Enteric Infections and Pathogenesis of Infectious Diarrhoea PDF

Enteric Infections and Pathogenesis of Infective Diarrhoea Dr Liam O’Connor School of Medicine University of Western Australia Enteric infections Outline: • The range of enteric pathogens • Outcomes of enteric infections • World impact of diarrhoeal disease • Definition of diarrhoea • Str...

Enteric Infections and Pathogenesis of Infective Diarrhoea Dr Liam O’Connor School of Medicine University of Western Australia Enteric infections Outline: • The range of enteric pathogens • Outcomes of enteric infections • World impact of diarrhoeal disease • Definition of diarrhoea • Structure of small & large intestine • Water flux in the intestine - secretion - absorption Mechanisms of diarrhoea production - types of diarrhoea - virulence factors of diarrhoeal pathogens Two examples - secretory diarrhoea – Vibrio cholerae - inflammatory diarrhoea - Shigella • • Range of enteric pathogens Bacteria Viruses Protozoa Helminths Range of enteric pathogens Bacteria Range of enteric pathogens Viruses • • • • • Rotavirus Norovirus Adenovirus Enterovirus Astrovirus Range of enteric pathogens Protozoa • • • • • • • • Giardia intestinalis Entamoeba histolytica Cryptosporidium parvum Cyclospora cayetanensis Isospora belli Microsporidia Dientamoeba fragilis (Blastocystis hominis) Range of enteric pathogens Helminths • • • • • • • • • • • • Enterobius vermicularis Trichuris trichiura Strongyloides stercoralis Necator americanus Ancylostoma duodenale Ascaris lumbricoides Taenia saginata Taenia solium Hymenolepis nana Hymenolepis diminuta Diphyllobothrium latum Fasciolopsis buski Outcomes of enteric infections Inconvenience Serious impact Healthy host Impaired host • immunocompromised • malnourished • extremes of age Death Outcomes of enteric infections Outcomes will vary according to the: • type of organism • infective load • acute, chronic or repeat infection • host factors, eg: - status of immune system - nutritional & general health status - age Significance very different in developing world compared to developed world. Outcomes of enteric infections Enteric infection Diarrhoea Dehydration Malabsorption (+ loss of electrolytes) Repeat infections: If severe, death • • • • malnutrition growth stunting cognitive impairment * impaired immunity (+ vaccine hyporesponsiveness) * Calculated that repeated bouts of diarrhoea in the first 2 years of life can lead to a loss of 10 IQ points World impact of diarrhoea • 1.5 million people died in 2019 from diarrhoea related illness • 34% of these are children under 5 (2017 data) World impact of diarrhoea % of all diarrhoeal deaths Worldwide deaths due to diarrhoea, by age, 2017 Age World impact of diarrhoea World impact of diarrhoea World impact of diarrhoea Diarrhoeal death rates in children <5 yrs per 100,000 against per capita income (2017) Definition of diarrhoea Increased stool water causing: • an increase in stool frequency • or the passage of soft stools Passage of greater than 3 stools per day or a stool volume of greater than 200mls Regardless of definition, generally results from increased water in the stool. Definition of diarrhoea Acute diarrhoea: • lasting less than 2 weeks Chronic diarrhoea • lasting more than 2 weeks Note: Not all diarrhoea is caused by infection. This lecture will discuss infectious diarrhoea. Structure of small intestine Small intestine: 6m long in adult human • duodenum • jejunum (~40% of length) • ileum (~60% of length) Absorbs • all amino acids • all sugars • most lipid • 80% of water Structure of small intestine Small intestinal length is 6 m. Surface area is 250 m2. This is achieved by: • mucosal folds • villi and crypts • microvilli on enterocytes Structure of small intestine Structure of small intestine Large intestinal structure The large intestine lacks folds or villi. Surface area 995 cm2 due to crypts It is a site of water absorption - residual water arriving from small intestine Dependent on Na+ absorption as in small intestine Water flux in the intestine The small intestine has a massive capacity to secrete and absorb water. Input • Daily oral intake • Salivary, gastric, biliary, pancreatic secretions Reabsorption • Small intestine • Colon Excretion in stool 9.0 L 2.0 L 7.0 L 8.8 L 7.5 L 1.3 L 0.2 L Water flux in the intestine The 8.8 L of water reabsorbed by the small intestine represents only the net water movement. The bidirectional flux of water in the small intestine is ~50 L per day. • water secretion into lumen • water absorption from lumen Even a small disturbance of this flux can result in diarrrhoea. Water flux in the intestine Water in the intestine follows osmotic gradients. Net water movement results from the movement of osmotically active molecules, eg. • electrolytes • sugars • amino acids Water flux in the small intestine - secretion Two distinct processes establish an osmotic gradient which pulls water into the small intestinal lumen: 1. Digestion of food into small molecules of high osmolarity. 