Natural conjugative plasmids induce bacterial biofilm development

Horizontal gene transfer is a principal source of evolution leading to chan ge in the ecological character of bacterial species(1). Bacterial conjugati on(2), which promotes the horizontal transfer of genetic material between d onor and recipient cells by physical contact, is a phenomenon of fundamenta l evolutionary consequence(3). Although conjugation has been studied primar ily in liquid, most natural bacterial populations are found associated with environmental surfaces in complex multispecies communities called biofilms (4). Biofilms are ideally suited to the exchange of genetic material of var ious origins, and it has been shown that bacterial conjugation occurs withi n biofilms(5,6). Here I investigate the direct contribution of conjugative plasmids themselves to the capacity of the bacterial host to form a biofilm . Natural conjugative plasmids expressed factors that induced planktonic ba cteria to form or enter biofilm communities, which favour the infectious tr ansfer of the plasmid. This general connection between conjugation and biof ilms suggests that medically relevant plasmid-bearing strains are more like ly to form a biofilm. This may influence both the chances of biofilm-relate d infection risks and of conjugational spread of virulence factors.