A lot of scientific works regarding Phage Therapy when describe the disadvantages of this application immediately list the appearance of bacterial resistance to used phages. The authors do not explain the cause and they consider the bacterial resistance as an immediate process but this is not true.The probability of appearance of bacterial resistance to phages is real but it is important to know how it reveals itself.
When there is a bacterial mutation before the appearance of the corresponding phenotypical effect in the bacterial cell it is necessary a bacterial growth for a long time ( this is the phenotypical delay phenomenon).
For one phage there are about 200 specific receptors on the bacterial cell wall.
If one bacterial cell has a mutation regarding one specific type of phage receptor this bacterial cell requires a long time both for the mutation appearance in its receptors and for a complete dilution of the old phage receptors.
In this moment this bacterial cell is still sensible to phage. This bacterial cell is sensible to phage up to a complete replacement of all receptors ( 200 ) with the new receptors. This will happen after 12 bacterial cell divisions ( R.Y.Stainier, M.L.Wheelis, J.L.Ingraham and P.R.Painter: “The Microbial World”, 1975) and for this reason we have the time for phage cocktail action.
