Methods for detecting and identifying unknown bioagents, including bacteria, viruses and the like, by a combination of microRNA containing nucleic acid amplification and molecular weight determination using primers which hybridize to conserved sequence regions of microRNA containing nucleic acids derived from a bioagent and which bracket variable sequence regions that uniquely identify the bioagent. The result is a “base composition signature” (BCS) or molecular mass which is then matched against a database of base composition signatures or molecular masses, by which the species of the bioagent is identified.