I'm not a doctor or research biologist, but I have the same question. My default assumption is that it depends on whether this is a locally evolved bacterium. I

I'm not a doctor or research biologist, but I have the same question. My default assumption is that it depends on whether this is a locally evolved bacterium. If it's a common strain found everywhere then the phage is probably pretty common too. If this is some unusual strain of Pseudomonas that's localized to his area then a phage that coevolved locally with that specific strain might be the most effective fighting it. Either way, it's a probability situation as there's no guarantee a particular phage will fight a particular bacterium until it's tried.