The imp operon is carried on a transmissible plasmid, Colla, in origin
al isolates of Salmonella typhimurium LT7. LT2 strain recipients of F'
factors from LT7 strains harboring Colla can acquire Colla and imp un
der nonselective conditions. Thus, S. typhimurium LT2 strains that hav
e received plasmids by conjugal transfer from LT7 strains might be ina
dvertently harboring Coll factors.