monopenems (aztreonam—activity against gram-negative aerobic bacteria) [30–32].

**4. Antimicrobial resistance mechanisms**

to various antimicrobial drugs [43, 46, 47].

Chloramphenicol Limiting uptake

*β*-lactams Penicillins Cephalosporins Monobactams Carbapenems

Aminoglycosides Amikacin Gentamicin Tobramycin

Tetracyclines Tetracycline

Fluoroquinolones Ciprofloxacin Norfloxacin

**4.1. Limiting drug uptake**

Metabolic pathway inhibitors Trimethoprim/Sulfamethoxazole

There are four general antimicrobial resistance mechanisms that bacteria use. These are limiting uptake of the drug, modifying the target of the drug, inactivating the drug, and active efflux of the drug. These mechanisms may be located on the bacterial chromosome and occur naturally in all members of a species (intrinsic) or come from other bacteria, usually via a plasmid (acquired). Intrinsic resistance genes may be expressed constitutively (usually at a low level) or be induced by the presence of antimicrobial drugs. Gram-negative bacteria widely use all four of these mechanisms and are very capable of horizontal transfer of resistance elements. **Table 2** shows which resistance mechanisms and genes are associated with resistance

**Antimicrobial agents Mechanisms of resistance Genetic basis**

Active efflux

Active efflux

Active efflux

Active efflux

**Table 2.** Common antimicrobial resistance genes and mechanisms in *Escherichia coli*.

Limiting uptake Modified target—gyrase

Limiting uptake Active efflux

β-lactamases—inactivate drugs

Aminoglycoside modifying enzymes

Modified target—topoisomerase IV

Target enzyme modification TMP—*dhfr*

Modify target—16S rRNA

*ampC*

*bla* genes—plasmid (TEM, SHV, CTX-M, NDM) *acrAB(tolC), acrAD(tolC)*

Antimicrobial Mechanisms of *Escherichia coli*

http://dx.doi.org/10.5772/67363

87

*aac*, *ant*, *aph*—plasmid

*acrAB(tolC), acrEF(tolC),* 

*mdtABC(tolC)*

SXT—*dhps*

*amrA, rmtB mdtEF(tolC)*

*ompF acrAB(tolC) tetA, tetB*—plasmid

*ompF acrAB(tolC)*

*ompF gyrA parC*

Gram-negative bacteria have an advantage in combating drugs because of the structure and functions of the LPS cell wall, which provides a natural barrier to certain molecules. The LPS is generally hydrophobic which limits access to small hydrophilic drugs, such as the β-lactams. These hydrophilic drugs gain access by traveling through the OMPs. The main

Differences in structure, metabolism, virulence factors, etc., between gram-negative and gram-positive bacteria predict which antimicrobial drug groups may be effective. Fewer of the drug groups have good activity against gram-negative bacteria. Those groups include some of the β-lactam drugs (especially second-, third-, and fourth-generation cephalosporins): aminoglycosides, fluoroquinolones, trimethoprim/sulfamethoxazole (TMP/SXT), and nitrofurans (for UTIs) [33]. Intestinal infections with *E. coli* are most commonly self-limiting and require supportive therapy (antiemetics, antidiarrhetics, rehydration) only. Severe or recurring infections (e.g., traveler's diarrhea) may be treated with fluoroquinolone drugs. Acute dysentery caused by EIEC strains may be treated with fluoroquinolones or appropriate cephalosporins. For infections caused by EHEC, antimicrobial therapy is contraindicated as it greatly increases the risk for development of HUS. UTIs caused by UPEC strains are usually treated with antimicrobial drugs and uncomplicated UTIs with nitrofurantoin or TMP/SXT; complicated UTIs may also be treated with fluoroquinolones [34–37]. Infections with ESBL or CRE strains severely limit treatment options. For ESBL strains, carbapenems may still be an option or newer β-lactam/β-lactamase inhibitor drug combinations; for CRE strains, gentamicin, amikacin, colistin, tigecycline, and fosfomycin may be options. Unfortunately, some ESBL and CRE strains may be resistant to even some of these drugs [38–40].

At the beginning of antimicrobial drug resistance, physicians did not realize how the various drugs affected the bacteria. In addition, in an effort to begin antimicrobial therapy as quickly as possible, physicians often ordered a broad-spectrum drug before knowing the causative agent of the infection. These issues (among others) have led to a large amount of resistance to β-lactam drugs, especially among the gram-negative bacteria. Bacteria that produced β-lactamases (enzymes that inactivate β-lactam drugs) were identified as early as the 1940s (around the same time as penicillin was discovered), and the number of different β-lactamases produced has increased over the years to around 1000 [2–4, 41].

The issue of resistance is not just with the β-lactam drugs. Over the 70 plus years that antimicrobial drugs have been in existence, resistance mechanisms have been seen for most of these drugs. It does not seem to take the bacteria very long from initial use of a drug to development of resistance to that drug. Important resistant milestones include resistance to aminoglycosides and tetracycline in the 1960s, vancomycin in the 1980s, fluoroquinolones in the 1990s, and linezolid in the 2000s [42]. In addition, some of the bacteria have become resistant to multiple antimicrobial agents from many of the drug classes. These multidrug-resistant bacteria, such as methicillin-resistant *Staphylococcus aureus* (MRSA), are currently a major cause of morbidity and mortality [43].

Similar to the threat of MRSA, members of the gram-negative Enterobacteriaceae, in particular *E. coli* and *Klebsiella pneumoniae*, include strains that are ESBLs and CREs. These organisms can be resistant to most commonly used antimicrobials, which makes the infections they cause extremely difficult to treat, leading to increased morbidity and mortality (and healthcare costs) [44, 45].
