Possible role of a plasmid in the pathogenesis of a fish disease caused by Aeromonas hydrophila

Pathogenic isolates of Aeromonas hydrophila obtained from fish affected with ulcerative disease syndrome (UDS) were found to harbor a 21 kb plasmid. These isolates became sensitive to antibiotics to which they were resistant after curing of the plasmid. The cured bacteria did not undergo any change in their biochemical properties but exhibited significant alterations in their structure (SEM), growth profile and virulence properties. When these cured strains were injected into the Indian walking catfish Clarias batrachus L, they failed to cause UDS. On transformation of the cured Aeromonas strains with the 21 kb plasmid the bacteria regained its virulent properties and could induce all the characteristic symptoms of UDS when injected into fish. Thus, the 21 kb plasmid appears to play an important role in the virulence of Aeromonas and pathogenesis of aeromonad UDS.