The present invention provides methods and compositions related to modulating the resistance of a cell against a target nucleic acid or a transcription product thereof. In some preferred embodiments, the present invention provides compositions and methods for the use of one or more cas genes or proteins for modulating the resistance of a cell against a target nucleic acid or a transcription product thereof. In some embodiments, the present invention provides methods and compositions that find use in the development and use of strain combinations and starter culture rotations. In additional embodiments, the present invention provides methods for labelling and/or identifying bacteria. In some preferred embodiments, the present invention provides methods for the use of CRISPR loci to determine the potential virulence of a phage against a cell and the use of CRISPR-cas to modulate the genetic sequence of a phage for increased virulence level. In still further embodiments, the present invention provides means and compositions for the development and use of phages as biocontrol agents.