2. Active secretion of electrolytes by crypt enterocytes Water flux in the small intestine - secretion 1. Digestion of food into small molecules of high osmolarity. Protein & starch Digestion into smaller molecules Absorption of small molecules relative osmolarity Water flux in the small intestine - secretion 2. Active secretion of electrolytes by crypt enterocytes SUBMUCOSA BOWEL LUMEN Cl cAMP + Na ATP ENTEROCYTE Adenyl cyclase enzyme is activated, ATP to cyclic AMP Water flux in the small intestine - secretion 2. Active secretion of electrolytes by crypt enterocytes Cl - Cl - cAMP + Na ATP Cyclic AMP activates the transmembrane chloride channel. Chloride pumped into lumen. Water flux in the small intestine - secretion 2. Active secretion of electrolytes by crypt enterocytes Electric potential attracts sodium. Net secretion of NaCl Cl - Cl Cl - Na+ Cl Cl - Cl Na+ Na+ - Na+ Cl Na+ Water flux in the small intestine - secretion 2. Active secretion of electrolytes by crypt enterocytes Na+Cl Na+Cl H2O H2O Na+Cl - H2O Na+Cl Water drawn H2O Na+Cl - into lumen by NaCl osmotic gradient. Water flux in the small intestine - absorption Sodium dependant hexose transporter: “coupled” transport of Na+ and glucose. Gl + Na Gl Gl + Na + + Na Na Gl Water flux in the small intestine - absorption Gl + Na Gl Gl + Na + + Na Na Gl Water flux in the small intestine - absorption Gl + Na Gl Gl + Na + + Na Na Gl Water flux in the small intestine - absorption + Gl Na Gl + Na Gl + Gl + + Na Na Gl Na Gl Gl Gl Gl + Na Gl + + Na + + Na + Na Gl Na Gl Gl + Na + Na Na Gl Water flux in the small intestine - absorption Gl + Na Gl Gl + Gl Na Na + Na Gl Gl + Na Gl + + Gl Na + + Gl Gl + Na Gl Gl Gl + Na Na + Na + Na + Na Na Gl Water flux in the small intestine - absorption Sodium removed from cell, extracellular concentration + Na + Na Gl + Na + + Na + Na Na + Na + Na Gl + Na + Na + Na + Gl + Na Na + + Na + Na + Na + Na + Na Na Gl + Na Water flux in the small intestine - absorption This high sodium concentration is largely responsible for absorption of water by the small intestine. + Na + Na + Na + Na + Na + + Na Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + + Na Na H2O + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + + + + Na + Na Na Na + Na Na Water flux in the small intestine - absorption As demonstrated, the absorption of water is linked to the coupled absorption of sodium + glucose. Water absorption can be increased by presenting the enterocytes with more sodium & glucose. This is the basis of oral rehydration solutions which have saved millions of lives. Mechanisms of diarrhoea production 1. Increased active secretion of electrolytes and thus water 2. Damage to brush border resulting in malabsorption of nutrients and electrolytes 3. Damage to brush border, loss of disaccharidase activity, increased osmolality of stool 4. Altered motility, less time for water absorption Mechanisms of diarrhoea production In broader terms, diarrhoea may be classified as: • • • • inflammatory secretory osmotic motility related The type of diarrhoea is dependant upon the virulence factors expressed by the pathogen. Mechanisms of diarrhoea production Virulence factors of enteric pathogens can be considered under the following headings: 1. Virulence factors enabling adhesion 2. Virulence factors enabling invasion 3. Toxins - enterotoxins - cytotoxins - neurotoxins Mechanisms of diarrhoea production Bacterial toxins Enterotoxins • Interfere with salt and water transport by enterocytes, resulting in net loss of water. Enterocytes are not damaged. Cytotoxins • Cause damage to the cells of the intestine. Neurotoxins • Central or local nervous system stimulation causing increased intestinal motility. Mechanisms of diarrhoea production Bacterial toxins Toxins may be preformed in food or produced by bacteria after ingestion. Preformed toxin disease usually has short incubation period. Toxins may be heat labile or heat stable (destroyed or not destroyed by cooking). Secretory diarrhoea – Vibrio cholerae Vibrio cholerae – curved Gram negative rod Some strains produce a powerful enterotoxin. Profuse watery diarrhoea. Severe dehydration, may cause death within hours in malnourished, debilitated hosts. Secretory diarrhoea – Vibrio cholerae Ingested vibrios are largely killed by stomach acid. Those that survive adhere to small intestinal enterocytes. Cholera toxin is secreted. The B subunit binds the toxin to specific receptors on the enterocytes. The A subunit enters the enterocyte. Secretory diarrhoea – Vibrio cholerae Vibrio cholera (does not invade host) Cl - Toxin ENTEROCYTE BOWEL LUMEN Secretory diarrhoea – Vibrio cholerae “A” subunit of cholera toxin enters cytoplasm and alters molecules which enhance adenyl cyclase activity( ). Cl cAMP ATP Overproduction of cyclic AMP results. Secretory diarrhoea – Vibrio cholerae Overstimulation of transmembrane chloride channel. Cl cAMP ATP Cl - Cl Cl - Cl Cl - Massive amounts Cl of chloride pumped Cl Cl into lumen. Secretory diarrhoea – Vibrio cholerae Overstimulation of transmembrane chloride channel. Cl + - cAMP Cl - Cl Cl - Cl Cl - Cl Cl Cl - Massive amounts of chloride pumped into lumen. Na ATP Na+ Na+ Na+ Na+ Followed by Na+ and other electrolytes Secretory diarrhoea – Vibrio cholerae HO HO HO HO Cl Na+ H O- Cl H O Cl Cl H O Na+ Cl H O Cl +H O Na HO Na+H O HO HO 2 2 2 HO HO HO HO HO HO HO HO HO HO 2 2 2 2 2 2 2 2 2 2 2 Massive amounts of water follow the sodium & chloride ions. 2 2 2 2 2 2 2 2 Cl - 2 Note that the enterocytes are not damaged by this process, allowing effective therapy with ORS. Secretory diarrhoea – Vibrio cholerae H2O + Na H2O Gl H2O + Gl H2O Gl H2O Gl + Na H2O Gl + Na H2O H2O H2O H2O H2O + Na Gl H2O H2O H2O Gl Na Gl H2O H2O + H2O H2O H2O H2O Gl H2O Na H2O + Na Gl H2O H2O Gl H2O H2O + Na + Na H2O Gl H2O H2O The small intestine can be forced to absorb water by presenting extra Na+ and glucose to the sodium dependant hexose transporter: (coupled transport of Na+ and glucose). Secretory diarrhoea – Vibrio cholerae Parents can be taught to make up oral rehydration solutions from available products and instructed how to administer it. Refer to: http://rehydrate.org/solutions/homemade.htm Inflammatory diarrhoea - Shigella Shigella – Gram negative rod, member of the Enterobacteriaceae Four species: • S. sonnei • S. dysenteriae • S. flexneri • S. boydii Cause of bacillary dysentery, an inflammatory colitis. Inflammatory diarrhoea - Shigella LUMEN Shigella bacteria in large bowel lumen Enterocyte M cell SUBMUCOSA Enterocyte Inflammatory diarrhoea - Shigella Parasite directed endocytosis The bacteria stimulate the M cell to engulf and internalise them. Inflammatory diarrhoea - Shigella Virulence plasmids encode Invasion Plasmid Antigens, • IpaA • IpaB • IpaC • IpaD Upon contact with host cell, Ipas are secreted and enter the host cytoplasm. Inflammatory diarrhoea - Shigella Ipa proteins cause the cell cytoskeleton to form filopods which engulf the bacterium. Inflammatory diarrhoea - Shigella The bacterium is carried through the cytoplasm in the resulting endocytic vacuole. Inflammatory diarrhoea - Shigella LUMEN The bacterium is released into the sub-epithelial tissue … Inflammatory diarrhoea - Shigella LUMEN … where it is engulfed by macrophages. Inflammatory diarrhoea - Shigella LUMEN Macrophage Inflammatory diarrhoea - Shigella LUMEN Macrophage IL-1 IL-1 Inflammatory diarrhoea - Shigella LUMEN The macrophages are stimulated to produce interleukin-1 which attracts PMNLs into the area. Macrophage IL-1 IL-1 PMNLs Inflammatory diarrhoea - Shigella LUMEN Shigella is not killed by the macrophages, which die. Macrophage PMNLs Inflammatory diarrhoea - Shigella LUMEN PMNLs Inflammatory diarrhoea - Shigella LUMEN PMNLs Ipa proteins stimulate endocytosis by the basilar membranes of the enterocytes. Inflammatory diarrhoea - Shigella LUMEN PMNLs Inflammatory diarrhoea - Shigella LUMEN PMNLs Inflammatory diarrhoea - Shigella icsA Genes on the plasmids encode outer membrane proteins (icsA) which cause condensation of host cell actin behind the bacterial cell. Inflammatory diarrhoea - Shigella LUMEN The actin propels the bacteria through the cytoplasm and into adjoining cells. PMNLs Inflammatory diarrhoea - Shigella LUMEN Damage to cells: • • PMNLs from inflammatory process from cytotoxins eg. Shiga toxin (Stx) Inflammatory diarrhoea. Blood & pus = dysentery Inflammatory diarrhoea - Shigella Depending on the species of Shigella involved, symptoms of infection may include: • • • • • • • prodromal upper intestinal watery diarrhoea (toxin?) frequent scanty dysenteric stools blood pus mucus tenesmus significant protein loss (malnutrition, impaired immunity) Inflammatory diarrhoea - Shigella RBCs PMNLs Inflammatory diarrhoea - Shigella Mechanisms of diarrhoea production Pathogenic mechanism Mode of action Examples Neurotoxin Action on autonomic nervous system; Increased motility S. aureus enterotoxin B Clostridium botulinum Bacillus cereus Enterotoxin Increased fluid secretion without mucosal damage Vibrio cholerae ETEC Salmonella spp. Campylobacter spp. Clostridium perfringens type A Cytotoxin Damage to intestinal mucosal cells Shigella spp. Salmonella spp. Campylobacter spp. Clostridium difficile toxin B Mucosal invasion Penetration and destruction of mucosal intestinal cells Shigella spp. Campylobacter spp. EIEC Yersinia enterocolitica