What sort of bacteriophage is it? With your second test in the liquid culture you were looking for impaired growth which can be observed with infections using non-lytic phages (eg. E. coli with filamentous bacteriophages like M13). The plaque assay relies on it being a lytic phage which kills the lawn. Try adding your phage dilution directly to the bacterial culture and then plating it to see if patches of reduced growth are observed (check 1-2 times a day). It may be that the phage is only lytic under certain stimuli. Are you using the same media for both your plates and liquid culture?

Robert Adolf Brinzer Thank you so much for your kind response. I also use the same media for both plates and liquid culture

Is the bacterial lawn growing nicely and highly turbid? Failure to have a good lawn will often hide phage lysis so that you can not see plaques or even phage spots.

Another simple and effective test is to cross streak. On a fresh plate you draw a line with your phage solution (using maybe 50ul undiluted) down the center of the plate.

Then using a sterile loop or toothpick you pick a fresh colony of cells (or a few microliter of culture) and streak across the line of the phage. Only go one time from left to right across it. Then if there is phage, you will see a notable inhibition of cell growth where it crosses the line of phage.

Michael J. Benedik so far, the bacterial lawn growing nicely, I usually used around 0.3-0.5 value of OD 600 nm, but then I also tried 1.0 OD 600 nm to make sure that my bacterial lawn is still nice or not. but then, the result showed no plaque. By the way, I used 0.5 % agar composition to make the top layer, is it really matter?

At 0.5% top agar you would get somewhat smaller plaques than with 0.3% due to diffusion rate, but I am not sure how much that will matter or not.

how do you know that there are any phages in your stock?

Michael J. Benedik Well, actually I bought phages from a company, I have followed their protocol to propagate phages. Unfortunately, it didn't really work, I stuck in this process.

Any details as to the type of phage product you purchased?

And do you know that your bacterial host strain is the proper strain for this specific phage? Phages are in general very host specific.

Michael J. Benedik Yes, I have checked many times and both bacteria and phages were well-matched. Before I bought them, the company also help me to check both the host and phages compatibility