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

Abstract

SUMMARYA variety ofKlebsiellastrains 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 alacoperon on a plasmid which can be transferred toEscherichia 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 theKlebsiellagroup derive from a common ancestor with a chromosomallacoperon of low efficiency, which was made good by the acquisition of alacoperon from another bacterial strain, probablyE. coli: the newlacgenes remained as a plasmid, possibly because they could not be integrated in the new host.