Contact-dependent growth inhibition systems in Acinetobacter

In bacterial contact-dependent growth inhibition (CDI) systems, CdiA proteins are exported to the outer membrane by cognate CdiB proteins. CdiA binds to receptors on susceptible bacteria and subsequently delivers its C-terminal toxin domain (CdiA-CT) into neighbouring target cells. Whereas self bacteria produce CdiI antitoxins, non-self bacteria lack antitoxins and are therefore inhibited in their growth by CdiA. In silico surveys of pathogenic Acinetobacter genomes have enabled us to identify >40 different CDI systems, which we sorted into two distinct groups. Type-II CdiAs are giant proteins (3711 to 5733 residues) with long arrays of 20-mer repeats. Type-I CdiAs are smaller (1900–2400 residues), lack repeats and feature central heterogeneity (HET) regions, that vary in size and sequence and can be exchanged between CdiA proteins. HET regions in most type-I proteins confer the ability to adopt a coiled-coil conformation. CdiA-CT and pretoxin modules differ significantly between type-I and type-II CdiAs. Moreover, type-II genes only have remnants of genes in their 3′ end regions that have been displaced by the insertion of novel cdi sequences. Type-I and type-II CDI systems are equally abundant in A. baumannii, whereas A. pittii and A. nosocomialis predominantly feature type-I and type-II systems, respectively.