**2. Pathogenic strains of** *Escherichia coli*

Commensal strains of *E. coli* are the predominant facultative organism in the human gut. Even though greatly outnumbered by the anaerobic organisms, the *E. coli* are vital to human health, playing roles in biofilm communities and subsequent digestion of oligosaccharides and polysaccharides, among other things [10, 11]. Unfortunately, there are also several pathogenic strains of *E. coli*. The classification names of these strains may vary some depending on the source, but for the purposes of this chapter, we will use the following names. There are six strains of potentially pathogenic intestinal-based diarrhea causing *E. coli*: diffusely adherent *E. coli* (DAEC), enteroaggregative *E. coli* (EAEC), enterohemorrhagic *E. coli* (EHEC), enteroinvasive *E. coli* (EIEC), enteropathogenic *E. coli* (EPEC), and enterotoxigenic *E. coli* (ETEC). In addition, there is one extraintestinal pathogenic strain, uropathogenic *E. coli* (UPEC), which causes urinary tract infections. There is some evidence that a second extraintestinal strain exists, the meningitis-associated *E. coli* (MNEC) strain. Findings on MNEC indicate that the infection starts as a blood infection and then gains access to the central nervous system. A majority of MNEC possess the K1 capsular antigen, and there is a high rate of mortality from the meningitis [12]. This chapter will focus on the six intestinal and the UPEC strains.

### **2.1. Diffusely adherent** *Escherichia coli* **(DAEC)**

Antimicrobial drugs are classified into groups according to the type of antimicrobial activity. These groups include drugs that inhibit bacterial cell wall synthesis, depolarize the cell membrane, inhibit bacterial protein synthesis, inhibit bacterial nucleic acid synthesis, and inhibit bacterial metabolic pathways. The improper use of these drugs has helped to create resistant bacterial strains. Factors that contribute to resistance include the increased use of all antimicrobial drugs and improper antimicrobial prescribing. Many of the less expensive drugs that have fewer side effects have been used too commonly. Improper prescribing may be choosing

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

Many strains of *Escherichia coli* are not harmful. In fact, these commensal strains in the gut are

there are pathogenic strains, and these strains may become a larger threat if they possess or acquire certain antimicrobial mechanisms. The main ones of concern are the extended-spectrum β-lactamase (ESBL)-producing strains and the carbapenem-resistant Enterobacteriaceae (CRE) strains. The ESBL strains are resistant to most β-lactam drugs, and the CRE strains are resistant to most carbapenem drugs. Greatly increased healthcare costs are associated with the ESBL and CRE strains. Various studies have shown a hospital stay of up to twice as long and increased costs of 1.5–2.5 times as much [6–8]. One study in the United States estimated

All of the pathogenic strains of *E. coli* are armed with the same types of potential virulence factors. These factors include a capsule (in some strains), flagella, the lipopolysaccharide (LPS) cell wall, fimbriae, outer membrane proteins (OMPs), a hemolysin, cytolysins, and siderophores. The specific types of some of these virulence factors plus the possession of other tox-

Commensal strains of *E. coli* are the predominant facultative organism in the human gut. Even though greatly outnumbered by the anaerobic organisms, the *E. coli* are vital to human health, playing roles in biofilm communities and subsequent digestion of oligosaccharides and polysaccharides, among other things [10, 11]. Unfortunately, there are also several pathogenic strains of *E. coli*. The classification names of these strains may vary some depending on the source, but for the purposes of this chapter, we will use the following names. There are six strains of potentially pathogenic intestinal-based diarrhea causing *E. coli*: diffusely adherent *E. coli* (DAEC), enteroaggregative *E. coli* (EAEC), enterohemorrhagic *E. coli* (EHEC), enteroinvasive *E. coli* (EIEC), enteropathogenic *E. coli* (EPEC), and enterotoxigenic *E. coli* (ETEC). In addition, there is one extraintestinal pathogenic strain, uropathogenic *E. coli* (UPEC), which causes urinary tract infections. There is some evidence that a second extraintestinal strain exists, the meningitis-associated *E. coli* (MNEC) strain. Findings on MNEC indicate that the infection starts as a blood infection and then gains access to the central nervous system. A majority of MNEC possess the K1 capsular antigen, and there is a high rate of mortality from

the meningitis [12]. This chapter will focus on the six intestinal and the UPEC strains.

ins and effectors may vary with each individual pathogenic strain [9].

