Molecular Evolution, Species Distribution, andClinical Consequences ofan EndemicAminoglycoside Resistance Plasmid

underwent a variety ofmolecular changes. Itwas mostprevalent initially inisolates ofKlebsiella pneumoniae, theninisolates ofSerratia marcescens,andfinally, after nearly disappearing, inisolates ofEnterobacter cloacae. Evolutionary changes intheplasmid didnotaccountforits shifts inspecies distribution, since theoriginal molecule was foundinisolates ofeachspecies. Thelate resurgenceofpBWHl occurred after a copy ofitsoriginal molecule entered a distinctive ornithine decarboxylase-negative strain ofE.cloacae, new tothehospital. Theresulting transconjugant strain, chromosomally resistant totopical silver salts andtocephalosporins, andwiththeaddition ofpBWH1-encoded aminoglycoside resistance, spread inthehospital bycausing anoutbreak ofsepsis intheburnunit, wherethese werecommonly usedantibacterial agents. Thus, an endemic plasmid becameprevalent ina new hostspecies because oneofits genessupplemented thefitness ofan uncommon strain ofthespecies for aparticular clinical niche.