**5. Diarrheagenic and extra intestinal** *E. coli* **pathotypes**

**4. Mechanisms of antibiotic resistance in Gram-negative bacteria**

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

a rapid spread of an antibiotic resistance.

have been acquired from food.

Bacterial antimicrobial resistance in both the medical and agricultural fields has become a serious problem worldwide. Resistant bacteria isolated from agriculture, farm or hospital can transfer the resistance genes to human pathogens [40]. The selection pressure applied by the antibiotics that are used in clinical and agricultural settings has promoted the evolution and spread of genes that confer resistance, regardless of their origins. Several factors can be implicated with resistance, sensibility, and antibiotic resistance dissemination such as: (i) impermeable barriers [6]; in this case, some bacteria are intrinsically resistant to certain antibiotics because they have an impermeable membrane or lack the target of the antibiotic; (ii) multidrug resistance efflux pumps; these pumps protect the bacterial cell against toxic molecules. It is an active transport mechanism for outside the cell. Some transporters, such as those of the resistance-nodulation cell division family, can pump antibiotics directly outside the cell, whereas others, such as those of the major facilitator superfamily, secrete them into the bacterial periplasm; (iii) resistance mutations; these mutations can cause a modification in the target protein, for example, by disabling the antibiotic-binding without changing the protein functionality. Specific examples include mutations in the gyrase, which cause resistance to fluoroquinolones, in RNA polymerase subunit B, which cause resistance to rifampicin, and in the 30S ribosomal subunit protein S12 (encoded by *rpsL*), which cause resistance to streptomycin; and (iv) antibiotic inactivation; inactivation can occur by covalent modification of the antibiotic, such as that catalyzed by acetyltransferases acting on aminoglycoside antibiotics, or by degradation of the antibiotic, such as the hydrolytic degradation of the β-lactam ring on antibiotics by the β-lactamases. The emergence of drug resistance among diarrheagenic *E. coli* is important, and in infant, is a cause of morbidity and mortality principally in developing countries. Analyzing stools of infants in India was verified that about 90% of *E. coli* strains presented resistance to the most antibiotics tested [41]. All isolates were resistant to ampicillin, imipenem, cotrimoxazole, and sensitive to amikacin, and presented 29 different antibiotic profiles. Most of the isolated *E. coli* harbored plasmids (64%) and up to 76% could transfer their plasmids. The transconjugant strains were carrying plasmids and presented resistance to ampicillin, imipenem, and cotrimoxazole. The authors found an increase in the prevalence of drug resistance among *E. coli* isolates, and conjugation transfer of plasmids contributed to

Cyclomodulins are a growing functional family of toxins, which hijack eukaryotic cell cycle. Four cyclomodulin types are actually known in *E. coli*: cytotoxic necrotizing factors (CNFs), cycle inhibiting factor (Cif), cytolethal distending toxins (CDTs), and the pks-encoded toxin. One interesting work [42] isolated ceftriaxone-resistant *E. coli* from 1.5% of participants in Maryland and Michigan, United States. One *E. coli* isolate collected from an apparently healthy person, presented resistance to eight antibiotics, and the resistance genes were contained on an incompatibility plasmid. These plasmid types are common among *Enterobacteriaceae* and can carry multiple resistance genes, generating multidrug resistance [43]. In Krueger's work [42], the source of the extensively resistant *E. coli* is not known, but the isolated strain may Several distinct pathogenic categories (i.e., pathotypes or virotypes) of diarrheagenic *E. coli* strains are recognized. Each pathotype is defined by a characteristic set of virulence-associated determinants that act in a concert to determine the clinical, pathological, and epidemiological features of the disease they cause [44].

By definition, the virulence determinants of each *E. coli* pathotype are distinct. However, they can generally be categorized as either colonization factors (adhesins), which enable the bacteria to bind closely to the intestinal mucosa and resist removal by peristalsis, or secreted toxins, which interfere with the normal physiological processes of host cells. The key virulence determinants of the primary pathotypes of diarrheagenic *E. coli* are summarized in **Table 3**.


