How Do You Determine Bacterial Cell Density Using A Spectrophotometer?
You are trying to measure cell density (i.e. the rough amount of cells in a set volume of liquid), filtering the growth broth would make the test meaningless because you would remove the cells. Diluting might help, especially if your reading is out of the reading range, but dilute using your clean broth (what you are zeroing with), not water. A reading drift is often caused by a poorly calibrated spec, get your Teacher, TA or Lab Tech to take a look at it. You should also ask what the proper OD600 reading range is where the numbers you get mean anything. There is a upper and lower reading limit for that. Remember to multiply your results through your dilution factor if you dilute and make sure you new reading falls between that upper and lower range on the spec output.
