Development of bacteriophage-resistant strains of lactic acid bacteria.

One exciting area that has emerged through genetic analysis of the lactococci is the definition and practical application of gene systems that provide phage resistance to these industrially important bacteria. Naturally occurring phage-insensitive strains have been characterized and found to harbour multiple defence systems which can act at different points of the lytic cycle to prevent the successful adsorption, infection, or replication of virulent phages [ 11. Although phage attack on lactococcal starter cultures is still a major problem for the cultured dairy products industries, this can now be successfully addressed using genetic approaches to construct strains which are resistant to the phages often encountered in the industry [2, 31. In this paper I will describe the various phage defence systems that are naturally present in lactococci, their individual and combined effects, and the genetic strategies which are currently available to construct phageinsensitive strains for dairy fermentations. In addition, I will discuss the molecular responses of virulent phages which have appeared in the industry following the introduction and use of specialized starter cultures carrying defined mechanisms of phage resistance. The genetic routes whereby industrial phages elicit counter-defences against lactococcal resistance mechanisms provide new and important information that will facilitate the development of improved starter culture systems and phage-resistant lactic acid bacteria. Historically, starter strains that perform well in dairy fermentations have been selected on the basis of long-lived and consistent fermentative activity in phage-contaminated environments. Genetic analysis of Lactococcus lactis subsp. lactis and subsp. eremot6 strains over the past ten years has revealed that the lactococci harbour a variety of natural phage-defence systems. These include plasmid-encoded defences that interfere with phage adsorption, numerous DNA restriction and modification systems of different specificity, and undefined mechanisms that abort the phage infection after injection of phage DNA into the cell (for reviews see [4-61). Numerous phage-resistance plasmids encoding the above classes of resistance