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, brxHH, 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 recombinantiy 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 recombinantiy expressing same. Also provided are compositions and methods for conferring phage resistance to bacteria or for conferring bacterial susceptibility to phages.