Note: Ref. [44] with modifications.

**Table 3.** Key virulence determinants of diarrheagenic *E. coli*.

It can be seen in **Table 3**, that the number of virulence traits varies from each pathotype and have implications on intestinal pathology. Besides Enteropathogenic *E. coli* (EPEC) causing intimate adhesion, attaching-effacing lesions throughout the intestine and loss of brush enterocytes, Enterotoxigenic *E. coli* (ETEC) do not present notable change to intestinal mucosa. Enterohaemorragic *E. coli* (EHEC) provoke a similar intestinal pathology, with necrosis. Enteroinvasive *E. coli* (EIEC) cause inflammation and necrosis, but Enteroaggregative *E. coli* (EAEC) present enterocytes changes. Atypical *E. coli* (A-EPEC) and Diffusely adherent *E. coli* (DAEC) can cause lesions in the intestine.

implies that they may be involved in virulence. The translocation of these proteins is essential

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The third stage of EPEC interaction with the eukaryotic cells is characterized by the intimate attachment with the host cell. A 94-kDa outer membrane protein and intimin, encoded by the *eae* gene [51], binds to a 90-kDa tyrosine phosphorylated protein in the host membrane. This receptor is of bacterial origin and has been designated as the translocated intimin receptor (Tir). Tir is translocated from the bacterial cell into the host membrane, where it becomes phosphorylated on one or more tyrosine residues and functions as a receptor for its binding partner, intimin. The resultant tight association is accompanied by the formation of actin pedestals. The most remarkable change in the cellular structure of the eukaryotic cell is the formation of typical actin pedestals. Within 3 hours of infection by EPEC, host-cell actin, a-actinin, talin, erzin, and villin accumulate directly under the bacteria. EPEC presents a strong and intimate adhesion to the intestinal mucosa leading to dissolution of the brush border by inducing vesiculation of the microvilli. This is the attaching and effacement step, and in the jejunum and ileum results in a loss of brush border disaccharidase enzymes and a large area of absorptive surface.

Typical kinds of EPEC are EPECs that have lost the EAF plasmid. ETEC strains are a major cause of secretory diarrhea in both humans and animals. They produce heat-labile and/or heatstable (STa and STb) toxins that also cause diarrhea. EHEC strains are implicated in foodborne diseases principally due to ingestion of uncooked minced meat and raw milk. These strains produce shiga-like toxin 1 (stx1), shiga-like toxin 2 (stx2), and variants thereof. These toxins can destroy colonic enterocytes and produce hemorrhagic colitis. EIEC can attach to enterocytes and penetrate by endocytosis and replicate therein. DAEC strains are diffusely adhering *E. coli* that are also implicated with episodes of diarrhea. EAEC damage and blunt colonic villi by hemorrhagic necrosis, although the precise pathogenic mechanisms are unclear. EAEC are a major cause of chronic diarrhea in children. ExPEC are the cause of a diverse spectrum of invasive human and animal infections, often leading to septicemia and sometimes to death.

Extraintestinal *E. coli* (ExPEC) strains have amazing behavior and possess virulence mechanisms to invade, colonize, and induce disease in sites outside of the gastrointestinal tract. Human diseases caused by the ExPEC include urinary tract infections, neonatal meningitis, sepsis, pneumonia, surgical site infections, as well as infections in other extraintestinal locations. ExPEC strains have been isolated from food products, in particular from raw meats, and poultry, indicating that these organisms potentially represent a new class of foodborne

Extraintestinal *E. coli* infections are associated with specialized strains presenting antimicrobial resistance. The food supply may disseminate ExPEC and antimicrobial-resistant *E. coli*. Retail foods may be an important vehicle for community-wide dissemination of antimicrobial-resistant *E. coli* and ExPEC, which may represent a newly recognized group of medically

*E. coli* contamination exhibited a prevalence gradient from miscellaneous foods (9%), through beef or pork (69%), to poultry (92%) [54]. Among *E. coli*-positive samples, similar prevalence gradients were detected for antimicrobial resistance (27, 85, and 94 of samples, respectively)

for activating a number of signal transduction pathways.

pathogens [52–53].

significant foodborne pathogens.

