Distinguishing a particular microorganism from all others demands finding a target sequence that is unique for that organism, is conserved in all members of that species or strain, and has limited or no plasticity. Such targets can be a nucleic acid sequence, constitutive protein, or an antigenic epitope. The target can be probed with PCR-based, immunochemical, or mass spectroscopy assays. Genomics has played an important role in developing new generations of probes. The genomes of an organisms of which is the nucleic acid content has been sequenced and annotated is of invaluable aid in the identifications of targets. Post genomic disciplines such as proteomics and glycomics are and will usher in a new generation probes. The proteome can be defined as a set of proteins produced by an organism under a defined set of conditions. The presence of a protein is the ultimate proof that a gene is being expressed. The glycome represents the glycan groups or saccharide chains attached to proteins or lipids. Surface proteins (S-layer) are ideal targets. Since they are often found at the surface of a cell they are probed with antibodies without breaking the cell.