What is Escherichia coli?

Escherichia coli, commonly referred to as E. coli, is a bacterium that belongs to the

Enterobacteriaceae family. E. coli, is a type of bacteria commonly found in the intestines of humans
and animals. While most strains of E. coli are harmless and even play a beneficial role in the digestive
system by helping with food digestion, some strains can cause illness. E. coli is a diverse species with
many different strains, some of which can cause various infections and diseases in humans.

Properties of E. coli

1.Classification: E. coli is a Gram-negative, facultative anaerobic bacterium belonging to the family

Enterobacteriaceae.

2.Motility: Many strains of E. coli are motile, thanks to flagella that allow them to move through
liquid environments. Motility helps E. coli to colonize and navigate within the intestinal tract.

3.Habitat: E. coli is primarily found in the lower intestines of warm-blooded animals, including
humans. It plays a crucial role in the human digestive system, aiding in the digestion of food and
producing certain vitamins.

4.Growth Conditions: E. coli thrives at temperatures around 37 degrees Celsius (98.6 degrees
Fahrenheit), which is the normal body temperature of humans.It can grow in a wide range of pH
levels, typically between 6.5 and 7.5.

5.Strains: E. coli comprises a diverse group of strains, some of which are nonpathogenic and others
that can cause diseases. Pathogenic strains are often categorized into different serotypes based on
the presence of specific surface antigens.

6.Pathogenicity: Some E. coli strains are pathogenic and can cause various diseases, including
gastrointestinal infections, urinary tract infections, and, in severe cases, kidney failure (hemolytic-
uremic syndrome). Notable pathogenic strains include enterohemorrhagic E. coli (EHEC),
enterotoxigenic E. coli (ETEC), and enteropathogenic E. coli (EPEC).

7.Virulence Factors: Pathogenic E. coli strains possess virulence factors such as toxins and adherence
molecules that enable them to attach to host cells, colonize tissues,

8.Metabolism: E. coli is a heterotrophic bacterium, meaning it relies on organic compounds as a

source of carbon and energy. It can ferment sugars like glucose to produce energy and various
metabolic byproducts.

Structure Of Escherichia coli

Size: They are rod shaped and are about 1.5 µm long and 0.5 µm wide.

Cell Wall: E. coli has a cell wall that provides structural support and protection. The cell wall consists
of a peptidoglycan layer, which is made up of sugar and peptide chains.

Cell Membrane (Plasma Membrane): Beneath the cell wall, there is a phospholipid bilayer
membrane known as the cell membrane or plasma membrane. It controls the passage of molecules
in and out of the cell and houses various transport proteins and receptors.
Cytoplasm: Inside the cell membrane is the cytoplasm, a gel-like substance where most of the
cellular processes take place. The cytoplasm contains various organelles, enzymes, and the bacterial
chromosome.

Bacterial Chromosome: E. coli has a single, circular DNA molecule located in the cytoplasm. This
chromosome contains the genetic information necessary for the bacterium's growth and
reproduction.

Ribosomes: E. coli has ribosomes in its cytoplasm that are responsible for protein synthesis. These
ribosomes are smaller than those found in eukaryotic cells.

Plasmids: In addition to the chromosomal DNA, E. coli can also carry small, circular pieces of DNA
called plasmids. Plasmids may contain genes that provide the bacterium with specific advantages,
such as antibiotic resistance.

Flagella: Some strains of E. coli possess flagella, which are whip-like appendages that allow the
bacterium to move in liquid environments. These are used for motility.

Pili (Fimbriae): E. coli may have pili or fimbriae on its surface. These hair-like structures are involved
in adherence to surfaces and other cells.

Capsule (in some strains): Certain strains of E. coli may have a protective capsule surrounding the
cell. The capsule helps the bacterium evade the host immune system.

Plasma Membrane: Inside the cell, there is a cell membrane that separates the cytoplasm from the
periplasmic space. It plays a crucial role in various cellular processes, including transport and energy
generation.
Escherichia coli Serological Types

Escherichia coli (E. coli) is a diverse species of bacteria that can be classified into various serological
types based on the presence of specific surface antigens. There are more than 700 different
serotypes of E. coli.

Serologically can be different based on-

1. O antigen
2. H antigen
3. K antigen

Most are harmless. Pathogenic strains are serotypes based on O and H Ag.

Here's a brief overview:

1. Antigens (Somatic Antigens): O antigens are found on the bacterial cell wall and are part of
the lipopolysaccharide (LPS) molecule. They are classified numerically and are used to
distinguish between different serogroups of E. coli. For example, E. coli O157:H7 is a well-
known serotype that causes foodborne illness.

2. H Antigens (Flagellar Antigens): H antigens are associated with the bacterial flagella, which
are responsible for bacterial motility. Like O antigens, H antigens are also classified
numerically. The combination of O and H antigens is used to identify specific serotypes of E.
coli.

3. K Antigens (Capsular Antigens): In addition to O and H antigens, some E. coli strains have a K
antigen, which is associated with the bacterial capsule. K antigens are less commonly used in
serotyping compared to O and H antigens.

