Isolated polynucleotides encoding a BREX system are provided. Accordingly there is provided an isolated polynucleotide encoding a BREX system comprising a nucleic acid sequence encoding the BREX system comprising brxC/pglY, pglZ and at least one of pglX, pglXI, brxP, brxHI, brxHII, brxL, brxD, brxA, brxB, brxF, and brxE, with the proviso that said BREX system does not comprise pglW, and wherein said BREX system confers phage resistance to a bacteria recombinantly expressing same; Also provided is an isolated polynucleotide encoding a BREX system comprising a nucleic acid sequence encoding the BREX system comprising brxC/pglY, pglZ, pglX, pglW and at least one of brxD and brxHI, and wherein said BREX system confers phage resistance to a bacteria recombinantly expressing same. Also provided are compositions and methods for conferring phage resistance to bacteria or for conferring bacterial susceptibility to phages.