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 .
 summary paragraphbacteria have developed mechanisms to communicate and compete with one another in diverse environments 1 . 
 a new form of intercellular communication , contact - dependent growth inhibition ( cdi ) , was discovered recently in escherichia coli 
 2 . 
 cdi is mediated by the cdib / cdia two - partner secretion system . 
 cdib facilitates secretion of the cdia  exoprotein onto the cell surface . an additional immunity protein ( cdii ) 
 protects cdi+ cells from autoinhibition 2 , 3 . 
 the mechanisms by which cdi blocks cell growth and cdii counteracts this growth arrest are unknown . 
 moreover , the existence of cdi activity in other bacteria has not been explored . here 
 we show that the cdi growth inhibitory activity resides within the carboxy - terminal region of cdia ( cdia - ct ) , and that cdii binds and inactivates cognate cdia - ct , but not heterologous cdia - ct . 
 bioinformatic and experimental analyses show that multiple bacterial species encode functional cdi systems with high sequence variability in the cdia - ct and cdii coding regions . 
 cdia - ct heterogeneity implies that a range of toxic activities are utilized during cdi . 
 indeed , cdia - cts from uropathogenic e. coli and the plant pathogen dickeya dadantii have different nuclease activities , each providing a distinct mechanism of growth inhibition . 
 finally , we show that bacteria lacking the cdia - ct and cdii coding regions are unable to compete with isogenic wild - type cdi+ cells in both laboratory media and upon a eukaryotic host . taken together 
 , these results suggest that cdi systems constitute an intricate immunity network that plays an important role in bacterial competition . 
Methods Summary
Supplementary Material