Based on genetic variation within *E. coli*, it was found [45] that pathogenic strains have accelerated the rates of mutation and recombination and virulence is the driving force for more frequent recombination. These characteristics can impulse the bacterial population to acquire more resistance. Some studies [45–46] proposed a model where commensal *E. coli* maintains low frequencies of homologous recombination and acquisition of novel genes that result in virulence by horizontal genetic exchange. The pathogenic condition results in exposure to immune system barriers and antibiotic selection. These population presents higher mutation and recombination rates. Epidemic strains are exposed to stronger selection by pressures imposed by immune defenses and antibiotic use, resulting in highest levels of mutation, recombination, and infection.

In one study, conducted in Ontario, Canada, the authors [15] showed that the most common bacteria identified on urine culture over a 5 year period were *Escherichia coli* (71.6%), *Enterococcus* spp. (5.7%), and *Klebsiella* spp. (5.0%) and that these bacteria were frequently resistant to ampicillin (54.4%) and trimethoprim-sulfamethoxazole (TMP-SMX) (40.4%) [15].

Another study showed that resistance was more commonly seen in typical EPEC than in atypical pathotypes. The most prevalent resistances observed were to ampicillin, tetracycline, streptomycin, and the sulfonamides [16].

EPEC, an established etiological agent of human infantile diarrhea, is a pathogen that subverts intestinal epithelial cell function to produce distinctive "attaching and effacing" (A/E) lesions. These types of pathogens are typically found on the surface of the host epithelial cell. They can cause severe lesions on intestinal microvilli. Other pathogens can display similar characteristics, which includes *Hafnia alvei, Citrobacter rodentium*, and enterohemorrhagic *E. coli*.

The interactions between EPEC and host cells have been divided into three stages. Initial adherence to cultured epithelial cells is mediated by the formation of type IV fimbriae known as bundle forming pili (BFP) [47]. Initial adherence helps bring the bacteria in intimate contact with the host cell. BFPs mediate bacterial interactions in a human intestinal organ culture model [48].

The genetic answer for the formation of A/E lesions can be explained by the presence of the *locus* of enterocyte effacement (LEE) [49]. This cluster includes the genes of following bacterial proteins: *E. coli* attaching and effacing that encodes the protein intimin (*eae*); *E. coli* secretion (*escs*); *E. coli*secreted protein (*esps*); secretion of *E. coli* proteins (*sep*), and translocated intimin receptor (Tir).

The second stage of EPEC pathogenesis involves the secretion of bacterial proteins, some into the host cell, including EspA, EspB, and EspD at the temperature of the body [50], and particularly the gastrointestinal tract, the expression of these proteins is maximal, which implies that they may be involved in virulence. The translocation of these proteins is essential for activating a number of signal transduction pathways.

It can be seen in **Table 3**, that the number of virulence traits varies from each pathotype and have implications on intestinal pathology. Besides Enteropathogenic *E. coli* (EPEC) causing intimate adhesion, attaching-effacing lesions throughout the intestine and loss of brush enterocytes, Enterotoxigenic *E. coli* (ETEC) do not present notable change to intestinal mucosa. Enterohaemorragic *E. coli* (EHEC) provoke a similar intestinal pathology, with necrosis. Enteroinvasive *E. coli* (EIEC) cause inflammation and necrosis, but Enteroaggregative *E. coli* (EAEC) present enterocytes changes. Atypical *E. coli* (A-EPEC) and Diffusely adherent *E. coli*

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

Based on genetic variation within *E. coli*, it was found [45] that pathogenic strains have accelerated the rates of mutation and recombination and virulence is the driving force for more frequent recombination. These characteristics can impulse the bacterial population to acquire more resistance. Some studies [45–46] proposed a model where commensal *E. coli* maintains low frequencies of homologous recombination and acquisition of novel genes that result in virulence by horizontal genetic exchange. The pathogenic condition results in exposure to immune system barriers and antibiotic selection. These population presents higher mutation and recombination rates. Epidemic strains are exposed to stronger selection by pressures imposed by immune defenses and antibiotic use, resulting in highest levels of mutation, recombination, and infection.

