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SUMMARY  A variety of Klebsiella strains examined all show either a strong (ML + ) or a weak (ML −/+ ) lactose-positive phenotype on MacConkey agar. ML + Klebsiellae have about 10 times the β-galactosidase activity of ML −/+ strains in cultures both induced and not induced for this enzyme. Of 14 ML + strains of diverse origin tested, at least 13 carry a lac operon on a plasmid which can be transferred to Escherichia coli . The seven plasmids so far studied in detail all belong to the F compatibility group but are unable to promote their own transfer. To explain these results it is suggested that the Klebsiella group derive from a common ancestor with a chromosomal lac operon of low efficiency, which was made good by the acquisition of a lac operon from another bacterial strain, probably E. coli : the new lac genes remained as a plasmid, possibly because they could not be integrated in the new host.

Lac+plasmids are responsible for the strong lactose-positive phenotype found in many strains ofKlebsiellaspecies

Lac⁺plasmids are responsible for the strong lactose-positive phenotype found in many strains ofKlebsiellaspecies