Phage therapy comes up every so often on HN. Some comments on why it's not more widespread and unfortunately not a cure-all for bacterial infections: - narrow

Phage therapy comes up every so often on HN. Some comments on why it's not more widespread and unfortunately not a cure-all for bacterial infections: - narrow range: phages have very specific host requirements, i.e. they usually work only on very specific subtypes of bacteria. This means you will need to spend a significant effort on finding a matching phage to a given pathogenic bacterial strain. It's almost a type of personalized medicine. Antibiotics on the other hand are usually very broad and work on a wide range of strains. - Resistance to phages: bacteria are much more adapted to deal with phages than with antibiotics. Bacteria can become immune to phages within a couple of generations, for antibiotics it takes significantly longer. So if the first problem can be overcome (a broadly acting phage cocktail), resistance will be an even bigger problem than with antibiotics. Fun fact: the well known CRISPR system used for genetic engineering originally stems from such a bacterial system that deals with phage immunity. - Manufacturing: antibiotics can rely on well established protocols for large scale production on the cheap mainly relying on chemical synthesis. Phages on the other hand will be needed to grow in a bacterial host culture (you can't build them synthetically) and will need extensive purification steps. You don't want to intravenously inject any bacterial remnants into a patient. Add to that the large diversity of phages with ever changing host requirements and it becomes a nightmare for production.