In one study, conducted in Ontario, Canada, the authors [15] showed that the most common bacteria identified on urine culture over a 5 year period were *Escherichia coli* (71.6%), *Enterococcus* spp. (5.7%), and *Klebsiella* spp. (5.0%) and that these bacteria were frequently resistant to ampicillin (54.4%) and trimethoprim-sulfamethoxazole (TMP-SMX) (40.4%) [15].

Another study showed that resistance was more commonly seen in typical EPEC than in atypical pathotypes. The most prevalent resistances observed were to ampicillin, tetracycline,

EPEC, an established etiological agent of human infantile diarrhea, is a pathogen that subverts intestinal epithelial cell function to produce distinctive "attaching and effacing" (A/E) lesions. These types of pathogens are typically found on the surface of the host epithelial cell. They can cause severe lesions on intestinal microvilli. Other pathogens can display similar characteris-

The interactions between EPEC and host cells have been divided into three stages. Initial adherence to cultured epithelial cells is mediated by the formation of type IV fimbriae known as bundle forming pili (BFP) [47]. Initial adherence helps bring the bacteria in intimate contact with the host cell. BFPs mediate bacterial interactions in a human intestinal organ culture model [48]. The genetic answer for the formation of A/E lesions can be explained by the presence of the *locus* of enterocyte effacement (LEE) [49]. This cluster includes the genes of following bacterial proteins: *E. coli* attaching and effacing that encodes the protein intimin (*eae*); *E. coli* secretion (*escs*); *E. coli*secreted protein (*esps*); secretion of *E. coli* proteins (*sep*), and translocated intimin receptor (Tir).

The second stage of EPEC pathogenesis involves the secretion of bacterial proteins, some into the host cell, including EspA, EspB, and EspD at the temperature of the body [50], and particularly the gastrointestinal tract, the expression of these proteins is maximal, which

tics, which includes *Hafnia alvei, Citrobacter rodentium*, and enterohemorrhagic *E. coli*.

(DAEC) can cause lesions in the intestine.

streptomycin, and the sulfonamides [16].

The third stage of EPEC interaction with the eukaryotic cells is characterized by the intimate attachment with the host cell. A 94-kDa outer membrane protein and intimin, encoded by the *eae* gene [51], binds to a 90-kDa tyrosine phosphorylated protein in the host membrane. This receptor is of bacterial origin and has been designated as the translocated intimin receptor (Tir). Tir is translocated from the bacterial cell into the host membrane, where it becomes phosphorylated on one or more tyrosine residues and functions as a receptor for its binding partner, intimin. The resultant tight association is accompanied by the formation of actin pedestals. The most remarkable change in the cellular structure of the eukaryotic cell is the formation of typical actin pedestals. Within 3 hours of infection by EPEC, host-cell actin, a-actinin, talin, erzin, and villin accumulate directly under the bacteria. EPEC presents a strong and intimate adhesion to the intestinal mucosa leading to dissolution of the brush border by inducing vesiculation of the microvilli. This is the attaching and effacement step, and in the jejunum and ileum results in a loss of brush border disaccharidase enzymes and a large area of absorptive surface.

