Four surface exclusion systems have been identified amongst a group of F-like plasmids in E. coli: SfxI(F), SfxII (ColV2 and R538-1fin−), SfxIII (ColVBtrp and R1-19) and SfxIV (R100-1 and R136fin−). None of these systems was expressed in stationary phase cells or, except for ColVBtrp, during fin+ transfer inhibition, showing that the surface exclusion gene(s) is usually co-controlled with the transfer genes.
Recipient cells carrying two plasmids specifying different surface exclusion systems did not always express both of these: the overall pattern suggested that the four systems and/or their sites of action are related. There was no surface exclusion between donor cells carrying two plasmids determining different surface exclusion systems and recipient cells carrying a plasmid determining either one of these. Our hypothesis to explain this and other results is that surface exclusion prevents interaction between the tip of the pilus on the donor cell and a receptor site on the recipient cell surface. Pili (probably mixed) with two types of tips would be present on cells carrying two different plasmids, the one unresponsive to the single surface exclusion system of the recipient cells allowing transfer of both plasmids.