BT8 Bacterial Pathogens Escherichia coli PDF Past Paper 2020

Lecture title: BT8 Bacterial Pathogens Escherichia coli Lecturer : Prof. DL Gally Learning outcomes 1. To have been introduced to the different genera within the Enterobacteriaceae. 2. To understand that different types of E. coli can cause different types of disease. 3. To know that enterot...

Lecture title: BT8 Bacterial Pathogens Escherichia coli Lecturer : Prof. DL Gally Learning outcomes 1. To have been introduced to the different genera within the Enterobacteriaceae. 2. To understand that different types of E. coli can cause different types of disease. 3. To know that enterotoxigenic Escherichia coli (ETEC) cause gastrointestinal disease in many host animals through the production of specific adhesins and heat labile (LT) and/or heat stable toxins (ST). To know the mechanism of action of LT and ST. 4. To know that certain strains of E. coli (APEC) can cause systemic disease in poultry. 5. To know that certain E. coli strains can cause urinary tract infections, especially in cats and dogs, and to understand the importance of adherence and toxin expression in this infection 6. To learn that enteropathogenic and enterohaemorrhagic E. coli (EPEC and EHEC) can cause gastrointestinal infections in many host animals by injection of proteins into host cells via a type 3 secretion system. Zoonotic threat of EHEC from ruminants to humans Enterobacteriaceae The majority of bacteria in the mammalian gastrointestinal tract are obligate anaerobes such as Bacteroides and Fusobacterium spp. By contrast, the Enterobacteriaceae are a family of different bacterial genera so-named due to their association with the gastro-intestinal tract, of which some are considered a very minor part of the commensal flora. This family contains some of the most researched and characterised bacterial species that have acted as paradigms for our understanding of biochemical pathways, genetics and pathogenicity. This grouping of bacteria also contains some of the more important pathogens of animals and humans. Many carry plasmids encoding antibiotic resistance and inheritance of multiple drug resistance continues to be a serious problem among these and other bacteria. This family of bacteria are all Gram negative, non-sporing rods, facultative anaerobes, with many being motile via peritrichous flagella. Many grow readily on simple nutrient media and are unusual in not containing cytochrome C (cytochrome oxidase). Approximate identification of genera can be based on simple characteristics such as fermentation of lactose, glucose, motility and urease production. Common genera example species Escherichia spp. - E. coli Salmonella spp. - S. typhimurium Shigella spp. - S. dysenteriae Yersinia spp. - Y. pestis Klebsiella spp. - K. pneumoniae Proteus spp. - P. mirabilis  Lactose Citrate Urease Indole E. coli + - - + Klebsiella sp. + + + - Salmonella sp. - + - - P. mirabilis - + + + Yersinia sp. -/+ - +/- +/- Shigella sp. - - - +/- Escherichia coli Colonises most mammals shortly after birth and different strains displace one another as members of the normal intestinal flora. Many of these strains are commensals and cause no disease. Other strains are able to express factors that contribute to both intestinal and extra- intestinal disease. E. coli strains are classically identified by surface antigens, for example E. coli O157:K5:H7. The somatic O- antigen (part of the lipopolysaccharide-LPS), the capsule antigen (K), and the flagella antigen (H). Traditional serotyping is being replaced by sequencing methods. E. coli infections in animals As stated. E. coli strains can be associated with diarrhoeal diseases as well as diseases away from the GI tract, including urinary tract infections (UTIs). Examples include: Colibacillary diarrhoea - multiple hosts including: calves, lambs and piglets, one to three days after birth. Associated with enterotoxigenic Escherichia coli (ETEC), large numbers of ETEC bind to the mucosa of the small and large intestine. Release ST and LT toxins (see below). Diarrhoea: in older animals and humans due to different types of E. coli in addition to ETEC, for example enteropathogenic E. coli (EPEC) and verotoxigenic E. coli (VTEC). Systemic colibacillosis: Multiple animals including calves, lambs and poultry. Invasion of bacteria through the GIT, resulting in a generalised infection. This is more likely to happen if animals are colostrum deprived. Different types of E. coli can cause this but often express a capsule providing serum resistance. Colibacillary toxaemia in pigs: oedema disease: 1-2 weeks following weaning, A type of verocytotoxin-producing E. coli (VTEC) but with a special variant of the VT toxin-VT2e+. ‘Shock in weaner syndrome’ and ‘haemorrhagic colitis’. Urinary tract infections – especially in cats, dogs and humans. Strains associated with UTIs sometimes known as uropathogenic E. coli (UPEC). Environmental Mastitis: common, but usually mild. Why are different diseases caused by the ‘same’ organism? Primary reason is carriage of different virulence-associated genes by the different types (ETEC, VTEC, UPEC). Key ‘differential’ virulence factors are toxins, adhesins and other secreted ‘effector’ proteins. Virulence genes are transmitted on ‘mobile’ genetic elements: bacteriophages and plasmids. They can enter the chromosome as pathogenicity islands or be carried independently as plasmids. Enterotoxigenic E. coli (ETEC) These strains cause diarrhoea in many young animals including calves, lambs, pigs and dogs. Adhere primarily in the small intestine (jejunum and ileum) and multiply to high numbers. Bacterial adherence is via F4 (K88) and F5 (K99) fimbriae. F4 more commonly associated with pigs, F5 with calves. The key factors associated with diarrhoeal disease are the enterotoxins_ LT:(heat) labile toxin (88 KDa) and ST (heat) stable toxin. LT is generally present in porcine and human strains. It is a two component (A/B5) toxin. The B subunits bind to the GM1 ganglioside receptor, allowing entry of the active A subunit that ADP-ribosylates certain small G-proteins and prevents shut down of adenylate cyclase. This leads to ion efflux and influx deregulation and loss of fluid into the gut. ST (a and b variants) are a small single