**Question:** Which of these are true of E. coli:

a) The L.T (labile toxin) in ETEC acts via cAMP
b) In types causing UTI the organism attaches via pili
c) The ST (stable-toxin) of ETEC is responsible for causing hemolytic-uremic syndrome (HUS)
d) EIEC invasiveness is under plasmid control
e) In EPEC, the toxin helps in invasion by the bacteria

**Core Concept:** Escherichia coli (E. coli) is a diverse group of bacteria that can cause various infections. E. coli can be classified into different types based on their virulence factors.

**Why the Correct Answer is Right:**

1. a) True: The L.T (labile toxin) in enterotoxigenic E. coli (ETEC) acts via cyclic AMP (cAMP). L.T is responsible for causing the diarrheal disease, and its action on cAMP leads to increased water secretion in the gut, causing diarrhea.
2. c) True: The stable toxin (ST) of ETEC is responsible for causing hemolytic-uremic syndrome (HUS). ST is a heat-stable toxin that damages the endothelial cells, leading to microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure, which characterizes HUS.
3. d) False: EIEC invasiveness is not under plasmid control. EIEC invasiveness is controlled by the virulence factors encoded by the pathogenicity islands (PAIs) in its genome, not plasmids.
4. e) False: In enteroaggregative E. coli (EAEC), the toxin helps in invasion of the bacteria into the host cells. In EPEC, the primary virulence factor is the attaching and effacing (AE) lesion, which disrupts the tight junctions, allowing the bacteria to invade the host cells.

**Why Each Wrong Answer is Incorrect:**

1. b) False: EPEC does not cause urinary tract infections (UTIs). EPEC primarily causes diarrheal illnesses, particularly in children, due to its ability to form attaching and effacing lesions in the intestinal tract, impairing the tight junctions and leading to diarrhea.
2. f) False: EPEC does not cause HUS. EPEC causes attaching and effacing lesions in the intestine, leading to bloody diarrhea rather than HUS, which is associated with other types of E. coli, such as ETEC and EHEC.

**Clinical Pearls:**

Escherichia coli (E. coli) strains can be classified based on their pathogenic mechanisms. The correct answer options (a, c, d, e) are true for different types of E. coli.

E. coli (ETEC) causes enterotoxigenic E. coli (ETEC) and produces enterotoxins that lead to diarrhea, not HUS.

Enteropathogenic E. coli (EPEC) causes attaching and effacing lesions in the intestine, impairing