E.coli-Epidemiology, Pathogenesis, and Treatment

Introduction

• E.coli belongs to the family Enterobacteriaceae, gram-negative rods, lactose fermenting (produces indole), facultative anaerobes.
• E.coli are ubiquitous in nature, commonly found in water, soil, and as commensal in normal intestinal microflora ( also are opportunistic pathogens).
• The culture media used for E.Coli are MacConkey agar, and Blood Agar (Both are Selective Media).
• There are 150 different O antigens & a diverse number of other species (denoted by number for e.g., serotypes antigenic formula denoted by using O, K, and H & antigen present in number such as O111: K76: H7 ).
• Pili is usually present in E.Coli strains, helps to mediate attachment to the epithelial cells of the host.
  • Type 1 Pili–—–> interact with the epithelial cell surface ( on D- mannose residues).
  • P pili ( known as Pap or Gal-Gal)——>binds to di-galactoside moieties.
  • Bundle forming pili (BFP) or Colonisation factor antigens——–>binds to the enterocytes.
    
    Antigenic structure of E.Coli
• E.coli strains produce four types of toxins that have a cytotoxic effect on the host cells.
  • • α-hemolysin- Pore-forming cytotoxin
    • Shiga toxin- AB-type toxin
    • Labile toxin (LT)- AB-type toxin
    • Stable toxin (ST)- Small Peptides activate membrane-bound guanylate cyclase.
      
      Virulence Factor associated with E.Coli strains

Epidemiology

• A diverse number of E.Coli are present in gastrointestinal microflora and act as an opportunistic pathogen.
• Because of effective virulence factors, it causes various gastrointestinal diseases.
• The five E.Coli pathogenic strains causing intestinal infections or gastroenteritis which lead to diarrhea are ETEC, EAEC, EIEC, EPEC, and EIEC.

• Some extraintestinal infections are also caused by E.Coli strains
  • Urinary Tract Infection (UTI)
  • Neonatal Meningitis
  • Septicemia

Pathogenesis

Enterotoxigenic E. coli (ETEC)

• Commonly observed in children of younger ages.
• Inoculum of the disease is high in number, primarily caused by fecal contamination of the food or water.
• Cause secretory diarrhea, the incubation period is 1-2 days, can persist for an average of 3 to 5 days.
• Symptoms: Watery diarrhea & abdominal cramp, sometimes nausea & vomiting
• The site of infection is the small intestines.
• Two types of Enterotoxin are produced by ETEC:
  • Heat-labile Toxins (LT-I, LT-II)
  • Heat Stable Toxins (STa and STb)
• LT-II is not related to human diseases and LT-I is an AB toxin, which consists of 1 subunit of A and 5 subunits of B
  • In LT-I toxin, B subunits bind to receptors as GM1 gangliosides and other glycoproteins present on epithelial cells.
  • While A subunit moves to cross the vacuole membranes, binds with Gs proteins that control adenylate cyclase.
• Because of interaction, cyclic adenosine monophosphate (cAMP) levels increase, which causes more secretion of chloride, and absorption of sodium and chloride is decreased.
• These modified alterations lead to watery diarrhea and also triggers the secretion of prostaglandin & produce inflammatory cytokines, which results in further water loss
• On the other hand, heat-stable toxin STa is a small peptide that interacts with the transmembrane guanylate cyclase receptors, which leads to the secretion of cGMP ( cyclic guanosine monophosphate ) which cause fluid hypersecretion.

Enteropathogenic E.Coli (EPEC)

• Cause infant diarrhea is commonly observed in poor countries.
• Infectious disease is low, person-to-person transmission occurs.
• Symptoms: Watery diarrhea, fever, and vomiting may be present.
• Initially, bacteria get attached to small intestine epithelial surface cells along with the disruption of the microvillus (known as effacement), which gives a lesion on the microvillus known as Attachment/ Effacement [A/E] histopathology.
  • First, microcolonies on the epithelium surface cells are formed by plasmid-encoded bundle forming pili (BFP).
  •  Subsequent attachment is carried out by genes present pathogenicity island known as “locus of enterocyte effacement”.
    • Consists of more than 40 genes that carry out the attachment & destruction of the host cell.

• • Bacterial type III secretion system, helps bacteria to secrete active proteins in the host cells.
  • One protein, Tir (translocated intimin receptors) is inserted on the epithelial cell for bacterial adhesin.
  • The binding of Tir receptors and intimin results in actin polymerization and cytoskeletal elements accumulation, under the attached bacteria, which cause cell death.

