SUMMARY: Most invasive strains of from man and domestic animals were lethal for chickens and mice. The lethal characteristic was not, in general, transferred when invasive strains were grown in mixed culture with non-pathogenic strains of , although two transmissible plasmids coding for pathogenic properties were discovered.

One plasmid, designated Vir, was found in an strain causing bacteraemia in a lamb. It transferred at high rate to several strains of , including a A-KI2 strain, to and Culture filtrates and, especially, bacterial ultrasonicates of Vir strains were toxic for chickens, mice and rabbits. The toxin was heat-sensitive, acid-sensitive and non-diffusible. Organisms producing it were agglutinated by specific Vir antisera; their toxic activity was not neutralized. The transfer factor of the Vir plasmid was fi and could transfer antibiotic-resistance determinants in addition to the Vir determinant.

The other plasmid was first discovered in an strain F120, isolated from an outbreak of bacteraemia in chickens. Organisms of K12 and of other strains acquiring this plasmid during mixed culture with FI20 were increased in lethality for chickens and mice; this was associated not with toxic activity but with greater ability to survive in blood and peritoneal fluids. Strain FI20 possessed transmissible ColV and Collb plasmids; increased lethality was closely associated with the ColV plasmid. When the ColV plasmids of another six wild strains of of varied origin were transferred to organisms of KI2, the lethality increase was similar to that for ColV transfer from FI20. No lethality change accompanied transfer of other Col plasmids. It was concluded that colicine V itself might be responsible for the increased lethality.

Strains of associated with bacteraemia in man and animals commonly produced colicine V.


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