Bacterial contact-dependent growth inhibition (CDI) systems are
   two-partner secretion systems in which toxic CdiA proteins are exported
   on the outer membrane by cognate transporter CdiB proteins. Upon binding
   to specific receptors, the C-terminal toxic (CT) domain, detached from
   CdiA, is delivered to neighbouring cells. Contacts inhibit the growth of
   not-self-bacteria, lacking immunity proteins co-expressed with CdiA, but
   promote cooperative behaviours in "self" bacteria, favouring the
   formation of biofilm structures. The Acinetobacter baylyi ADP1 strain
   features two CdiA, which differ significantly in size and have different
   CT domains. Homologous proteins sharing the same CT domains have been
   identified in A. baumannii. The growth inhibition property of the two A.
   baylyi CdiA proteins was supported by competition assays between
   wild-type cells and mutants lacking immunity genes. However, neither
   protein plays a role in biofilm formation or adherence to epithelial
   cells, as proved by assays carried out with knockout mutants. Inhibitory
   and stimulatory properties may be similarly uncoupled in A. baumannii
   proteins.