The average size of the openings between pieces of the filter media are represented in microns. For example, a 20-micron filter has larger openings than a 5-micron filter. Consequently, the 20-micron filter element will let larger particles pass through the filter than the 5-micron element.
Bacteria range in size from 0.2 to 2 microns in width or diameter and from 1 to 10 microns in length for the nonspherical specie, so a 1-micron filter will remove most bacteria and cysts. If you are on municipal water, the water has already been treated with chlorine, chloramines and/or fluoride to remove bacteria, so chances are you do not have a bacteria problem unless there is a problem with the water pipes between the water reclamation plant and your home. If you are on well water and have a bacteria problem, installing a 1-micron absolute water filter at the point of use will remove most of the bacteria (99.9%), however, if your water has tested positive for bacteria, it is safer to install a chlorine injection water sanitizing tank to purify the water before it enters the house, followed by a whole-house filter to remove the chlorine.
As a general rule, the smaller micron rating for a filter is better, especially if you are on well water, but as with most everything, there is a trade-off. Flow capability usually drops off as the micron rating gets smaller, especially if the water has a lot of sediment, which well water often has. If the filter at the end of your kitchen sink spout needs to be cleaned frequently, it's a good bet that your water has a lot of sediment.
To overcome sediment-causing flow rate problems, low micron-rating filters must have larger elements to keep from sacrificing precious flow, like a sediment prefilter to remove larger particles that clog the 1-micron filter and reduce water flow. In a well water situation, it is common to see two or three sediment prefilters in the water flow, starting with the higher micron rating to remove the larger particulate, followed by lower micron rating filters. For example, first filter the water through a 20-micron sediment prefilter, then a 5-micron and finally through a 1-micron or a sub-micron filter. This process extends the life of all filters, including the carbon filters which do the work of removing chemicals, herbicides, pesticides and other water pollutants.