Evolutionary and ecological role of extracellular contractile injection systems: from threat to weapon

Abstract

Contractile injection systems (CISs) are phage tail-related structures that are encoded in many bacterial genomes. These devices encompass the cell-based type VI secretion systems (T6SSs) as well as extracellular CISs (eCISs). The eCISs comprise the R-tailocins produced by various bacterial species as well as related phage tail-like structures such as the antifeeding prophages (Afps) of Serratia entomophila, the Photorhabdus virulence cassettes (PVCs), and the metamorphosis-associated contractile structures (MACs) of Pseudoalteromonas luteoviolacea. These contractile structures are released into the extracellular environment upon suicidal lysis of the producer cell and play important roles in bacterial ecology and evolution. In this review, we specifically portray the eCISs with a focus on the R-tailocins, sketch the history of their discovery and provide insights into their evolution within the bacterial host, their structures and how they are assembled and released. We then highlight ecological and evolutionary roles of eCISs and conceptualize how they can influence and shape bacterial communities. Finally, we point to their potential for biotechnological applications in medicine and agriculture.

Fragmented micro-growth habitats present opportunities for alternative competitive outcomes

“…The production of tailocins is indeed a highly heterogeneous process at a population level, initiated in less than 1% of cells 48,58 . In addition, tailocins, like other specialized bacterial killing devices such as the type VI secretion system, act very locally 48 , which has raised the question of their ecological importance 58 . Our droplet observations suggest that tailocins are particularly helpful in overcoming competition in spatially restricted microhabitats.…”

Breaking genetic shackles: The advance of base editing in genetic disorder treatment

“…They actively inject various cargo proteins into prokaryotic or eukaryotic cells, utilizing the stored energy in sheath proteins for this purpose. Based on differences in their mechanisms of action, bacterial CIS can be broadly categorized into the cell-based type VI secretion systems (T6SSs) and the extracellular CIS (eCIS) ( Heiman et al, 2023 ). The Photorhabdus Virulence Cassette (PVC), a subtype of eCIS, primarily consists of an outer sheath and an inner tube, which together play an important role in the delivery and injection of effector molecules into host cells ( Wang X. et al, 2022 ).…”

Control of a gene transfer agent cluster inCaulobacter crescentusby transcriptional activation and anti-termination

“…Viruses and mobile genetic elements are major drivers of evolution in bacteria 1,2 . In some cases, there is evidence that these elements might be co-opted to perform biological functions for the host, thus providing selective advantages for the producing organisms [3][4][5][6][7] . Virus-like Gene Transfer Agents (GTAs) are among these cases 3,4 .…”

“…GTAs were first discovered and characterized in the non-sulfur photosynthetic bacterium Rhodobacter capsulatus 8 , but have now been found in a wide range of bacterial species and archaea 3,4,9,10 . Homologs of GTA core genes are common in abundant environmental organisms, and it has been speculated that ~10 6 virus-like particles per milliliter in nature could be GTAs 9,[11][12][13][14] . GTAs are thought to be domesticated prophages that can no longer self-multiply and be infectious, but can still transfer DNA via a DNA-filled capsid head 3,[15][16][17][18][19][20] .…”