Enteric Gram Negative Bacilli

Question 1

Enterobacteriales generally share what characteristics?

Answer 1

• Short, plump Gram-negative bacilli
• Often found in the microbiota or as pathogens in the GI tract, but can also be found at other body sites (blood and urinary tract are also common sites for these organisms)
• Similar biochemical characteristics: Facultative anaerobes, ferment glucose, oxidase negative, reduce nitrate
• Grow rapidly (overnight) on many culture media

Question 2

pathotypes

Answer 2

Groups of species or strains within a genus that are associated with specific manifestations of infection and defined by the presence of virulence traits (toxins or adherence mechanisms)

Question 3

Secretory diarrhea

Answer 3

Caused by the loss of electrolytes and fluids from the small intestine. The intestinal mucosa is basically undamaged, and there is no inflammation or bleeding.

Question 4

Travelers’ diarrhea

Answer 4

A mild form of secretory diarrhea caused by enterotoxic E. coli acquired via travel to another area where these organisms originated. Transmitted via fecal-oral route.

Question 5

Enterobacteriaceae

Answer 5

Family of Gram-negative rods known for causing diarrheal disease.

Question 6

How enterotoxic E. coli cause pathology

Answer 6

Adheres to jejunum/upper ileum and multiplies, not invading the mucosa. Then, produces two types of exotoxin: LT (heat labile) and ST (heat stabe).

Question 7

LT Toxin

Answer 7

Enterotoxic E. coli heat labile toxin. An AB toxin.

B-domain facilitates entry.A domainmodifies (ADP-ribosylates) a G protein.LTpromotes the active GTP-bound formof the G protein by the ADP ribosylation of one of itsarginine residues. ADP-ribosylated G protein islocked in the active conformationthat stimulatesadenylate cyclase.

The elevated intracellular synthesis of cAMP provokes the movement of massive quantities of ions and water across the intestinal membrane and into the lumen of the gut leading to watery diarrhea.

Question 8

AB Toxin

Answer 8

The B (binding) domain of an AB toxin binds a host receptor and mediates toxin entry into the cell by endocytosis. When the endosome is acidified, the B domain mediates transport of the A (active) domain out of the endosome and into cytoplasm of the host cell.

Once in the cytoplasm, the A domain modifies or cleaves a host protein, leading to damage to the host cell. There are many AB toxins produced by many different bacteria.

Question 9

ST Toxin

Answer 9

Enterotoxic E. coli heat stable toxin

ST interacts with and activates the guanylate cyclase (GC) of intestinal cells. This interaction leads to an increase in the cyclic guanosine monophosphate (GMP) and altered sodium and chloride transport, much like LT

Question 10

Clinical presentation of Traveler’s diarrhea

Answer 10

Patient presents with watery diarrhea beginning in or after returning from an area in which enterotoxic E. coli is endemic. Duration is typically a few days and symptom treatment with oral rehydration is usually adequate.

Counter drugs such as Imodium or Lomotil (both mu opioid receptor agonists) may relieve symptoms by slowing peristalsis and reducing ion flow. Antibiotics will reduce the duration of illness by about a day, but are generally not required.

Traveler’s diarrhea can be prevented by avoiding foods that are likely to be contaminated with ETEC

Question 11

Dysentery

Answer 11

Bloody diarrhea

Question 12

Enterohemorrhagic E. coli

Answer 12

Pathotype of E. coli. Also called shiga-toxin producing E. coli. (Note: It does not actually produce shigella toxin, but its toxin mimics the symptoms of shigella toxin)

Often aquired via zoonosis. Part of the normal microbiota of cows and may contaminate meat or other products when used as fertilizer. The estimated infectious dose is very low.

Utilizes a Type III secretion system to inject a receptor of its own manufacture into host cell membranes, then uses it as a channel to pump in enzymes that destroy the 60S ribosome and cause apoptosis of the host cell.

Question 13

Type III secretion system

Answer 13

Used by enterohemorrhagic E. coli, Shigella, and Salmonella.

Effectors injected by type III secretion systems often interfere with the ability of the host to respond to infection by the modulation of the actin cytoskeleton or inhibition of phagocytosis.

Question 14

Tir

Answer 14

Receptor injected by enterohemorrhagic E. coli’s Type III Secretion System.

Tir inserts into the host cell membrane, where it binds to the bacterial receptor, intimin. Binding of Tir stimulates polymerization of actin in the host cell just below the bacterium, leading to formation of a “pedestal” that projects from the host cell and associates tightly with the bacterium.

Question 15

Enterohemorrhagic E. coli exotoxins

Answer 15

Shiga-toxin 1 and 2. Both AB toxins.

Encoded by prophages incorporated into the E. coli genome. Shiga toxins 1 and 2 cleave the mammalian 60S ribosomal subunit, thereby stopping protein synthesis. The toxins also cause cells to undergo apoptosis. Effects of the toxin on mucosal endothelial cells contribute to the bloody diarrhea that follows

Question 16

Clinical presentation of enterohemorrhagic E. coli infection

Answer 16

Patient presents with dysentery after consuming contaminated, undercooked beef. Stool culture demonstrates enterohemorrhagic E. coli with high CFU. Sorbitol-resistance positive culture supports O157:H7 EHEC.

In approximately 5% of severe clinical cases, small blood vessels are ruptured. Often in children and sometimes in adults, small blood vessel rupture may lead to hemolytic-uremic syndrom (HUS, 5-10% mortality in children, many go on dialysis) due to toxin action on the glomerulus or release of disease-causing mediators from damaged endothelial cells. If this occurs in the brain, thrombotic thrombocytopenic purpura may result.

DO NOT treat with antibiotics, as they increase risk of thrombotic thrombocytopenic purpura. This may be due to increased toxin production stimulated by antibiotics.