Bacteriophage and anti- phage mechanisms in lactic acid bacteria

Bacteriophages (phages) are the principal cause of starter culture failure in many food fermentations carried out by lactic acid bacteria (LAB). These bacterial viruses were first characterized in the early 20th century by the work of Felix d’Hérelle in 1917, although their inhibitory activity had previously been documented by Ernest Hankin in 1896, Nikolay Gamaleya in 1898, and Frederick Twort in 1915 (O’Flaherty et al. 2009). Phage morphology remained undetermined until electron microscopy became available during the 1940s. Nevertheless, phages attacking Lactococcus lactis starter cultures have been known since the 1930s, when Whitehead and Cox in New Zealand observed that they were responsible for failed lactic acid development in cheese production (Whitehead and Cox 1935). In the area of food fermentations, the permanent threat of phage contamination is particularly manifested in the dairy field. Here phage infections of LAB during the fermentation process result in an unacceptably low production rate of lactic acid and flavor compounds along with reduced proteolysis. Thus, starter activity is either severely affected (“slow vats”), or in extreme cases, a complete failure of starter growth may occur (“dead vats”). Due to resulting financial losses for the dairy fermentation industry, control of phages is an area of concern in handling starter cultures. It is worthy of mention that phages have also been isolated from other food fermentations (e.g., sauerkraut, coffee, and wine), but their role in those situations is not as destructive as in the dairy environment due to its liquid nature, its relatively short fermentation time scale, and the large volumes of milk involved, usually with multiple successive uses of fermentation vats daily (Coffey and Ross 2002). Phage infections begin with adsorption of the phage to receptor molecules on the bacterial cell surface (Figure 9.1), followed by injection and replication of the phage genome, intracellular assembly of progeny phages, and finally cell lysis with release of progeny phages (Figure 9.2). Research efforts in a variety of research laboratories worldwide have focused on understanding the complex and dynamic mechanisms of phage–host interactions as detailed below primarily using examples from L. lactis and Streptococcus thermophilus where most of the phage resistance research in LAB has been performed.