Typical kinds of EPEC are EPECs that have lost the EAF plasmid. ETEC strains are a major cause of secretory diarrhea in both humans and animals. They produce heat-labile and/or heatstable (STa and STb) toxins that also cause diarrhea. EHEC strains are implicated in foodborne diseases principally due to ingestion of uncooked minced meat and raw milk. These strains produce shiga-like toxin 1 (stx1), shiga-like toxin 2 (stx2), and variants thereof. These toxins can destroy colonic enterocytes and produce hemorrhagic colitis. EIEC can attach to enterocytes and penetrate by endocytosis and replicate therein. DAEC strains are diffusely adhering *E. coli* that are also implicated with episodes of diarrhea. EAEC damage and blunt colonic villi by hemorrhagic necrosis, although the precise pathogenic mechanisms are unclear. EAEC are a major cause of chronic diarrhea in children. ExPEC are the cause of a diverse spectrum of invasive human and animal infections, often leading to septicemia and sometimes to death.

Extraintestinal *E. coli* (ExPEC) strains have amazing behavior and possess virulence mechanisms to invade, colonize, and induce disease in sites outside of the gastrointestinal tract. Human diseases caused by the ExPEC include urinary tract infections, neonatal meningitis, sepsis, pneumonia, surgical site infections, as well as infections in other extraintestinal locations. ExPEC strains have been isolated from food products, in particular from raw meats, and poultry, indicating that these organisms potentially represent a new class of foodborne pathogens [52–53].

Extraintestinal *E. coli* infections are associated with specialized strains presenting antimicrobial resistance. The food supply may disseminate ExPEC and antimicrobial-resistant *E. coli*. Retail foods may be an important vehicle for community-wide dissemination of antimicrobial-resistant *E. coli* and ExPEC, which may represent a newly recognized group of medically significant foodborne pathogens.

*E. coli* contamination exhibited a prevalence gradient from miscellaneous foods (9%), through beef or pork (69%), to poultry (92%) [54]. Among *E. coli*-positive samples, similar prevalence gradients were detected for antimicrobial resistance (27, 85, and 94 of samples, respectively) and ExPEC contamination (4, 19, and 46%, respectively). Indirect evidence suggested on-farm selection of resistance.

that resistance of commensal *E. coli* to almost all agents studied increased rapidly over time [59]. Additionally, urban residents in Nigeria, Ghana, and Zimbabwe were more likely to carry multidrug-resistant *E. coli* than were rural or provincial residents [60]. This finding has important consequences in light of the rapid rate of urbanization in these countries and other parts of the

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Most antibiotic-producing strains carry genes encoding resistance to the antibiotics that they produce, and they are located in the same gene cluster as the antibiotic biosynthesis pathway genes. The sources by which antibiotic resistance genes can be found are presented in **Table 4**. Resistance genes exist naturally in the environment owing to a range of selective pressures in nature. Humans have applied additional selective pressure for antibiotic resistance genes because of the large quantities produced, consumed, and applied in daily activities. Physical and biological forces also cause widespread dissemination of resistance throughout many

In lifetime, humans are exposed to antibiotic resistance bacteria. The potential routes for human exposition with wild animals and its microbiota include [6]: (i) translocation of wildlife into suburban areas, habit destruction, pollution ,and changes to water storage, irrigation or climate changes; (ii) human contact with nature such as hunting and camping; (iii) consumption of exotic foods, bushmeat and game farms; (iv) acquisition of exotic pets and transport of live animals from long distances; (v) incorporation of animal's habitats in

Some microorganisms and some environments harbor antibiotic resistance genes irrespective of the human use of antibiotics. The prevalence and diversity of resistance genes in the environment inspire hypotheses about the native roles of so-called resistance genes in natural

– –

Industrial antibiotic production Utilization of antibiotics onto fields

Antibiotic consumption Antibiotic consumption

human life as zoos; and (vi) trapping of fur-bearing animals.

**Selection for antibiotic resistance Environment Utilization** Nature Medicine Agriculture

**Spread of antibiotic resistance genes** – – Physical forces Biological forces – Air currents Human activities – Water Animal presence –

**Table 4.** Sources and movement of antibiotic resistance genes in the environment.

continent. Travel networks have become more efficient and are more extensively used.

natural environments.

microbial communities.

