Escherichia coli

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:

- 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
- 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