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Escherichia coli

Shigella spp.

Salmonella enterica

Yersinia spp.

large group of medically important Gram-negative, facultative anaerobic bacilli and coccobacilli

many are normal inhabitants of the human colon, some are pathogens and not part of the normal flora

Salmonella spp., Shigella spp., and Yersinia spp.

many are also opportunistic pathogens


Escherichia coli

part of normal flora of colon

frequent cause of urinary tract infections, uropathogenic E. coli (UPEC)

most common cause of Traveler's diarrhea


five types of Escherichia coli that cause diseaes

enterotoxigenic E. coli (ETEC)

enterohemorrhagic E. coli (EHEC)

enteropathogenic E. coli (EPEC)

enteroinvasive E coli (EIEC)

enteroaggregative E. coli (EAEC)


Escherichia coli determinants of pathogenicity

alpha-hemolysin - a pore forming toxin

aeorbactin - iron siderophore

polysaccharide capsule - reduces phagocytosis (K1 capsule associated with meningitis and bacteremia)

pili - type I binding int he bladder and P pili binding in the upper urinary tract and causes pyelonephritis






heat labile toxin (LT)

  • A-B toxin
  • similar to cholera toxin
  • stimulates adenylate cyclase in gut epithelial cells
  • plasmid encoded

heat stable toxin (ST)

  • stimulates guanylate cyclase in gut epithelial cells
  • plasmid encoded

causes Traveler's Diarrhea, spread through contaminated food and water, 24-72 hours incubation period

Escherichia coli



Bundle-forming pili (BFP)

  • initial attachment to intestinal epithelium


  • bacterial surface protein
  • adhesin

Type II secretion system

Escherichia coli


Type III secretion systems

directly inject bacterial proteins int ocytoplasm of host cells

secretion apparatus in the bacterial cell envelope, secretes toxins to the exterior of the bacterium

translocation complex inserts into the host cell membrane and translocates toxins into the host cell cytoplasm

effector proteins are toxins

Escherichia coli


pathogens that harber type III secretion systems

P. aeruginosa








EPEC Type III Secretion

cause attachment and effacement lesions due to loss of normal microvilli structure and pedestal formation

secretes a number of proteins including Tir, which inserts into the host cell membrane and acts as a receptor for intimin, which allows for tight adherence



type III secretion system, also forms attachment and effacement lesions

Shiga-like toxin, similar to toxin made by Shigella spp., HUS (hemolytic uremic syndrome)

O157:H7 is the most frequent serotype

cattle are reservoir, acquire from undercooked meat, unpasteurized milk, and juices

bloody diarrhea, abdominal pain, and usually no fever or low-grade fever

leads to HUS in 10% of cases - hemolytic anemia, decreased platelets, renal failure, CNS dysfunction, 3-5% mortality

Escherichia coli


Escherichia coli clinical disease

UTIs, 90% of bladder infections and ppyelonephritis in healthy individuals

meningitis in neonates

nosocomial infections such as pneumonia, UTI, bacteremia, and intra-abdominal infections



causes Diarrhea in developing countries

Escherichia coli


Escherichia coli diagnostic laboratory tests

growth on routine media, differentiated from other enterobacteriaceae by biochemical tests

ETEC - ELISA and PCR assays for ST and LT

EHEC - can differentiate O157:H7 strains from commensal E. coli in that the former fail to ferment sorbitol

enzyme immunoassay for Shiga-like toxin in stool


lactose fermentation

MacConkey Agar

test used to differentiate bacteria

Lactose fermentors - Escherichia, Klebsiella, Enterobacter

Lactose non-fermentors - Proteus, salmonella, SHigella, Yersinia


Escherichia coli treatment

ETEC - rehydration, tetracycline/doxycycline, trimethoprim/sulfamethoxazole (TMP-SMX), bismuth subsalicylate (Pepto-Bismol), but increasing resistance, ciprofloxacin drug of choice in some countries

EHEC - no antibiotics

Uropathogenic E. coli - TMP-SMX, nitrofurantoin, ciprofloxacin


Escherichia coli prevention

avoid raw vegetables, fruits you do not peel yourself, unpasteurized dairy products, undercooked foods, tap water

rifaximin or bismuth subsalicylate if avoidance of illness is crucial

EHEC - avoid undercooked meat or unpasteruized milk or juices


Shigella spp.

