I read this Nature news piece about phage therapy today. It's great that phage might finally be taken seriously as an alternative to carpet-bombing bacterial infections with antibiotics.

This sentence caught my eye: "Nature provides an almost inexhaustible supply: no two identical phages have ever been found." That claim isn't entirely true. While bacteriophage genomes are staggeringly diverse, many contain very similar conserved sequences. If we're only considering two phages to be identical if each nucleotide in one genome is present in the other, then it's very close to true. We could, of course, pick two phage out of the same culture and have a good chance of their genomes being 100% identical. Depending on the phage, we may only be talking about few thousand nucleotides (ΦX174 is only about 5 thousand base pairs, for example!). 

Let's not even start on the issue of how frequently we isolate genetically identical bacteria. We'd like to think that a culture of any one bacterial species is a monoculture, but how many cells in that culture are completely genetically identical at any one time? How about their transcriptomes? I suppose that the original idea remains true: bacteria can produce great genetic variation, so bacteriophage can do so just as well.