viknesh
Hi Justin and Ryan,

One explanation for what you are observing is called "lysis from without". This happens when phages that are not able to successfully infect and replicate in particular bacteria are nevertheless still able to lyse those bacterial cells when they are applied to cells at high concentrations. Such lysis is not the result of replication and lysis from within the cells but instead is happeneing from the outside of cells, hence the term "lysis from without". Such lysis is likely due to the mass action of lots of hydrolase enzymes that phages typically have as part of their tails and that are used to locally depolymerase the cell wall in order to deliver the phage genome into the cell. At some point, there is too much of these "local" depolymerization activity that the cells are overwhelmed and lyse as a result.

Because lysis from without only happens at high concentrations, you will not see lysis when the phage sample is diluted. Because lysis from without does not involve phage replication, you will not see individuals plaques form.

When performing host-range assays, it is typical to observe clearing of spots at high phage concentrations but no individual plaques at the lower concentrations. Such results are regularly interpreted as phage not being able to infect that particular host.

I hope this helps explain some of your observations.