In a new paper in Cell Reports, Beatriz Beamud (Universidad Complutense de Madrid) and colleagues report on their study of the genetic determinants of host tropism in Klebsiella phages. They isolated 46 phages to challenge 138 representative clinical Klebsiella pneumoniae isolates, and found that <2% of 6,319 phage-host combinations tested yielding detectable interactions. Capsule diversity was found to be the main factor restricting phage host range.