Table 1.

Suggested principles in the choice of phages for therapy.

	Principle	Benefits	Drawbacks	Reference
1	Broad host range	Reduces need to diagnose infecting strains; may simultaneously attack multiple strains of a single pathogen	Broad host range may conflict with other optimal phage characteristics	Balogh et al. (2010), Gill (2010)
2	Phage mixtures targeting different host receptors	Development of resistance to phage less likely as mutants resistant to one phage remain sensitive to others in the mixture	Greater development time, may limit the repertoire of phage available for some bacteria	Balogh et al. (2010), Gill (2010), Chan and Abedon (2012), Chan et al. (2013)
3	Non-temperate	Avoids lysogeny as an easy form of bacterial insensitivity; avoids pathogenicity genes commonly found in temperate phages	Limits the repertoire of phage available for some bacteria	Gill and Young (2011)
4	Ability to clear liquid cultures	Simple in vitro assay	Applicability to in vivo success unknown	Smith and Huggins (1982), Henry et al. (2013)
5	Non-lysing phages	Cells are killed without releasing toxins	Phage do not amplify, so huge numbers of phage must be inoculated	Matsuda et al. (2005)
6	Target surface virulence determinants or otherwise impose high cost of resistance	Difficult bacterial escape; resistant cells become unfit/avirulent	Limits the repertoire of phage available	Capparelli et al. (2010a,b), Viertel et al. (2014), this paper
7	Tailspike de-polymerase	Acapsular bacterial mutants are often avirulent (see item 6); unassembled tailspikes released at lysis as free enzyme, digest capsule of nearby cells and expose them to immune system	Possibly limited host range of such phage, limits the repertoire of phage available for some bacteria	Francis et al. (1934), Sutherland (1995), Mushtaq et al. (2004), Malnoy et al. (2005), Bull et al. (2010)
8	Non-transducing	Will not mobilize pathogenicity or antibiotic resistance determinants	Limits the repertoire of phage available for some bacteria	Gill (2010)
9	Slow in vivo virion decay rate	Increases phage longevity in the animal host, increasing probability of phage encountering bacteria	None obvious, may allow for faster development of acquired immunity to phage	Merril et al. (1996), Balogh et al. (2010)