Shigella flexneri

Shigella flexneri provides the clearest illustration of how horizontal gene transfer can transform an organism's ecological role. Genetically, Shigella is E. coli—the two are so similar that by standard bacterial taxonomy criteria, they should be the same species. Yet Shigella causes severe dysentery while most E. coli strains are harmless gut commensals. The difference lies almost entirely in acquired genetic elements: a large virulence plasmid and several pathogenicity islands that Shigella picked up through horizontal transfer.

These acquired genes encode a Type III secretion system—a molecular syringe that injects bacterial proteins directly into human intestinal cells. The injected proteins force cells to engulf the bacterium, rupture neighboring cells to enable spread, and suppress inflammatory responses. None of these capabilities existed in the E. coli ancestor; all were imported from other pathogenic bacteria. Shigella essentially downloaded a pathogenicity software package that completely changed its business model from commensal to pathogen.

The Shigella story also demonstrates gene loss as adaptive strategy. As Shigella acquired virulence capabilities, it lost genes needed for survival outside hosts—it can no longer grow on many nutrients that E. coli uses readily. This streamlining reflects commitment to the pathogenic lifestyle; unnecessary capabilities were jettisoned to optimize the new strategy. For businesses, Shigella illustrates both the transformative power of strategic acquisitions and the importance of subsequent integration—acquiring new capabilities means little without reorganizing around them and shedding what no longer fits.