微生物学美国IndianaUniversityPurdueUniversity授课12

– Some are always associated with disease
• e.g., Shigella, Salmonella, Yersinia pestis
– Some are normal flora that can become opportunistic pathogens
• e.g., E. coli, K. pneumoniae, P. mirabilis
• Infants< 1 year affected
– Enterohemorrhagic (EHEC; hemorrhagic colitis)
• Produces cytotoxin (verotoxin) • Severe abdominal pain, bloody diarrhea, little or no
• Can involve all body sites • 5% hospitalized patients develop nosocomial
infections, primarily caused by Enterobacteriaceae
such as Escherichia
• Sites of infection
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Virulence Factors
• Capsular K
– Either protein or polysaccharide
• Heat-labile • May interfere with detection of “O” • Removed by boiling organisms
• Cross reactions
– E. coli K1 with N. meningitidis and Haemophilus meningitidis
– K. pneumoniae with S. pneunomiae
– Organisms with specific antigens have been associated with increased virulence
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Enterobacteriaceae
• Epidemiology
– Animal reservoir: most Salmonella infections – Human carrier: Salmonella typhi, Shigella
– Endogenous spread in a susceptible patient
– Somatic “O” LPS
• Major cell wall Ag; heat stable • Specific “O” antigens associated with each genus;
however, cross reactions are common
– Salmonella and Citrobacter – Escherichia and Shigella
• Role that Ag’s play in these diseases is not clear
– Some capsular Ag are poor immunogens
• Protect against antibody-mediated phagocytosis
– Flagellar Ag probably play a role in adherence
normal flora
• Bacterial sepsis (multiplication in blood)
– Primary focus-infection of urinary tract or spread from gastrointestinal tract
– Death can occur in immunocompromised patients and infections resulting from intestinal perforation
(e.g., E. coli K1 with neonatal meningitis)
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Virulence Factors
• Flagella H
– Heat-labile proteins
• Can be absent or undergo antigenic variation (present in two phases)
– Enteropathogenic (EPEC; childhood diarrhea)
• Organism adheres to enterocyte plasma membrane and causes destruction of microvilli producing watery diarrhea
is common; disease is not
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Pathogenesis of Escherichia
• Urinary tract infections
(80% community and most nosocomial)
– Originate from gastrointestinal tract – Important virulence factors
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Microbial Physiology and Structure
• Cell morphology
– Moderate-sized Gram— rods
• th peritrichous flagella) or non-motile
• Specific H antigens assocated with disease
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Virulence Factors
• General role in pathogenesis of “O,” “K,” and “H” antigens
– Specific antigens associated with meningitis, gastroenteritis, and urinary tract infections
• Resistance to serum-killing • Production of hemolysins
• Pili-mediated binding (not demonstrated in vivo)
• Production of slime layer that participates in cell adhesion
– Physiology
• All are facultative anaerobes • Simple nutritional requirements:
– Ferment glucose – Reduce nitrates to nitrites
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Distinguishing Characteristics
– Capsular antigen of Salmonella typhi referred
to as Vi antigen
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Virulence Factors
• Capsular K, continued
– Shared by different genera both inside and outside of family
stable exotoxins (activates guanylate cyclase and
stimulates secretion of fluid)
– Both are coded from plasmid-borne genes
• World-wide:both adults and children
– Incubation 1-2 days; persists 3-4 days
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– Mild symptoms, including cramps, nausea, vomiting,
watery diahrrea 16
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Pathogenesis of Escherichia
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Pathogenesis of Escherichia
• Gastroenteritis
(countries with poor hygiene)
– Enterotoxigenic (ETEC)
• Mediated by heat-labile (like cholera) and heat-
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Virulence Factors
• Pili
– Attachment to host cells
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Pathogenesis of Escherichia
• E. coli present in gastrointestinal tract in
flora in this group
• Eosin Methylene Blue (EMB)
– Lactose, eosinY, methylene blue; Lac+; grow with green sheen
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Virulence Factors
• Antigens
• Gastroenteritis, continued
– Enteroinvasive (EIEC)
• Invade and destroy colonic epithelium
– Fever and cramps with blood and leukocytes in stool – Uncommon; often food-borne
from other lactose— Enterobacteriaceae
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Distinguishing Characteristics
• Resistance to bile salts
– Separate Shigella and Salmonella from normal
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Pathogenesis of Escherichia
• Neonatal meningitis
– E. coli and group B streptococci most common – 75% E. coli possess Capsular K1 antigen – Colonization of infants with E. coli at delivery
• Oxidase¯:
– Distinguishes among other fermentative and non-fermentative Gram— bacilli
• Lactose fermentation (red colonies on MacConkey agar)
– Separate Escherichia, Klebsiella, Enterobacter
Medical Microbiology
Enterobacteriaceae
BIOL 533 Lecture 12
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Enterobacteriaceae
• Diversity of species
• Ecology
– Found worldwide in soil, water, vegetation, and microbial flora of animals and humans
– Adhesiveness mediated by plasmid-encoded pili
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Pathogenesis of Escherichia
• Gastroenteritis, continued
– Enteropathogenic (continued)