Escherichia coli


Eschericia coli (E. coli) overview

Ecoli.jpg
  • Member of Enterobacteriacae family of Gram-negative bacilli
  • Facultative anaerobe
  • One of predominant bacterial species in colonic flora
  • Colonisation of intestinal tract from environmental sources shortly after birth
  • Abundant in the environment
  • Most strains have low virulence
  • Found in many non-specific, endogenous infections, eg. wound infections, upper respiratory tract infections, infections of the urinary tract, mammary glands and uterus and septicaemia
  • An enteropathogen, causing neonatal diarrhoea in young animals and enteric colibacillosis
  • Enterotoxigenic E. coli is the most common cause of diarrhoea in calves, lambs and pigs
  • Pathogenic strains possess virulence factors allowing colonisation of mucosal surfaces


E. coli characteristics

  • Usually motile with flagella and fimbriae
  • Oxidase negative (do not possess cytochrome C oxidase)
  • Grow on MacConkey agar (in presence of bile salts), producing pink colonies
  • 'Haemolytic activity on blood agar characteristic of certain strains
  • Lactose fermenter
  • Reduce nitrates to nitrites and ferment glucose to produce acid and gas
  • Possess a lipopolysaccharide (O) antigen, a flagellate (H) antigen, polysaccharide capsule (K) antigens and fimbrial (F) antigens
  • Epidemiological typing of E. coli uses antigen combinations, eg. O125:K12:H42


Pathogenesis

  • Virulence factors include capsules, endotoxin, enterotoxins and colonisation factors
  • Capsular polysaccharides produced by some strains prevent phagocytosis and interfere with complement
  • Endotoxin is a lipolysaccharide component of the cell wall of Gram-negative bacteria, composed of lipid A, a core polysaccharide and various side chains
  • Endotoxin is realeased when bacteria die, and causes endothelial damage leading to disseminated intravascular coagulation and endotoxic shock; it is also a pyrogen
  • Enterotoxins, verotoxins and cytotoxic necrotising factors produced by many pathogenic E. coli; these produce cell damage at their site of action
  • Alpha-haemolysin may increase iron availability for invading organisms
  • Siderophores are made by certain pathogenic strains, and are responsible for iron aquisition; they include aerobactin and enterobactin

Extra-intestinal infection

  • Soft tissue infections in adult animals
  • Pathogenesis:
    • Produces an alpha-haemolysin which may be cytotoxic
    • Iron aquisition system
    • K antigens prevent phagocytosis or mimic host antigens and resist complement
    • Fimbriae permit adhesion to mucosal surfaces
    • May enter blood to cause septicaemia
  • Clinical infections:
    • Urogenital tract infections
      • Most common organism infecting urinary tract
      • Ascending infections of urinary tract
      • Causes pyometra in the dog and cat and pyelonephritis
      • Cystitis in the bitch
      • Prostatitis in dogs via opportunistic infection
      • Colonisation of mucosa aided by fimbriae
    • Mastitis
      • Opportunistic infection of mammary glands of sows and cows
      • Endotoxaemia in the acute form often fatal
      • Death within 24-48 hours during peracute disease
      • Animals depressed with sunken eyes
    • Colibacillosis:
      • Avian:
        • Septicaemia in newly-hatched chickens
        • Infection enters via faecal contamination of the egg surface or via the ovary of the hen
        • Infection enters via the respiratory tract
        • A bacteraemia develops
        • Acute colisepticaemia, subacute fibrinopurulent serositis or chronic granulomatous disease of the viscera
        • Occurs in older birds via inhalation of E. coli in dust; respiratory infection spreads to the blood to cause acute colisepticaemia
        • Airsacculitis, pericarditis and perihepatitis during acute phase
        • Often secondary to virus or mycoplasma infection or environmental stress
    • Colisepticaemia:
      • Systemic disease in young calves, piglets, foals, lambs
      • Penetration of intestinal mucosa and entrance into the blood
      • Invasive strains survive the host defences
      • Virulence related to adhesive properties, complement resistance and ability for iron aquisition
      • Ammonia, dust, viral infections and temperature changes enhance likelihood of disease
    • Oedema disease of pigs:

Intestinal infection

  • E. coli is part of the flora of the large intestine, but is not usually found in the small intestine
  • Some strains possess fimbrae which attach the bacteria to the small intestinal epithelium of particular animal species
  • E. coli may cause diarrhoea via attaching and effacing lesions, where bacteria adhere intimately to the enterocyte, and cause localised effacement of the brush border microvilli; the epithelial erosion causes dysentery


