All correct . Initially, when this and coming next questions were framed (AI 95), the best answer was shigella dysentriae because at that time new shigella enterotoxins were not discovered. . But, with discovery of new enterotoxins it is clear that all four shigella can produce enterotoxin (exotoxin) - Read text below. SHIGELLA TOXINS . Shigella dysenteriae type I forms a toxin (shiga toxin). . It is the earliest example of an exotoxin produced by a Gram negative bacilli. . Shigella has three type of toxic activity 1) Neurotoxicity Primary sites of action are blood vessels of the CNS, not the nervous tissue. Neurological effects are secondary. 2) Enterotoxicity . There are two enterotoxins - Sh. ET -1 ---> Is restricted almost exclusively S. flexneri 2a - encoded by chromosome - Sh. Et - 2 --> More widely distributed to all shigella - encoded by plasmid. . Both these enterotoxins alter electrolyte transpo by gut segment in vitro and cause net fluid secretion in ligated ileal loops in vivo. 3) Cytotoxicity . It is due to production of Shiga toxin (verocytotoxin) which is similar to VT1 expressed by EHEC. . It is produced by shigella dysentriae type 1. . Genes encoding shiga toxin are located on chromosome, while the VT1 of EHEC is phage-encoded. . Toxin constits of two subunits - b Subunits, helps is binding and A subunit which has the active poion of toxin. Subunit 'A' is divided into A1 and A2 fragment. A l fragments inhibits protein synthesis by inactivating 60s ribosome. Remember . The major factor in the pathogenicity of shigella is its ability to penetrate and multiply in colonic mucosa. . Invasive propey is related to the presence of large plasmids coding for the outer membrane protein responsible for cell penetration - Virulence marker antigen. Detection of VMA by ELISA serves as a virulence test for shigella, as for enteroinvasive E. col i. . Invasive propey of the bacillis can be demonstrated by its ability to penetrate cultured HeLa or Hep - 2 cells. . Majority of lesions occur in distal colon. . Enterotoxins are produced by all four type of shigellae.