Abstract
In addition to carrying determinants conferring resistance to at least two antibiotics, chloramphenicol and streptomycin, a Klebsiella aerogenes strain contains a plasmid responsible for increased beta-galactosidase activity. The plasmid can be transferred to Escherichia coli and Salmonella typhimurium strains. K. aerogenes segregants without the plasmid grow on lactose one-half as fast as the parent strain and contain only one-tenth to one-fifth as much beta-galactosidase.