**7. Problem of phage contamination in dairy industry**

There are no commercial LAB cultures available which would be completely insensitive to all phages. Even when a starter culture that is launched on the market appears to be phage resistant, phages are detected usually after a certain period of use.

Lactic Acid Bacteria Resistance to Bacteriophage and Prevention Techniques

is suspected (Fig.1). However, in this method a delay in acidification can also result from

**Figure 1.** Example of pH curve during milk fermentation in the presence of virulent phages incubated

The most common and most useful method of phage enumeration is the plaque assay. The method is quite old and was first described by d'Herelle shortly after the discovery of bacteriophages. Currently it is used in many labs with some modifications, but its principle has not changed [103]. The most common, practical, cheap, without using large numbers of plates and sufficiently accurate method in the dairy industry is the semi-quantitative spot test method. Using this approach, results are available after 24-48 h. The method is well suitable for detection of phages of pure lactic acid bacterial strains at relatively low levels (< 100 phages ml-1). Plague assays allow detecting the presence of phages as well as determining the number of phages in dairy samples against all individual strains present in the applied defined cultures. In case of phage contamination in a dairy plant, the method is a good tool for selecting the best phage-resistant alternative cultures. The method can also be used for hygiene monitoring by enumeration of phages in samples collected from critical places if the plant. For dairy culture producers, permanent phage monitoring can identify strains which are most sensitive in defined cultures. These strains can be systematically replaced with more phage resistant strains. Semi-solid medium supporting

with the multistrain and multispecies culture.

**8.2. Routine service at culture supplier level** 

other inhibitors than phages (e.g. antibiotics, detergents) present in the sample.

to Lower Phage Contamination in Dairy Fermentation 45

Phage contaminations in dairy plants can cause 3 main serious drawbacks:

