Escherichia coli

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Eschericia coli (E. coli) overview

• 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:
    • Acute, frequently fatal enterotoxaemia of weaned pigs
    • Associated with particular haemolytic serotypes of E. coli
    • Verotoxin (Shiga toxin II e) released in the small intestine and carried in the bloodstream
    • Haemolysin production
    • Subcutaneous and subserosal oedema
    • Peracute disease affecting particulary healthy piglets
    • Mortality rate 30%-90%
    • Antimicrobial treatment effective if administered in time
  • Watery mouth of lambs:
    • Affects lambs under three days old
    • Lack of colostrum allows collonisation and overgrowth of E. coli in the small intestine
    • Systemic invasion by E. coli
    • Absorption of endotoxin leads to death
    • Severe depression, anorexia, salivation and abdominal distension
    • Morbidity and mortality high
  • Histiocytic ulcerative colitis in the dog and cat.
  • Causes peritonitis in dogs and peritonitis in pigs
  • Found in osteomyelitis
  • Associated with neonatal polyarthritis of calves
  • In deep pyoderma
  • Found in arthritis of horses

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
  • Immunuty:
    • 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

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