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 ...

Twenty-two phages that infect Stenotrophomonas species were isolated through sewage enrichment and prophage induction. Of them, S1, S3, and S4 were selected due to their wide host ranges compared to those of the other phages. S1 and S4 are temperate siphoviruses, while S3 is a virulent myovirus. The genomes of S3 and S4, about 33 and 200 kb, were resistant to restriction digestion. The lytic cycles lasted 30 min for S3 and about 75 min for S1 and S4. The burst size for S3 was 100 virions/cell, while S1 and S4 produced about 75 virus particles/cell. The frequency of bacteriophage-insensitive host mutants, calculated by dividing the number of surviving colonies by the bacterial titer of a parallel, uninfected culture, ranged between 10 ؊5 and 10 ؊6 for S3 and 10 ؊3 and 10 ؊4 for S1 and S4. The 40,287-bp genome of S1 contains 48 open reading frames (ORFs) and 12-bp 5 protruding cohesive ends. By using a combination of bioinformatics and experimental evidence, functions were ascribed to 21 ORFs. The morphogenetic and lysis modules are well-conserved, but no lysislysogeny switch or DNA replication gene clusters were recognized. Two major clusters of genes with respect to transcriptional orientation were observed. Interspersed among them were lysogenic conversion genes encoding phosphoadenosine phosphosulfate reductase and GspM, a protein involved in the general secretion system II. The attP site of S1 may be located within a gene that presents over 75% homology to a Stenotrophomonas chromosomal determinant.