Lec#20 .pptx

Lecture #20- E. coli Coloured scanning electron micrograph (SEM) of Escherichia coli (green) taken from the small intestine of a child. E. coli are Gram-negative rod-shaped bacteria that are part of the normal flora of the human gut. However some strains, including this one, produce a toxin that ca...

Lecture #20- E. coli Coloured scanning electron micrograph (SEM) of Escherichia coli (green) taken from the small intestine of a child. E. coli are Gram-negative rod-shaped bacteria that are part of the normal flora of the human gut. However some strains, including this one, produce a toxin that causes severe diarrhoea and can be fatal, especially in the very young or elderly. Lecture Overview Escherichia coli is probably one of the most well-known bacteria. In this lecture we will look at the 5 different virotypes ot E. coli. Additionally we will look at the way that E. coli strains are identified. Lecture Objectives After this lecture and reading appropriate portions of the text you should be able to identify the 5 different virotypes of E. coli. You should also be aware of the role that interspecific horizontal gene transfer plays in pathogen evolution. Microorganisms and Concepts Escherichia coli Salmonella sp. Proteus sp. Enterobacter sp. Shigella sp. • Proteobacteria • classification of diseases in E. coli • antigenic classification • plasmid exchange between bacterial “species” Gram- stain (pink) This photo shows a half eaten hamburger that was infected with E. coli. The young man that ate it become very ill, but survived. Rod-Shaped Bacterium, hemorrhagic E. coli, strain 0157:H7 -Purple bacteria (Proteobacteria) -largest and most diverse of all Bacteria -E.coli belongs to enteric bacteria within the gamma group of Proteobacteria Key genera: Escherichia, Salmonella, Proteus, Enterobacter, Shigella -live in the intestinal tracts of animals -characteristics: gram-, nonsporulating rods, non-motile or motile by peritrichous flagella, facultative aerobes, ferment sugars to various end products- most can ferment lactose -strains pathogenic to humans animals and plants -E. coli is the best known prokaryote -mixed acid fermentation in E. coli -ability to ferment lactose; enteric bacteria all ferment lactose to acid end products while similar Gram-negative bacteria cannot ferment lactose Lactose -under anaerobic conditions three acids are formed in significant amounts: acetic, lactic and succinic. Ethanol, CO2 and H2 are also formed. E. coli colonies on MacConkey Agar. Lactose fermenters are red. Contains, amongst other ingredients, lactose and neutral red. Below pH 7 the dye turns dark red. -E. coli is an indicator of fecal pollution of drinking water supplies, swimming beaches, foods, etc. -E. coli is the most studied of all organisms in biology because of its occurrence, and the ease and speed of growing the bacteria in the laboratory. -The GI tract of most warm-blooded animals is colonized by E. coli within a few hours or days after birth -The human bowel is usually colonized within 40 hours of birth. E. coli can adhere to the mucus overlying the large intestine. -most strains of E. coli are harmless commensals HOWEVER some strains are virulent pathogens. Pathogenic E. coli Invade Attach but do not invade Toxins 1. ETEC 4.EAggEC No Toxins 2. EIEC 3. EPEC Toxins 5. EHEC O157:H7 Intestinal Diseases Caused by E. coli -As a pathogen, E. coli, of course, is best known for its ability to cause intestinal diseases. -Five classes (virotypes) of E. coli that cause diarrheal diseases are now recognized: 1. enterotoxigenic E. coli (ETEC) 2. enteroinvasive E. coli (EIEC) 3. enterohemorrhagic E. coli (EHEC) 4. enteropathogenic E. coli (EPEC) 5. enteroaggregative E. coli (EAggEC) and one uropathogenic virotype (UPEC). -Each class falls within a serological subgroup and manifests distinct features in pathogenesis. 14 Pathogenic E. coli Strains • enterotoxigenic E. coli (ETEC) – produces one or both enterotoxins responsible for diarrhea – distinguished by their heat stability • enteroinvasive E. coli (EIEC) – multiplies within intestinal epithelial cells – may produce a cytotoxin and an enterotoxin • enteropathogenic E. coli (EPEC) – causes effacing lesions • caused by destruction of brush border microvilli on intestinal epithelial cells 15 Pathogenic E. coli Strains… • enterohemorrhagic E. coli (EHEC) – produces effacing lesions, leading to hemorrhagic colitis – releases shiga-like toxins • implicated in hemolytic uremic syndrome and thrombotic thrombocytopenic purpura – e.g., E. coli 0157:H7 16 1.Enterotoxigenic E. coli (ETEC): causative agent of diarrhea (without fever) in humans, pigs, sheep, goats, cattle, dogs, and horses. • produces two proteinaceous enterotoxins: the larger of the two proteins, LT enterotoxin, is similar to cholera toxin in structure and function, while the smaller protein, ST enterotoxin causes cGMP accumulation in the target cells and a subsequent secretion of fluid and electrolytes into the intestinal lumen. • non-invasive, and they do not leave the intestinal lumen. ETEC is the leading bacterial cause of diarrhea in children in the developing world, as well as the most common cause of traveler's diarrhea. Each year, ETEC causes more than 200 million cases of diarrhea and 380,000 deaths, mostly in children in developing countries.