The combination of O, H, and sometimes K antigens allows for the classification of E. coli strains into
numerous serotypes. These serotypes are important for epidemiological and diagnostic purposes, as
they can help trace the source of outbreaks and determine the virulence potential of different E. coli
strains. It's important to note that E. coli serotyping is just one aspect of bacterial classification, and
genetic techniques like DNA sequencing are increasingly used for more precise identification and
characterization of E. coli strains.
Diarrheagenic strains of Escherichia coli

Diarrheagenic Escherichia coli (DEC) are a group of pathogenic strains of the bacterium Escherichia
coli that can cause gastrointestinal infections and diarrhea in humans. DEC strains are classified into
several pathotypes based on their virulence factors and mechanisms of causing disease. Here are
some of the major types of diarrheagenic E. coli:

Enterotoxigenic E. coli (ETEC): ETEC strains are a common cause of traveler's diarrhea and are often
associated with contaminated food and water. They produce two types of enterotoxins, heat-labile
toxin (LT) and heat-stable toxin (ST), which can lead to watery diarrhea.

Enteropathogenic E. coli (EPEC): EPEC strains primarily affect infants and young children, causing
watery diarrhea. They attach to the intestinal epithelial cells, leading to the formation of
characteristic attaching and effacing lesions.

Enterohemorrhagic E. coli (EHEC): EHEC, including the infamous strain E. coli O157:H7, can cause
severe illness and is associated with outbreaks of foodborne illness. EHEC produces a toxin called
Shiga toxin, which can lead to bloody diarrhea and, in severe cases, hemolytic uremic syndrome
(HUS).

Enteroaggregative E. coli (EAEC): EAEC is associated with persistent diarrhea, particularly in children
and immunocompromised individuals. They adhere to the intestinal lining and produce toxins and
biofilms.

Enteroinvasive E. coli (EIEC): EIEC strains are similar to Shigella bacteria in their ability to invade and
cause inflammation of the intestinal epithelial cells. They can lead to dysentery-like symptoms.

Diffusely Adherent E. coli (DAEC): DAEC strains are linked to childhood diarrhea, particularly in
developing countries. They adhere diffusely to the intestinal lining and may cause prolonged
diarrhea.

Cytotoxic Necrotizing Factor (CNF)-Producing E. coli: These strains produce toxins that can cause
cell damage and are associated with various clinical symptoms.

Shiga Toxin-Producing E. coli (STEC): STEC is a subgroup of EHEC strains that produce Shiga toxin. In
addition to E. coli O157:H7, there are non-O157 STEC strains that can also cause illness.

Each of these pathotypes has distinct virulence factors and mechanisms of causing diarrhea, and
they can vary in terms of severity and clinical presentation. It's important to note that proper
hygiene, food safety practices, and access to clean drinking water are essential for preventing
diarrheal illnesses caused by these pathogenic E. coli strains

Features of O157:H7

 A strain of E. coli that produces the Shiga toxin

 The grow well at temperatures lower than 44.5°C
 Many strains tolerate acidic environment with a minimum pH:4.0-4.5
 Acid tolerance increases tolerance to heating, radiation and antimicrobials
 They can ferment Sorbitol and produce b-glucornidase
 Possesses an attaching and effacing gene, eae
 Contain a 60-Mda plasmid
Shiga-like toxin (Stx):

Shiga toxin is a potent toxin produced by certain strains of bacteria, particularly Escherichia coli (E.
coli) and Shigella dysenteriae. It is named after the Japanese city of Shiga, where it was first
discovered in the early 20th century.

There are two main types of Shiga toxin, Shiga toxin 1 (Stx1) and Shiga toxin 2 (Stx2), with subtypes
and variants within each type. These toxins are primarily associated with pathogenic strains of E.
coli, such as E. coli O157:H7, which is known for causing foodborne illnesses.

Shiga toxin is responsible for the symptoms of diseases like hemolytic uremic syndrome (HUS) and
bloody diarrhea, which can occur in individuals infected with Shiga toxin-producing bacteria. The
toxin damages the lining of blood vessels, particularly in the kidneys, leading to kidney damage and
potentially life-threatening complications.

Mode of action of Shiga toxin:

1) Binding of the bacterium to the gb-3 layer of the cell.

(2) Production of the shiga toxin.

(3) Stx is transported to the Golgi complex.

(4) Endocytosis of the toxin in the cell.

(5) Clathrin-enveloped vesicle formation.

(6) The toxin is transported to the Golgi complex.

(7) Vesicle breaking and separation of pentamer B from the toxic A1 fraction.

(8)Action of the A1 portion on the rRNA in the 28S portion, acting as N- glyocsidase, replacing an
Adenine moiety.

(9) Inability to translate the RNA tape.

(10) Cell death.

Reservoirs of E. coli O157:H7

Animals:

 Cattle: main animal reservoir

 Sheep, goats, deer, horses, swine, cats
 Also isolated from seagulls and rats
 Human:
 Infected people excrete in feces
 Patients with hemorrhagic colitis or HUS can shed bacteria for 13-21 days after onset of
symptoms
 Excretion can continue for weeks
 No incidence of asymptomatic long-term carrier state

Seasonality of infection

Outbreaks peak during the warmest months of the year.

