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Phage resistance
- Phage-Host Interaction: an Ecological Perspective

Phages are a nuisance in the dairy industry, where they interfere with industrial milk fermentation when they attack starter bacteria. Therefore, food microbiologists have intensively studied phage-host interactions. Lactic acid bacteria were under substantial evolutionary pressure to develop antiphage strategies, as demonstrated by numerous phage resistance systems naturally found in lactococci. These mechanisms range from the blocking of phage DNA injection (45) to abortive infection systems (abiA to abiU) interfering with phage DNA replication, RNA transcription, phage development, and morphogenesis. The evolutionary arms race between bacteria and their phages has been well documented by dairy microbiologists. For example, against bacterial restriction systems, phages used different escape strategies covering either the loss of restriction sites (47) or the gain of a methylase gene (34). Acquisition of chromosomal DNA by lactococcal phages confronted with abi mechanisms has been repetitively demonstrated (6, 24). The high number of prophages in Lactococcus lactis offers in fact a large supply of new genetic material to superinfecting phages which is accessible via homologous recombination. Interestingly, the rich knowledge on phage resistance mechanisms acquired in the dairy field could also contribute to the understanding of the coexistence of phages and bacteria in the marine environment or the management of phage infections in humans (e.g., the cigarette smoke-induced prophage propagation purportedly linked to a decline of the protective Lactobacillus flora in the vagina which might open the way to vaginosis and candida superinfections) (36).

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