Protection against endogenous

Protection against naturally occurring antibiotics and heavy

Alternative cellular functions of the

antibiotics

metals

resistance protein

Note: Ref. [6] modified.

Uropathogenic strains can invade bladder cells and at this local, form reservoirs, which is possibly the storage local of the bacterium. *E. coli* causing infant meningitis is resistant to host immune responses and has the ability to cross the blood-brain barrier and cause disease. ExPEC from human and avian hosts encounter similar challenges in establishing infection in extraintestinal locations.

Extended spectrum beta-lactamases (ESBLs) are the bacterial enzymes that make them resistant to advanced generation cephalosporins and might lead to the failure on therapy.

The importance of this resistance in one children population in India was studied. CTX-M-15 enzyme is increasingly being reported from this part of the world together with TEM-1 [55]. TEM-1 is the most commonly encountered beta-lactamase in Gram-negative bacteria. Up to 90% of ampicillin, resistance in *E. coli* is due to the production of TEM-1 and *E. coli* is the most common cause of neonatal sepsis. The authors found that 97 ESBL-producers were identified among 266 *E. coli* strains isolated from 238 neonates. The isolates were screened for blaCTX-M, blaTEM, armA, rmtA, and rmtB, the last three genes being responsible for aminoglycoside resistance. The authors [55] concluded that male neonates colonized or infected by ESBL-producing *E. coli* have longer stay in NICU compared to their female counterparts. This happened because of male neonates getting colonized and/or infected earlier than their female counterparts do. Plasmid-mediated-conjugal transfer was found to be the mechanism of transfer of blaCTX-M-15 resistance marker in the described setting [55].

Antimicrobial drug resistance is a large and growing problem among organisms that cause diarrheal disease. Although most diarrheal diseases are self-resolving and should not be treated with antimicrobial agents, invasive or protracted infections require chemotherapy and are typically managed empirically [56].

The more recently defined enteroaggregative *E. coli* are typically multidrug-resistant and are one of the most common causes of childhood diarrhea, particularly persistent infections [56]. Antimicrobial drug-resistant diarrheagenic *E. coli* pathotypes, including enteroaggregative *E. coli*, are also emerging as important diarrheal pathogens in AIDS patients [57].

According some data [18, 20] about *E. coli* outbreaks, new pathotypes can emerge and cause disease and death in different populations in both, developed [20] and in developing [53] countries. Other authors [58] observed that both EHEC O157 and non-O157 STEC infections can occur at the same time. These authors presented some interesting reasons to this, such as: (i) they are common and may be increasing in frequency; (ii) could be associated with high morbidity and mortality; (iii) utilizing ideal laboratory conditions these pathogens should be detected by both, culture procedures and using protocols to detect Shiga toxin; and (iv) these strains cannot be readily detected with certainty by selective targeting of patients age, time of year or presence of blood in the stool. These observations can be understood in a globalized world. Humans are embedded into the microbial world.

In healthy populations, saprophytic microorganisms constitute a rich source of genetic material which pathogens can readily acquire resistance. The study conducted by NIS in Nigeria showed that resistance of commensal *E. coli* to almost all agents studied increased rapidly over time [59]. Additionally, urban residents in Nigeria, Ghana, and Zimbabwe were more likely to carry multidrug-resistant *E. coli* than were rural or provincial residents [60]. This finding has important consequences in light of the rapid rate of urbanization in these countries and other parts of the continent. Travel networks have become more efficient and are more extensively used.

and ExPEC contamination (4, 19, and 46%, respectively). Indirect evidence suggested on-farm

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

Uropathogenic strains can invade bladder cells and at this local, form reservoirs, which is possibly the storage local of the bacterium. *E. coli* causing infant meningitis is resistant to host immune responses and has the ability to cross the blood-brain barrier and cause disease. ExPEC from human and avian hosts encounter similar challenges in establishing infection in