Reasons are unknown.

i. An increases prevalence of the pathogen in cattle or other livestock or vehicles during summer
ii. Greater exposure to ground beef or other contaminated foods

iii. Improper handling or inadequate cooking might result in higher contamination due to the
warmth

Age of patients

 All age groups

 Most frequent in very young and the elderly
 Study shows highest age specific incidence in 2-10yrs.
 Reasons:
I. Increased exposure to contaminated foods, environments and infected animals
II. More chance of person-person contamination (less knowledge of hygiene)
III. Less protection against shiga toxins
 High frequency among young women
 Indicates to the influence of socioeconomic factors-
 Women are more likely to prepare food
 Primary care providers for infected young children
Bovine carriage of E. coli O157:H7

One study found increased carriage of the bacteria in cattle resulting from-

 Grouping claves before weaning.

 Sharing among calves feeding utensils without sanitation.
 Early feeding of grain.
 Other factors include-
 Water
 Feed sources
 Farm management practices: manure handling
 Bacteria can survive in bovine feces and water for weeks & months

Transmission

Several ways of transmission:

 Person to person (via contaminated hands) 100 , even as few as 10 cells can cause disease in
highly susceptible populations)
 Water borne
 Contact with human or bovine feces
 Variety of foods
 Ground, roasted beef, cooked meats
 Raw milk, yogurt, cheese
 Ice cream bars and cakes
 Unpasteurized apple or cider juice
 Raw vegetables, eg. lettuce, sprouts, coleslaw

pathogenesis - AttachinPg and effacing adherence

The pathogenesis of Escherichia coli (E. coli) infection involves the mechanisms by which this
bacterium causes diseases in humans. E. coli is a diverse species of bacteria, with most strains being
harmless and even beneficial, residing in the human gut. However, some strains can cause various
illnesses, depending on the specific virulence factors they possess and the site of infection. Here is
an overview of the pathogenesis of E. coli:

Localized adherence

 EPEC initially attach to the host cell through a plasmid-encoded bundle forming pilus (BFP)
  mutants lacking this plasmid still attach to host cells, but produce fewer AE lesions than wild-
type
 BFP mutant strains are less able to cause diarrhea in human volunteers
 EHEC lacks BFP
 attaches through the E. coli common pilus (ECP) and the haemorrhagic coli pilus (HCP)

Signal transduction

Esps

 corrupt host cell systems and redirect the cell’s own structural components to support the
attachment of EPEC
 induce changes in host cell signaling pathways that mediate pedestal formation
 Tyrosine phosphorylation of Tir (not in EHEC)
 block phagocytosis
 Mediate the diarrheal response

Intimate adherence

I. Tir binds with the outer membrane protein intimin

 II. Intimate attachment
III. Tir-intimin interaction induces signal transduction
IV. signals cytoskeletal rearrangement and actin assembly
V. destruction of brush border microvilli
VI. The epithelial membrane beneath the adherent organisms is raised
VII. form pedestal-like structures
VIII. can extend up to 10 mm away
IX. the bacterium resides upon the pedestral
X. AE lesion formation thus firmly anchors the bacterium to the host cell
Symptoms

Signs and symptoms of E. coli O157:H7 infection usually begin three or four days after exposure to
the bacteria. But you may become ill as soon as one day after exposure to more than a week later.
Signs and symptoms include:

 Diarrhea, which may range from mild and watery to severe and bloody
 Stomach cramping, pain or tenderness
 Nausea and vomiting, in some people

Risk factors

 E. coli can affect anyone who is exposed to the bacteria. But some people are more likely to
develop problems than are others. Risk factors include:
 Age. Young children and older adults are at higher risk of experiencing illness caused by E.
coli and more-serious complications from the infection.
 Weakened immune systems. People who have weakened immune systems — from AIDS or
from drugs to treat cancer or prevent the rejection of organ transplants — are more likely to
become ill from ingesting E. coli.
 Eating certain types of food. Riskier foods include undercooked hamburger; unpasteurized
milk, apple juice or cider; and soft cheeses made from raw milk.
 Time of year. Though it's not clear why, the majority of E. coli infections in the U.S. occur
from June through September.
 Decreased stomach acid levels. Stomach acid offers some protection against E. coli. If you
take medications to reduce stomach acid, such as esomeprazole (Nexium), pantoprazole
(Protonix), lansoprazole (Prevacid) and omeprazole (Prilosec), you may increase your risk of
an E. coli infection.

Treatment:

 For illness caused by E. coli, no current treatments can cure the infection, relieve symptoms
or prevent complications. For most people, treatment includes:
 Rest
 Fluids to help prevent dehydration and fatigue

Prevention:

No vaccine or medication can protect you from E. coli-based illness, though researchers are
investigating potential vaccines. To reduce your chance of being exposed to E. coli, avoid swallowing
water from lakes or pools, wash your hands often, avoid risky foods, and watch out for cross-
contamination