polypeptide that once internalised act on guanylate cyclase activity. This again also impacts on ion transfer and results in water loss leading to diarrhoea. Avian pathogenic E. coli (APEC) Avian colibacillosis is a serious welfare & productivity issue across the world in the commercial poultry sector. It is a major cause of mortality & reduced egg yield & quality. It is caused by a complex of E. coli types, some are most closely related to strains that can cause urinary tract infections in humans. Mostly due to serogroups O1, O2 & O78, but many others can be found. The infection often involves inflammation of visceral organs: peritonitis, salpingitis (oviduct), perihepatitis, pericarditis & sepsis Often secondary to respiratory infections, but also associated with stress (onset of lay) & poor ventilation. It is unclear how systemic E. coli infections are started. May involve translocation of APEC from the gut, depending on bacterial factors, host immune status &/or co-infections. Another important route is inhalation of contaminated faecal dust, especially if accompanied by viral (e.g. infectious bronchitis virus) or Mycoplasma infections, Can follow injury due to pecking as social hierarchy is formed. Anecdotal evidence of higher incidence in free-range systems. Some vaccines have been developed (and better ones required) now that antibiotic prophylaxis should not be used to control these infection. Urinary tract infections involving E. coli (UPEC) UTIs are a common infection, particularly in cats and dogs, with E. coli often identified associated with the infection. Most infections will clear, but some animals may be more prone to infections due to anatomical differences and/or genetic susceptibility. Infection often originates from strains present in the gastrointestinal tract. Many infections may be asymptomatic but others induce considerable inflammation in the urinary tract and therefore pain and discomfort for the patient. Simplistically, strains that cause UTIs have to be able to bind and persist in the tract. Some strains also secrete toxins, potentially to release nutrients from cells or to aid invasion. The more damage/pathology that a strain induces the more likely it will provoke a strong inflammatory response. If a strain causes inflammation in the bladder, then this is referred to as cystitis. If the kidneys are involved then this is referred to as pyelonephritis. UPEC need to adherence to epithelial or bladder (umbrella) cells using long fimbriae/pili such a P-pili or type 1 fimbriae. Flagella also aid colonisation. Damage to cells can be induced by release of toxins such as haemolysins or cytotoxic necrotising factor (CNF). E. coli strains associated with UTIs can carry multiple adhesins that are phase variable, so their expression can be reversibly switched off and on at the single cell level. This control may be important to avoid immune recognition and to allow expression of different bacterial surface factors by different individuals in the bacterial population – ‘don’t put all you eggs in one basket’! Verotoxigenic or Shiga-toxin producing E. coli (VTEC or STEC) If an E. coli strain is infected by a bacteriophage that carries a particular type of toxin (Verotoxin or Shiga toxin) and the phage becomes lysogenic, i.e. enters the chromosome, then that strain is now know as a verotoxigenic or Shiga toxigenic E. coli (VTEC or STEC). ( We will use the STEC terminology). These Shiga toxin- carrying phages appear to be very common in ruminants and the toxin may provide an advantage for E. coli in colonising the gastrointestinal epithelium of ruminants. However, these phages have also infected certain EPEC strain (see below) and when a strain can express both a type III secretion system and Shiga toxins then it can be very pathogenic to humans that are exposed. These are then known as human enterohaemorrhagic E. coli (EHEC) strains, a good example being EHEC O157 that has caused infection from contaminated meats, private water supplies, petting farms etc. EHEC O157:H7 colonises the terminal rectum of cattle BUT the Shiga- toxin does not have a serious negative impact on the animal as cattle do not express the Gb3 receptor for the toxin on their endothelial cells (that line blood vessels). However, humans do and so if EHEC colonise the human GIT and Shiga toxin is released then it can cause bloody diarrhoea followed by kidney and brain damage by inducing capillary damage at these sites. Our ongoing work is aimed at restricting EHEC O157 colonisation of ruminants, for example by vaccination) to therefore limit human exposure. Please see below for description of the type III secretion system that is also expressed by EPEC strains. Enteropathogenic E. coli. (EPEC) Cause diarrhoeal disease in many mammals including calves, pigs, cats, dogs and humans. Characteristic pathology as it induces the formation of attaching and effacing (A/E) lesions in the gastrointestinal tract. These lead to a reduction in the integrity of the epithelium and diarrhoea. Initial attachment driven by fimbrial adhesins, including the bundle-forming pilus. Bacteria inject their own receptor, the translocated intimin receptor (Tir) into the host cell via a type III protein secretion system. This binds to intimin on the bacterial surface resulting in intimate attachment. Further ‘effector’ proteins are injected into the host cell that for example can interfere with cellular water uptake leading to diarrhoea. Other secreted effector proteins alter cell signalling and lead to apoptosis of the epithelial cells, this induces inflammation with neutrophil influx to try and control the infection. Increased mucus secretion and sloughing off of epithelial cells are defences against pathogens colonising the gastrointestinal epithelium. Summary of E. coli enteric infections: Oral infection (faecal-oral cycle) Colonisation of intestinal mucosa requiring expression of specific adherence factors Toxin release and Brush border and Systemic infection pathology of toxin mucosal damage by via ‘permeable’ action, ETEC. intimate association of mucosa (often in bacteria and T3S of very young effectors EPEC/VTEC animals), APEC Fluid electrolyte Acute disease, imbalance, diarrhoea, Diarrhoea fever, endotoxic can be fatal shock, death