described herein is a bacterial cell containing a recombinant bacteriophage defense mechanism . the defense mechanism comprises a bacteriophage promoter operatively associated with a heterologous dna encoding a product lethal to the bacterial cell . the bacterial cell is susceptible to infection by a bacteriophage , and the promoter is activated upon the infection of said bacterial cell by that bacteriophage . bacteria useful in carrying out the invention include both gram negative and gram positive bacteria ; the heterologous dna may encode an enzyme that degrades nucleic acid . recombinant dnas useful for making the foregoing cells , cultures prepared from such cells , and fermentation methods carried out with such cells are also disclosed .