<p>The genus Yersinia contains just three species: <taxon tax_id="630">Yersinia enterocolitica</taxon>, <taxon tax_id="632">Yersinia pestis</taxon>, and <taxon tax_id="633">Yersinia pseudotuberculosis</taxon> [<cite idref="PUB00010303"/>]. Although the three use different routes to infect their host, each targets the lymphoid tissue for invasion, and all have developed specific systems to evade host immune cells [<cite idref="PUB00010303"/>]. PYV, a major virulence plasmid common to all members of this family, harbours the genes necessary for type III secretion in the host and the exotoxins translocated by them. </p><p>One of the proteins encoded within this plasmid is the Yersinia YadA non-fimbrial adhesin, a moiety that facilitates cell interaction between the host and pathogen [<cite idref="PUB00010288"/>]. Mutational studies indicate that this protein allows intimate attachment and subsequent uptake by host macrophages of the bacterial cell. Synergistic mechanisms by two other PYV-encoded proteins,YopH and YopE, inhibit the action of YadA. Electron microscopy of the mature YadA adhesins suggest that they form distinct "lollipop" shaped structures on the cell surface [<cite idref="PUB00010380"/>]. This is a trait shared by the adhesins of another pathogen, namely <taxon tax_id="480">Moraxella catarrhalis</taxon> UspA1 and UspA2 [<cite idref="PUB00010380"/>]. </p><p>The YadA protein itself exists as a homotrimer of 45kDa subunits, anchored in the outer bacterial membrane by its C terminus [<cite idref="PUB00010396"/>]. The lollipop's globular head is formed by the N terminus in the extracellular space. A Yersinia bacterial cell thus coated with YadA can bind a number of host cell macromolecules, including collagen, laminin, mucus and fibronectin, enhancing its capacity for infection.</p> Outer membrane adhesion, Yersinia