strains , plasmids , and oligonucleotides used in this study are shown in supplementary table 2 . d. dadantii competition assays on chicory were carried out as described in supplementary methods . ec93 and d. dadantii 3937 cdia - ct - cdii deletions and cdia chimeras the 3 end of cdia and all of cdii from upec 536 kpsk15 aracba specrexbad - cdibai ( dl5646 ) were replaced with cdia - ct ( sequence immediately following vennx ) and cdii from e. coli ec93 , y. pestis co92 ( accession number q7cgd9 ) , or d. dadantii 3937 region 2 ( see supplementary methods ) . the kpsk15 capsule mutation was used to increase the efficacy of cdi , based on our previous results showing that capsule production blocks cdi 3 . immunity plasmids were constructed by ligating pcr - amplified cdii genes into plasmid pbr322 under tet promoter control ( fig . the immunity plasmids were constructed by ligating pcr - amplified cdii genes into the minitn7 delivery plasmid puc18r6kt - minitn7 t under tet promoter control ( see supplementary methods ) . 2b ) was carried out by cloning specific sequences amplified by pcr into plasmid plac1114 under lac promoter control . all plasmids were propagated in epi100 acrb mutant strain dl5154 to mitigate toxic effects . in vivo interactions between cdia - ct and cdii were determined using a modified bacth bacterial two - hybrid system ( euromedex ) 8 . -galactosidase14 and fluorescence3 analyses were carried out as previously described . in vitro affinity pull - downs with his6-tagged cdii / cdia - ct cdia - ct was released by denaturation in buffer containing 6 m guanidine - hcl , and his6-tagged cdii was released in native buffer supplemented with 250 mm imidazole .