Enteroaggregative E. coli (EAEC)

• Cause persistent watery diarrhea along with dehydration in infants.
• Cases reported in the US, Europe, Japan, and other developing countries
• Some E.Coli strains can cause chronic diarrhea which also causes retardation in the infant.
• The characterization of the bacteria is done by autoagglutination.
  • Its mediated by AAF1 (The bacteria are characterized by their autoagglutination adherence fimbriae I, adhesins).
• EAEC attaches to the intestine surface, which stimulates mucus secretion and leads to thick biofilm formation.
• Biofilm protects aggregated bacteria from antibiotics & phagocytic cells.
• EAEC produces enteroaggregative heat-stable toxic and plasmid-encoded toxin which induces secretion of the fluid.

Enterohemorrhagic E. coli (EHEC)

•  EHEC is the common E.Coli strain, causes gastrointestinal disease in developed countries.
• The infection reported is 73000 cases/ yr, mortality reported per year is 60 cases in the US.
• Caused in warm months, affecting children under 5 yrs.
• Usually, consumption of undercooked beef, or other meat products, unpasteurized milk, uncooked vegetables can lead to the cause of infections.
• Inoculum dose is low, even 100 bacteria can cause disease, transmission can take place by person-to-person contact.
• Symptoms: Mild diarrhea to hemorrhagic colitis which can cause bloody diarrhea ( known as dysentery) and sharp abdominal pain, in a few patients vomiting can be observed.
• The incubation time period is 3 to 4 days, symptoms like diarrhea & abdominal pain are developed.
• 30% to 65% of patients experience bloody diarrhea & severe abdominal pain at the onset of 2 days.
• EHEC is related to HUS (Hemolytic uremic syndrome), which leads to acute renal failure, deficiency of the platelets ( thrombocytopenia), intravascular hemolysis ( microangiopathic hemolytic anemia), these symptoms complicate the case.
• EHEC serotype, such as O157:H7 is a common strain to cause disease
• EHEC produces Shiga toxins (Stx-1 and Stx-2, ), both are AB-type toxins,
  • Here B subunits bind to the specific glycolipid such as globotriasylceramide[Gb3] present on the host renal endothelial cell and intestinal villi.
  • A subunit is internalized, and cleave into two molecules, one part binds to 28S rRNA and ceases protein synthesis.
• EHEC strains are more pathogenic if it has both Shiga like a toxin and  Attaching and Effacing activity.

Enteroinvasive E. coli (EIEC)

• The occurrence of disease caused by EIEC is rare both in developing & developed countries.
• Few serotypes of EIEC strains are pathogenic such as O124, O143, and O164.
• Bacteria invade and cause destruction in the colonic epithelium. (Associated with large intestines).
• Symptoms: Watery diarrhea, may progress into dysentery, fever, abdominal cramps, and blood and leukocyte in the stool samples.
• Bacterial invasion is mediated by pInv genes.
• Bacteria cause lysis of phagocytic vacuoles and multiply in the cytoplasm.
• The formation of actin tails, helps bacteria to move within the cytoplasm and across the neighboring epithelial cells.
• Colonic ulceration occurs because of epithelial cell destruction with inflammatory cytokines stimulated by bacteria

Escherichia coli (Strains)	Site of Infection	Disease	Remarks
Enteropathogenic, E. coli (EPEC)	Small Intestine	Classic infant diarrhea	Epidemics in hospitals, children’s homes
Enterotoxigenic, E. coli (ETEC)	Small Intestine	Traveler’s Diarrhea	Cause of travelers’ diarrhea ( 50 %)
Enteroaggregative, E. coli (EAEC)	Small Intestine	Watery diarrhea, mainly in infants	Adhesion to the small intestine mucosa; production of a toxin
Enterohemorrhagic, E. coli (EHEC)	Large Intestine	Hemorrhagic colitis	Hemolytic–uremic syndrome (HUS) in 5% of EHEC cases
Enteroinvasive, E. coli (EIEC)	Large Intestine	Dysentery	Invasion and verocytotoxins

Diagnosis

• Immunoassay and nucleic acid detection can be used to determine the toxins and their associated genes. It’s expensive to be used practically for diagnosis.
• MacConkey agar is used as a culture medium( Pink colonies is obtained), for EHEC in media instead of lactose, sorbitol is incorporated. (Colourless Colonies is obtained)

Treatment

• Rehydration and supportive measures are taken into consideration.
• Hemorrhagic colitis and HUS are treated by using hemodialysis or hemapheresis.
• To treat diarrhea caused by ETEC, EIEC, and EPEC, trimethoprim/sulfamethoxazole (TMP-SMX), doxycycline, or quinolones.

References and Sources

• https://quizlet.com/292896388/unit-5-gi-flash-cards/
• https://www.researchgate.net/publication/5850838_Escherichia_coli_STb_toxin_and_colibacillosis_Knowing_is_
  half_the_battle
• https://www.sciencedirect.com/topics/nursing-and-health-professions/methyl-red
• https://quizlet.com/289127010/micro-final-diagram/

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