S. dysenteriae, S. flexneri, S. boydii, S. sonnei

cause dysentery - cramps, painful straining to pass stools (tenesmus), and frequent, small-volume, bloody, mucoid stools


Salmonella enterica

formerly, many species

now known that all constitude a single species

former species designations now called serovars

facultative intracellular pathogens

common cuase of diarrhea in the US (15% of food-borne illness)

causes gastroenteritis

farma nimals (especially chickens) and turtles and other reptiles are reservoirs

serovar Typhi causes typhoid fever


Salmonella enterica determinants of pathogenicity

SPI-1 type III secretion system induces ruffling of enterocytes -> bacterial internalization

survive and multiply in phagosomes

leads to strong inflammatory response and diarrhea

occasionally disseminates to bloodstream, SPI-2 type III secretion system

serovar typhi capable of surviving inside macrophages -> aids in dissemination


Salmonella enterica clinical disease

majority of bacteria killed by acid in stomach, so large inoculum required for disease

diarrhea, nausea, vomiting 24048 hours after ingestion

usually, self-limited, lasts 7 days

fever in 50%

bacteremia in 8% of normal healthy patients

typhoid fever


typhoid fever

prolonged fever

persistent bacteremia

constipation or diarrhea

abdominal pain

occasionally a rash consisting of a few pink macules (rose spots) is observed

occasionally, carrier state develops and gallstones get infected, leads to shedding in feces, sometimes for years

Salmonella enterica


Salmonella enterica diagnostic laboratory tests

growth on agar with routine stool culture

typhoid fever, culture organism from blood


Salmonella enterica treatment

diarrhea is usually self-limited and has no treatment

treatment may predispose to carrier state

treat immunocompromised patients or severe infections

fluoroquinolones, ampicillin, chloramphenicol

always treat typhoid fever and bacteremia


Salmonella enterica prevention

prevent typhoid fever with parenteral capsular vaccine

live attenuated oral vaccine

recommended for travelers to Central and South America, Asia, and Africa who plan to stay for >4 weeks or live in conditions of poor hygiene


Yersinia spp.

Yersinia enterocolitica causes infectious diarrhea

Y. pestis causes plague


Yersinia pestis

cuase of plague ("The Black Death")

infection acquired in three ways:

1) transmission from rats/fleas (bubonic plague)

2) human-to-human transmission by aerosols (pneumonic plague)

3) direct contact with infected animal tissues (primary septicemic plague)

endemic cases still occur in the US (about 15 cases/year)

usually hikers or hunters who acquire the illness from wild animals

90% of US cases occur in NM, AR, CO, and CA


 Yersinia pestis determinants of pathogenicity

hms locus (hemin storage locus) - bacterial colonization of flea foregut

multiplies in flea foregut, blocks passage of blood meals, leads to regurgitation of blood meal along with bacteria

bacteria inoculated into host and travel to nearest lymph node

bacteria multiply and form bubos, leak into blood stream and releases LPS and causes septic shock and DIC

eventually, make their way to macrophages in the lungs

spread of disease to close contacts by aerosols

adhesins - Ail, chromosomally encoded, type III secretion system (on plasmid) - secretes several Yops (Yersinia outer proteins) - YopE and YopH

Fra1 - antiphagocytic protein that forms part of a capsule around Y. pestis

Pla - plasminogen activator -> cleaves fibrin clots -> dissemination



chromosomally encoded adhesin

Yersinia pestis



antiphagocytic protein that forms part of a capsule

Yersinia pestis



plasminogen activator -> cleaves fibrin clots -> dissemination

Yersinia pestis