MOLECULAR-GENETICS OF BACTERIOPHAGE AND NATURAL PHAGE DEFENSE SYSTEMSIN THE GENUS LACTOCOCCUS

Bacteriophage infection of starter cultures used in a range of milk fe rmentation processes, particularly those involving Lactococcus lactis, poses a significant problem in industrial practice. The application o f genetic and molecular technologies to the study of lactococcal bacte riophages has proven to be very rewarding in terms of understanding th e nature of phage with respect to their physical and genetic organisat ion. The availability of the full genomic sequence of a number of phag es provides an unambiguous basis for determining the relationship betw een them, for elucidating their evolutionary progression and will also yield strategies for obstructing successful phage proliferation on pr eviously sensitive hosts. The genetic analysis of phage/host interacti ons has also highlighted the presence of natural defence systems (e.g. adsorption blocking, inhibition of phage DNA entry, restriction modif ication and abortive infection) in lactococci. A number of restriction modification systems and abortive infection mechanisms have been char acterized at a molecular level and the genes involved have been cloned and sequenced. Plasmid-encoded phage resistance mechanisms can be exp loited to generate strains which can successfully counter phage prolif eration and will provide a basis for understanding the complex interac tions between phages and their target hosts at a molecular level.