Many examples. Eg diphtheria toxin and cholera toxin genes are encoded by phages.

Just check out this paper, its interesting.

http://www.ncbi.nlm.nih.gov/pubmed/?term=Antibiotic+treatment+expands+the+resistance+reservoir+and+ecological+network+of+the+phage+metagenome

If the viirus infected previously a more virulent bacteria , then it can take some genes in the viral replication. Then in a new bacteria infection, it can incorporate the virulent genes.

I would like to add with Mooi that Vibrio phages is one of the important example in this field. Non virulent Vibrio cholerae can converted in to virulent strain due to the incorporation of different virulent genes by vibrio phage.

http://www.ncbi.nlm.nih.gov/pubmed/12615217

http://www.ncbi.nlm.nih.gov/pubmed/23076327

I just read what Mr Uddin wrote and I have to kindly disagree. Incorporation of phage DNA in Vibrio cholerae genome to yield a toxigenic ergo choleragenic strain, was actually an evolutionary event taken place successfully centuries ago when cholera epidemics first started to hunt humanity. Fortunately it is not a common thing to happen, as it was suggested. Otherwise there would be a lot of epidemic and pandemic strains causing cholera everywhere. What we see in nature, is the one an only clonal/pandemic strain with perhaps small genetic variations, just like the most recent epidemic phenotype Vc O139, which is no other than the Vc O1 genetic background that incorporated rather important changes in the genes for the biosynthesis of the O antigen. And it does not mean that phages are not being the vehicle for very efficient horizontal genetic transfer occurring everywhere and all the time as we are speaking, just the cassette encoding the cholera toxin does not have such a great mobility (again, fortunately!).

Certainly DNA transformation by the uptake of foreign DNA seems an important feature of many bacterial pathogens, where it may be phage mediated or not. For example, type 4 pili (T4P) that are used for bacterial mobility, attachment, colony formation, and transformation undergo genetic variation that is dependent upon the uptake of DNA and influenced in at least some systems by phage (Mol Microbiol. 2010 Aug;77(3):755-70. doi: 10.1111/j.1365-2958.2010.07244.x ).

Absolutist yes. For example Corynebacterium diphtheriae earns virulence gene from phage. The diphtheria toxin gene is encoded by a bacteriophage found in toxigenic strains, integrated into the bacterial chromosome.

From our practice, Bacterial isolates which is in continuous exposure to phages are more virulent and resist more than other isolates... also Pseudomonas that isolated from hospitals are less susceptible to phage and more virulent to burns injured rats than strains isolated from river and environmental sources..... best wishes