the steripen should not be trusted. The military has a report that lists the major players in the market and ran their own, unbiased, tests MP-1s always came in second only to sweetwater purifier. here is the pdf

http://usachppm.apgea.army.mil/WPD/PDFDocs/FinalReport.pdf

and here is the military website with easier to read results.

http://chppm-www.apgea.army.mil/wpd/CompareDevices.aspx