Revisiting lactococcal abortive infection-like antiphage systems: Diversity, structures and mechanisms

Lactococcus spp. are cornerstone species in the production of fermented dairy products such as cheese. However, the non-sterile dairy environment exposes these bacteria to phage infection, leading to potential environmental and economic losses. To counteract such phage threats, bacteria have evolved multiple defence mechanisms, including abortive infection (Abi) systems, of which many of the first (AbiA to AbiZ) were discovered in Lactococcus. Recent discoveries have expanded the list of Abi-like systems experimentally confirmed in Lactococcus species. In this review, we examine and revisit the antiphage activity spectrum, the interference with the phage lytic cycle, and the emergence of phage escape mutants associated with these systems. Since many of these systems were identified before the development of advanced functional and structural prediction tools, their domains and mechanisms have largely remained unexplored. Here, we group certain Abi-like systems based on structural superimposition and use predicted domain information to hypothesize their potential mechanisms. Additionally, we discuss established mechanisms for some Abi-like systems. Our findings confirm that the co-evolution of Lactococcus species with their phages, driven by their extensive application in dairy fermentation, has led to the selection of a diverse array of phage defence mechanisms.