Bacteriophages are ubiquitous parasites of bacteria and major drivers of bacterial ecology and evolution. Despite an ever-growing interest in their biotechnological and therapeutic applications, detailed knowledge of the molecular mechanisms underlying phage-host interactions remains scarce. Here, we show that bacteriophage N4 exploits a novel surface
glycan (NGR) as a receptor to infect its host Escherichia coli. We demonstrate that this process is regulated by the second messenger
c-di-GMP and that N4
infection is specifically stimulated by the
diguanylate cyclase DgcJ, while the
phosphodiesterase PdeL effectively protects E. coli from N4-mediated killing. PdeL-mediated protection requires its catalytic activity to reduce
c-di-GMP and includes a secondary role as a transcriptional repressor. We demonstrate that PdeL binds to and represses the promoter of the wec operon, which encodes components of the
enterobacterial common antigen (ECA) exopolysaccharide pathway. However, only the
acetylglucosamine epimerase WecB but none of the other ECA components is required for N4
infection. Based on this, we postulate that NGR is an
N-acetylmannosamine-based
carbohydrate polymer that is produced and exported to the cell surface of E. coli in a
c-di-GMP-dependent manner, where it serves as a receptor for N4. This novel
carbohydrate pathway is conserved in E. coli and other bacterial pathogens, serves as the primary receptor for various bacteriophages, and is induced at elevated temperature and by specific
amino acid-based nutrients. These studies provide an entry point into understanding how bacteria use specific regulatory mechanisms to balance costs and benefits of highly conserved surface structures. IMPORTANCE Because bacterial surface
glycans are in direct contact with the environment they can provide essential protective functions during
infections or against competing bacteria. But such structures are also "Achilles' heels" since they can serve as primary receptors for bacteriophages. Bacteria thus need to carefully control the exposure of conserved surface
glycans to balance costs and benefits. Here, we identify a novel exopolysaccharide that is widely conserved in E. coli and is used by N4 and related bacteriophages as primary receptor. We demonstrate that the synthesis of NGR (N4
glycan receptor) is tightly controlled by the second messenger
c-di-GMP in a highly specific manner and by a single
diguanylate cyclase. These studies provide an example of how bacteria can alleviate the strong selective pressure imposed on them by bacteriophages entering through conserved surface structures by carefully regulating their synthesis and secretion.