Shiga toxigenic Escherichia coli (STEC)

Investigators: Dr. Adrienne Paton, Prof. James Paton, Dr. Hui Wang, Dr. Sylvia Herold

Shiga toxin-producing strains of E. coli (STEC) are known to cause serious gastrointestinal disease in humans, with potentially life-threatening complications such as haemolytic uraemic syndrome (HUS). Dr. Adrienne Paton is directing an extensive STEC research program, involving characterisation of novel virulence factors, particularly those produced by STEC strains that lack the LEE pathogenicity island, to determine their precise contribution to the pathogenesis of disease.  Our data indicate that human-virulent LEE-negative STEC strains carry large (>150 kb) plasmids, which are distinct from those of O157 STEC strains, and have a number of additional genes which may function in pathogenesis.  One such gene encodes an auto-agglutinating adhesin designated Saa (STEC autoagglutinating adhesin), which mediates semi-localised adherence to host cells. Another region of the plasmid contains a novel type IV pilus biosynthesis locus, which directs expression of long thin pili on the STEC surface, and facilitates conjugative transfer of the plasmid. 

We have recently discovered that in addition to Shiga toxin, certain STEC strains produce another AB5 toxin which we have named Subtilase cytotoxin. Three families of AB5 toxins have been recognized to date, exemplified by Shiga toxin, cholera toxin and pertussis toxin. They comprise a catalytic A subunit which acts on specific intracellular components of eukaryotic cells, and a pentameric B subunit which binds to glycolipid receptors on the target cell surface. Subtilase cytotoxin was discovered in an STEC strain that caused an outbreak of HUS in Adelaide in 1998, where neurologic manifestations were prominent in affected children. It belongs to a new class of AB5 cytotoxins, because its A subunit has distinct enzymic activity (it is a subtilase-like serine protease), and it has no sequence similarity with any of the other AB5 toxin families. Subtilase cytotoxin is extraordinarily toxic for cultured cells; it is 10-100 times more potent than Shiga toxin in Vero cells. Moreover, when injected, it is lethal for mice at nanogram doses, with histopathological features similar to those seen in human cases of HUS. It is not understood why some STEC are more likely to cause severe disease than others.  Aside from Shiga toxin expression, the virulence factors associated with STEC that may contribute to development of HUS are as yet poorly defined. Thus, there is a strong possibility that expression of this emerging toxin contributes significantly to the pathogenesis of disease caused by bacteria that produce it. Our current activities are aimed at comprehensive characterization of the new toxin, including identification of its intracellular target, and examining target cell recognition and trafficking of the toxin within the cell. We are also collaborating with structural biologists at Monash University to determine its crystal structure. These studies will provide comprehensive basic information on the biology of this hitherto unknown toxin, including knowledge of its mechanism of action, the tissues targeted, and the nature of the pathology it causes in an animal model. Structural data will also provide functional insights, as well as facilitating future design of inhibitory drugs with therapeutic potential. These data will also inform future studies exploring a potential synergistic relationship between Subtilase cytotoxin and Shiga toxins in human disease pathogenesis.