Milk contamination by phages, the susceptibility of the phages to pasteurization, and the high levels of resistance to phage infection of starter strains condition the evolution dynamics of phage populations in dairy environments. Approximately 10% (83 of 900) of raw milk samples contained phages of the quasi-species c2 (72%), 936 (24%), and P335 (4%). However, 936 phages were isolated from 20 of 24 (85%) whey samples, while c2 was detected in only one (4%) of these samples. This switch may have been due to the higher susceptibility of c2 to pasteurization (936-like phages were found to be approximately 35 times more resistant than c2 strains to treatment of contaminated milk in a plate heat exchanger at 72°C for 15 s). The restriction patterns of 936-like phages isolated from milk and whey were different, indicating that survival to pasteurization does not result in direct contamination of the dairy environment. The main alternative source of phages (commercial bacterial starters) does not appear to significantly contribute to phage contamination. Twenty-four strains isolated from nine starter formulations were generally resistant to phage infection, and very small progeny were generated upon induction of the lytic cycle of resident prophages. Thus, we postulate that a continuous supply of contaminated milk, followed by pasteurization, creates a factory environment rich in diverse 936 phage strains. This equilibrium would be broken if a particular starter strain turned out to be susceptible to infection by one of these 936-like phages, which, as a consequence, became prevalent.Bacteriophage infection of the starters used in dairy fermentations is the main cause of disturbance in the manufacture of products such as cheese, yogurt, etc. (11, 23). The problem was initially recognized in 1935 (30) and led to the design and application of good manufacturing practices, such as direct inoculation of the starters in closed fermentation vats, use of antiphage media for starter propagation, and rotation of starter cultures (1). In addition, genes that encode natural resistance mechanisms were introduced into starter strains (2,11,14). As a consequence of all these measures, total loss of the final product is infrequent nowadays, although phages are still responsible for quality defects that affect the flavor, texture, and even safety of dairy foods (1, 11).Of the variety of phage species that infect Lactococcus lactis, only three, c2, 936, and P335, are commonly found in dairy plants, and these phages are responsible for most milk fermentation failures (4, 7, 24). All three belong to the family Siphoviridae, although 936 and P335 have isometric capsids (morphotype B1), whereas c2 has a prolate head (morphotype B2) (17, 18). Only virulent representatives of the c2 and 936 groups are known, while P335 includes both temperate and lytic viruses. Of the three, phages belonging to the 936 quasi-species are most frequently isolated from dairy environments, followed by phages belonging to the c2 group.The question of the origin ...

The ability of specific bacteriophages to inhibit Staphylococcus aureus growth in curd manufacturing processes was determined. Two lytic bacteriophages specific against S. aureus were obtained by DNA random deletion from the milk-isolated temperate phages, FH5 and FA72. A cocktail of these lytic phages, F88 and F35, at multiplicity of infection (MOI) of 100, produced a complete elimination of 3 Â 10 6 cfu mL À1 of the pathogen in ultra-high-temperature (UHT) whole milk at 37 1C. Furthermore, the frequency of emergence of bacteriophage-insensitive mutants was reduced up to 200-fold in the presence of the two lytic phages compared with that detected with the combination of the temperate counterparts. The lytic phage derivatives, added to milk, were able to decrease rapidly the viable counts of S. aureus during curd manufacture. In acid curd, the pathogen was not detected after 4 h of incubation at 25 1C, whereas pathogen clearance was achieved within 1 h of incubation at 30 1C in renneted curd. These results indicate that lytic bacteriophages could be used as biopreservatives in the manufacture of particular dairy products. r

The prevalence of bacteriophages infecting Staphylococcus aureus in dairy samples was assessed. Fourteen Staph. aureus strains were used in enrichment cultures of 75 dairy samples. All samples grew specific Staph. aureus bacteriophages. According to the host range, 8 different phages were isolated. Three of them, phages PhiH5, PhiG7, and PhiA72, were found in 89% of the samples; all the isolated phages were temperate. Phages PhiH5 and PhiA72 were used in preliminary bacterial challenge tests against Staph. aureus in milk. A phage mixture (1:1) was more effective than each single phage, most likely by preventing the survival of lysogenized cells. Phages inhibited Staph. aureus in UHT and pasteurized whole-fat milk. However, the phages were less active in semi-skimmed raw milk and little inhibition was achieved in whole, raw milk. Killing of Staph. aureus was observed at room temperature and at 37 degrees C, but not at refrigeration temperature.

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