Extended spectrum beta-lactamases (ESBLs) are the bacterial enzymes that make them resis-

The importance of this resistance in one children population in India was studied. CTX-M-15 enzyme is increasingly being reported from this part of the world together with TEM-1 [55]. TEM-1 is the most commonly encountered beta-lactamase in Gram-negative bacteria. Up to 90% of ampicillin, resistance in *E. coli* is due to the production of TEM-1 and *E. coli* is the most common cause of neonatal sepsis. The authors found that 97 ESBL-producers were identified among 266 *E. coli* strains isolated from 238 neonates. The isolates were screened for blaCTX-M, blaTEM, armA, rmtA, and rmtB, the last three genes being responsible for aminoglycoside resistance. The authors [55] concluded that male neonates colonized or infected by ESBL-producing *E. coli* have longer stay in NICU compared to their female counterparts. This happened because of male neonates getting colonized and/or infected earlier than their female counterparts do. Plasmid-mediated-conjugal transfer was found to be the mechanism

Antimicrobial drug resistance is a large and growing problem among organisms that cause diarrheal disease. Although most diarrheal diseases are self-resolving and should not be treated with antimicrobial agents, invasive or protracted infections require chemotherapy and

The more recently defined enteroaggregative *E. coli* are typically multidrug-resistant and are one of the most common causes of childhood diarrhea, particularly persistent infections [56]. Antimicrobial drug-resistant diarrheagenic *E. coli* pathotypes, including enteroaggregative *E. coli*,

According some data [18, 20] about *E. coli* outbreaks, new pathotypes can emerge and cause disease and death in different populations in both, developed [20] and in developing [53] countries. Other authors [58] observed that both EHEC O157 and non-O157 STEC infections can occur at the same time. These authors presented some interesting reasons to this, such as: (i) they are common and may be increasing in frequency; (ii) could be associated with high morbidity and mortality; (iii) utilizing ideal laboratory conditions these pathogens should be detected by both, culture procedures and using protocols to detect Shiga toxin; and (iv) these strains cannot be readily detected with certainty by selective targeting of patients age, time of year or presence of blood in the stool. These observations can be understood in a globalized

In healthy populations, saprophytic microorganisms constitute a rich source of genetic material which pathogens can readily acquire resistance. The study conducted by NIS in Nigeria showed

tant to advanced generation cephalosporins and might lead to the failure on therapy.

of transfer of blaCTX-M-15 resistance marker in the described setting [55].

are also emerging as important diarrheal pathogens in AIDS patients [57].

world. Humans are embedded into the microbial world.

selection of resistance.

extraintestinal locations.

are typically managed empirically [56].

Most antibiotic-producing strains carry genes encoding resistance to the antibiotics that they produce, and they are located in the same gene cluster as the antibiotic biosynthesis pathway genes. The sources by which antibiotic resistance genes can be found are presented in **Table 4**.

Resistance genes exist naturally in the environment owing to a range of selective pressures in nature. Humans have applied additional selective pressure for antibiotic resistance genes because of the large quantities produced, consumed, and applied in daily activities. Physical and biological forces also cause widespread dissemination of resistance throughout many natural environments.

In lifetime, humans are exposed to antibiotic resistance bacteria. The potential routes for human exposition with wild animals and its microbiota include [6]: (i) translocation of wildlife into suburban areas, habit destruction, pollution ,and changes to water storage, irrigation or climate changes; (ii) human contact with nature such as hunting and camping; (iii) consumption of exotic foods, bushmeat and game farms; (iv) acquisition of exotic pets and transport of live animals from long distances; (v) incorporation of animal's habitats in human life as zoos; and (vi) trapping of fur-bearing animals.

Some microorganisms and some environments harbor antibiotic resistance genes irrespective of the human use of antibiotics. The prevalence and diversity of resistance genes in the environment inspire hypotheses about the native roles of so-called resistance genes in natural microbial communities.


**Table 4.** Sources and movement of antibiotic resistance genes in the environment.
