Please don't rely on these as your sole means of water filtration. The 'absolute micron' rating is at only 99% capture.
From Filterbag's website:
"Filter bags made out of monofilament mesh or micro-fiber are given absolute ratings. A filter bag with an absolute rating will capture 99% of its rated particles in a single pass."
The EPA standard is 99.9999% for bacteria and 99.9% for cysts.
The EPA testing protocol dopes test water with 100,000,000 bacteria per liter (10^7/100mL):
Fewer than 100 bacteria are allowed to pass through a filter meeting the EPA standards. The Filterbag 'filter' could potentially let 1,000,000 bacterial through!
If you chose your water source carefully, you're not likely to encounter those concentrations of bacteria (something like one gallon of raw sewage in 10,000 gallons of water), but I don't thinks it's worth the risk.
However, the Filterbag filter would make a great pre-filter.
Hope this helps.