Use of the ssrA gene or tmRNA, an RNA transcript of the ssrA gene, or fragments thereof as target regions in a nucleic acid probe assay for the detection and identification of prokaryotic and/or eukaryotic organisms is described. Nucleotide sequence alignment of tmRNA sequences from various organisms can be used to identify regions of homology and non-homology within the sequences which in turn can be used to design both genus specific and species specific oligonucleotide probes. These newly identified regions of homology and non-homology provide the basis of identifying and detecting organisms at the molecular level. Oligonucleotide probes identified in this way can be used to detect tmRNA in samples thereby giving an indication of the viability of non-viral organisms present in various sample types.