Genome-wide Identification and Characterization of a Superfamily of Bacterial Extracellular Contractile Injection Systems

Several phage-tail-like nanomachines were shown to play an important role in the interactions between bacteria and their eukaryotic hosts. These apparatuses appear to represent a new injection paradigm. Here, with three verified extracellular contractile injection systems (eCISs), a protein profile and genomic context-based iterative approach was applied to identify 631 eCIS-like loci from the 11,699 publicly available complete bacterial genomes. The eCIS superfamily, which is phylogenetically diverse and sub-divided into six families, is distributed among Gram-negative and -positive bacteria in addition to archaea. Our results show that very few bacteria are seen to possess intact operons of both eCIS and type VI secretion systems (T6SSs). An open access online database of all detected eCIS-like loci is presented to facilitate future studies. The presence of this bacterial injection machine in a multitude of organisms suggests that it may play an important ecological role in the life cycles of many bacteria.