'Species concept' in microbial taxonomy and systematics

Species is the basic unit of identification of a large diversity of organisms. The concept of species is old and has undergone several changes depending on the kind of organism(s). Earlier concepts were formulated considering mostly macroorganisms (plants or animals). In 21st century the microbial species concept took shape which changed from morphological, ecological, biological and phylogenetic to more agreeable polyphasic species concept. Polyphasic species concept involves a combination of characters such as morphology, physiology, biochemical markers and sequencing of various regions of the DNA known as molecular markers. There are also specific criteria such as DNA-DNA hybridization for complex genera of Bacteria and Archaea, extrolite profiling for complex genera of fungi, etc. Although species concept and taxonomy of Bacteria are more systematic than fungi, taxonomy in two domains (bacteria, archaea) and fungi of eukarya domain is undergoing changes. The time is not far when genome sequencing will be considered as another criterion for delineating species. This article also discusses the role of various techniques (MALDI, FAME, etc.) in species identification. The concept of Candidatus species and species annotation for metagenomic studies is also discussed. We briefly describe the criteria used to delineate species in various microbes and the need for different species concepts based on the kind of organism.