  • Enterotoxigenic E. coli (ETEC):
    • General:
      • Contributes to undifferentiated neonatal calf diarrhoea, a mixed viral enteritis in calves, also known as enteric colibacillosis
      • Causes scours in pigs, calves and lambs
      • 'Traveller's diarrhoea' in humans
    • Pathogenesis:
      • Oral infection, intestinal colonisation and toxin production
      • Fimbrial antigen or colonisation factor antigens (CFAs)determine species specificity
      • Fimbrial adhesins allow bacteria to attach to mucosal surfacesin the small intestine and lower urinary tract; this prevents expulsion by peristalsis and flushing of urine
      • K88 (F4) is associated with adhesion to the small intestinal mucosa of pigs
      • K99 (F5) associated with adhesion in pigs and cattle (these fimbrial adhesins were originally thought to be capsular (K) antigens)
      • The fimbriae are encoded by plasmids
      • These strains carry a plasmid which encodes an enterotoxin
      • Two types of enterotoxin: heat-labile (LT) and heat-stable (ST) toxins
      • The plasmids which produce these toxins are responsible for the pathogenicity of these strains
        • LT
          • An oligometric toxin composed of an enzymatically-active A subunit (30KDa; 2 fragments - A1 and A2) and 5 identical B subunits (12KDa) forming the binding portion (B oligomer)
          • It attaches to the brush border of the epithelial cells of the small intestine
          • Causes ADP-ribosylation of the stimulatory subunit of guanine nucleotide binding proteins of the adenylate cyclase complex in eukaryotic cell membranes
          • This causes irreversible activation of adenylate cyclase in target cells
          • This raises the cAMP level and causes hypersecretion of water and chloride ions into the lumen of the small intestine and inhibits reabsorption of sodium
        • ST
          • Activates guanylate cyclase in enteric epithelial cells, stimulating fluid secretion
    • Clinical signs:
      • The gut becomes distended with fluid and a secretory diarrhoea which lasts several days results
      • Watery diarrhoea, dehydration, acidosis, death
    • Immunity:
      • LT is antigenic
      • Immunity is developed via production of antibody to LT protein and fimbrial antigen
      • Parenteral vaccination of pigs and cattle protects offspring from scours via antibody production in the colostrum (passive immunity)
      • ST is not immunogenic; it is small, with only 19 amino acids


  • Enteropathogenic E. coli (EPEC):
    • Attaching and effacing strains of E. coli
    • Attach to small intestinal epithelial cells and cause necrosis of enterocytes and stunting and fusion of villi
    • Possess E. coli adherence factor plasmid
    • An adhesin, intimin is required for attachment to enterocytes
    • Secrete signalling proteins that activate a tyrosine kinase, causing rearrangement of cytoskeletal proteins and effacement of microvilli
    • Intracellular calcium levels increase and production of protein kinase C causes loss of chloride ions and water from the intestinal epithelial cells
    • Diarrhoea results


  • Enteroinvasive E. coli:
    • Cause colisepticaemia in calves during their first week of life; occasionally in lambs, piglets and puppies
    • Infection via ingestion or umbilicus; inadequate colostrum increases susceptibility
    • Invade epithelial cells of small intestine by inducing endocytosis
    • Traverse gut wall to lamina propria and enter lymphatics
    • Resistant to complement-mediated killing
    • Bacteraemia or septicaemia and endotoxaemia
    • Widespread petechial haemorrhages of organs and serosa
    • Abscesses, pneumonia in long term
    • Death occurs in absense of treatment


  • Enterohaemorrhagic E. coli:
    • Possibly carried by cattle
    • Produce shiga-like toxin, a vero toxin
    • Plasmid-coded fimbriae important for virulence
    • Intimin produced allowing intimate attachment to intestinal epithelial cells
    • Strains do not product LT or ST and are not enteroinvasive
    • Attaching and effacing lesions, unrelated to toxin production
    • Disseminated intravascular coagulation and thrombus formation
    • E. coli O157:H7 causes haemorrhagic collitis-haemolytic uraemic syndrome in humans


  • Cytotoxin necrotising factor-producing E. coli
    • Infrequently cause diarrhoea in calves, pigs and humans
    • Important virulence factors include toxin and fimbriae

in cattle

  • Neonatal polyarthritis:
    • Coliforms
      • Localises in joints and meninges in severe non-fatal neonatal colibacillosis
      • May remain as chronic arthritis in larger joints