[18] 2. ･ Enteroinvasive E. coli (EIEC): found only in humans. EIEC infection causes a syndrome that is identical to Shigellosis, with profuse diarrhea and high fever. • EIEC are highly invasive, and they utilize adhesin proteins to bind to and enter intestinal cells. They produce no toxins, but severely damage the intestinal wall through mechanical cell destruction. 3.Enteropathogenic E. coli (EPEC): causative agent of diarrhea in humans, rabbits, dogs, cats and horses. Like ETEC, EPEC also causes diarrhea, but the molecular mechanisms of colonization and etiology are different. • EPEC lack fimbriae, ST and LT toxins, but they utilize an adhesin known as intimin to bind host intestinal cells. This virotype has an array of virulence factors that are similar to those found in Shigella, and may possess a shiga toxin. • Adherence to the intestinal mucosa causes a rearrangement of actin in the host cell, causing significant deformation. EPEC cells are moderatelyinvasive (i.e. they enter host cells) and elicit an inflammatory response. 4. Enteroaggregative E. coli (EAggEC): found only in humans. So named because they have fimbriae which aggregate tissue culture cells, EAggEC bind to the intestinal mucosa to cause watery diarrhea without fever. • EAggEC are non-invasive. They produce a hemolysin and an ST enterotoxin similar to that of ETEC. 5. ･ Enterohemorrhagic E. coli (EHEC): found in humans, cattle, and goats. The sole member of this virotype is strain O157:H7, which causes bloody diarrhea and no fever. • EHEC can cause hemolytic-uremic syndrome and sudden kidney failure. It uses bacterial fimbriae for attachment, is moderately-invasive and possesses a phage-encoded Shiga toxin that can elicit an intense inflammatory response. -EHEC has recently been recognized as a cause of serious disease (e.g. Walkerton, ON and the Jack-inthe-Box outbreak in the Northwest USA- XL Foods Brooks, AB). -produces Shiga-like toxin (Vero toxin) probably acquired by plasmid transfer from a strain of Shigella. What does O157:H7 mean? Somatic antigens (O)- LPS of gram- bacteria Flagellar antigens (H) Capsular antigens (K) About 170 O antigens and 55 H antigens Over 900 strains identified - E. coli O157:H7 produces one or two toxins (called Stx1 and Stx2) that are closely related to the toxin produced by Shigella dysenteriae type 1. Thus, these toxins are called Shiga-like toxins or simply SLT's. -Recent studies suggest that the toxins damage the cells that line the walls of blood vessels in certain organs. -E. coli O157:H7 is also able to attach itself to the cells that line the large intestine. The ability to attach to intestinal cells is thought to be essential to the organism's ability to cause disease. -E. coli O157:H7 is able to attach to intestinal cells because of a specific gene, called the eae gene (eae stands for "E. coli attaching and effacing"). Phenol-red lactose broth How is E. coli O157:H7 spread? -The organism can be found on a small number of cattle farms and can live in the intestines of healthy cattle. -Meat can become contaminated during slaughter, and organisms can be thoroughly mixed into beef when it is ground. -Bacteria present on the cow's udders or on equipment may get into raw milk. -Eating meat, especially ground beef, that has not been cooked sufficiently to kill E. coli O157:H7 can cause infection. -Contaminated meat looks and smells normal. Although the number of organisms required to cause disease is not known, it is suspected to be very small. https://www.youtube.com/watch?v=j87XBrt3Vy0 How did E. coli evolve pathogenicity determinants? Adhesins Invasins hemolysins siderophores Shigella-like "invasins" for intracellular invasion and spread Motility/chemotaxis Toxins Antiphagocytic surface properties Defense against serum bactericidal reaction Defense against immune responses Genetic attributes Http://www.Youtube.Com/watch?V=fkcd3ejdze4 -plasmids are genetic elements that replicate independently of the chromosome -literally thousands of different types of plasmids -300 naturally occurring plasmids from E. coli alone -phenotypes conferred by plasmids include: antibiotic production, antibiotic resistance, heavy metal resistance, degradation of herbicide and other organics, virulence factors such as coagulase, hemolysin. or other bacterial species If bacteria can transfer DNA (i.e. have sex) between different species then what defines a bacterial species?