Ans. is 'a' i.e., O 157 : H7 of EHEC o Diarrhea is caused by intestinal pathogenic E. coli.o At least five distinct "pathotypes" of intestinal pathogenic E. coli exist:1) Enteropathogenic E. coli lEnteroadherent E. colil# It causes watery diarrhoea in infants and children usually occuring as institutional out breaks.# It does not produce enterotoxin, nor are they invasive. They adhere to the mucosa of small intestine and cause disruption of the brush border microvilli.# These strains can be identified by their adhesion to HEP - 2 cells.2) Enterotoxigenic E. coli# It causes traveller's diarrhoea [ETEC is the most common cause of traveller's diarrhea).# ETEC is the most frequent bacterial cause of diarrhea in infant and children of developing countries, especially in tropical countries.# It produces enterotoxins. They can produce heat labile toxin (LT) or heat stable toxin or both.# Toxin production alone may not lead to illness. The strain should first be able to adhere to intestinal mucosa. This adhesiveness is medicated by funbrial or colonisation factor antigen (CFA).# It is non-invasive (does not invade mucosa).3) Enteroinvasive E. coli# They themselves resemble shigella and their infection resembles shigellosis (remember : shiga like toxin is elaborated by enterohemorrhagic E. coli).# They produce mild diarrhoea to frank dysentry and occur in adult as well as in children.# They have been termed enteroinvasive because they have the capacity to invade intestinal epithelial cells in vivo and penetrate HeLa or HEP - 2 cells in tissue culture.# This ability of penetration is plasmid determined which codes for outer membrane antigens called the 'virulence marker antigen' (VMA). The detection of plasmid can be diagnostic.# For laboratory diagnosis of EIEC, the sereny test used to be employed.# These strains are non motile, do not ferment lactose or ferment it late with acid without producing any gas.4) Enterohemorrhagic E. coli or verotoxigenic E. coli# These strains produce verocytotoxin (VT) or shiga like toxin (SLT)# They can cause mild diarrhoea to fatal hemorrhagic colitis.# Shiga like toxin belongs to class ribosome inactivating proteins (RIPs). It inhibits protein synthesis by inhibiting ribosomal function.# This toxin also acts on vascular endothelium to promote the synthesis of coagulation factor VIII, vWF - Platelet aggregation.# They can cause hemolytic uremic syndrome particularly in young children and the elderly.# O 157 : H7 is the most prominent serotype of EHEC, associated with HUS, but 06, 026, 055, 091, 0103, 0111, 0113 and OX3 have also been associated with this syndrome.# The primary target for V.T is vascular endothelium.# The typical EHEC is serotype 0157: H7 which does not ferement sorbital unlike majority of E. coll Therefore sorbitol Mac Conkey media (a modified Mac Conkey media containing sorbitol in place of lactase) is used for EHEC 0157: H7.# Some other serotype like 026 : HI also belongs to this group.# Laboratory diagnosis of VIEC diarrhea is established by demonstration of the bacilli or VT in feces directly or in culture.5) Enteroaggregative E. coli# In developing countries and young children.# Can cause traveller's diarrhea (2nd me cause ofter ETEC) and persistent diarrhea.# In vitro, they exhibit a "stacked - brick" adherence pattern on Hep - 2 cells.# Enteroaggregative E. coli forms a low molecular weight heat stable enterotoxin EAST - 1 (enteroaggregative heat stable enterotoxin - 1).