, which is an important clotting factor [5]. However,

broad spectrum or ineffective drugs [2–4].

necessary for the synthesis of vitamin K2

the increased costs to be \$16,450 per patient [6].

**2. Pathogenic strains of** *Escherichia coli*

These strains are sometimes referred to as enteroadherent *E. coli* (EAEC) (not to be confused with the enteroaggregative strains, which are sometimes referred to as EAggEC). While not known to cause severe diarrheal disease, the DAEC, which are probably a group of related strains, are thought to be responsible for some types of persistent diarrhea in infants. Whether they possess true virulence factors is still under debate, but these bacteria are able to bind to enterocytes (probably via adhesins such as Afa/Dr) and elicit a response in which the microvilli extend and wrap around the bacteria. Diarrhea in association with DAEC has also shown an ability to induce the production of inflammatory cytokines, such as IL-8 [13–15].

### **2.2. Enteroaggregative** *Escherichia coli* **(EAEC)**

The EAEC strains were so named because of their tendency to adhere to enterocytes in dense clusters. The bacteria attach to the microvilli and also to other EAEC bacteria. The EAEC are also a heterogeneous group of strains with a similar pathogenesis, resulting in non-bloody diarrhea. Infection is established by adhering to the microvilli via fimbriae (the aggregative adherence fimbriae—AAF), inducing increased mucus production and biofilm formation, inducing an inflammatory response and production of toxins. The main toxins of the EAEC strains are the EAEC heat-stable enterotoxin (EAST1), which enters the enterocytes and activates guanine cyclase, resulting in increased levels of cGMP in the cell and loss of fluid into the intestinal lumen; the plasmid-encoded toxin (Pet), which disrupts the enterocyte cytoskeleton, resulting in cell detachment; and a *Shigella*-like enterotoxin (ShET1), a heat-stable toxin which may also result in fluid secretion [16–18].

### **2.3. Enterohemorrhagic** *Escherichia coli* **(EHEC)**

The most publicized strain of pathogenic *E. coli* is the EHEC strain. EHEC serotype O157:H7 is well known as the causative agent of outbreaks of food-associated severe diarrhea. Infection with O157:H7 results in sever abdominal cramps and bloody diarrhea and may lead to hemolytic-uremic syndrome (HUS) which can be life-threatening. The most common foods associated with transmission of these bacteria are undercooked meat (especially ground beef), raw milk, and raw vegetables. The EHEC strains do not directly invade the enterocytes, but produce toxins that do enter and severely damage these cells. The responsible cytotoxins are verotoxins I and II (designated as Shiga toxins, Stx-1 and Stx-2). The Shiga toxins are capable of inactivating ribosomes, blocking protein synthesis, and emerging through the basolateral membrane into the subepithelial region. Stx-2 is seen most often in the EHEC strains that cause HUS [19–21].

### **2.4. Enteroinvasive** *Escherichia coli* **(EIEC)**

In infections with EIEC, the bacteria invade by directly entering M cells. The bacteria pass through these cells and then are able to invade the enterocytes via the basolateral membrane, causing severe damage to the intestinal mucosa. The bacteria are also able 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 laboratory [19, 22, 23].

**3. Antimicrobial therapy**

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

Antimicrobial Mechanisms of *Escherichia coli*

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

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Carbapenems Cephalosporins Monobactams Penicillins Glycopeptides

Aminoglycosides Tetracyclines

Chloramphenicol Lincosamides Macrolides Oxazolidinones Streptogramins

Fluoroquinolones

Trimethoprim

Bind to 50S ribosomal subunit

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

few side effects, physicians commonly treated their patients initially with these drugs.

Inhibit cell wall synthesis β-Lactams

Depolarize cell membrane Lipopeptides

Inhibit nucleic acid synthesis Quinolones

Inhibit metabolic pathways Sulfonamides

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

Inhibit protein synthesis Bind to 30S ribosomal subunit

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

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 inhibit the transport of Na+ and Cl in the cell, which results in the subsequent exodus 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 [24–26].

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

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 impaired absorption of Na+ , which causes hypersecretion of water into the intestinal lumen [19, 24, 27].

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

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) [19, 28, 29].
