Introduction to taxa

Classification of taxa of Bisgaard

Taxonomy is the study of classification, nomenclature and identification, while systematics may include genetics, evolution, physiology and ecology. The main task of classification is to outline taxa defined as “a real group of organisms recognized as a formal unit at any level of a hierarchic classification” (Simpson 1961, Principles of animal taxonomy).

Taxa might be named according to the Prokaryotic Code, provided they are circumscribed by a unique set of characters. This is a very time-consuming process, one of the preconditions of which is a geographically broad and epidemiologically unrelated strain collection. To overcome this problem, my strategy in the diagnostic laboratory has been based on isolating as well as receiving and keeping atypical isolates, to generate enough isolates to allow publication of new taxa, to address the international microbiological society and to speed up collection of similar isolates, the ultimate goal being a safer basis for an unambiguous diagnosis in microbiology. This is a prerequisite for proper treatment, and for improving our knowledge of the complexity of bacterial-host interactions leading to peaceful coexistence or ultimately fatal disease.

Since I started working with organisms now classified with Pasteurellaceae some fifty years ago, the roots of bacterial systematics have changed from morphology and biochemistry to molecular biology. Identification at that time was based upon only a few phenotypical characters, and for the same reason the literature was confusing as to their host reservoir, epidemiology and disease potential.

To investigate the phenotypic diversity observed for isolates originating from the same animal species or identical  lesions, I extended the number of phenotypical characters significantly and soon discovered, what I believed was a much more restricted host reservoir for most species I worked with, and a good colleague of mine, Dr. Wilhelm Frederiksen, SSI, suggested to name and publish new groups as taxa followed by an Arabic number, to increase the interest for new groups of bacteria of interest for the diagnostic laboratories. Colleagues were subsequently expected to isolate and identify the new taxa, and ultimately classify and name the organism properly, improve the diagnosis and understanding of their disease potential and epidemiology.

However, during the past decades, we have noticed an increasing difficulty of aligning phenotypic differentiation with the differentiation that can now be achieved with molecular methods. We are fast approaching a time, when the ability to separate valid genera and species of the Family Pasteurellaceae is becoming increasingly difficult. Therefore, the numbers of genomospecies are increasing, making it very difficult for veterinary diagnostic laboratories to correctly identify an isolate, as long as they only have access to phenotypic identification.

To investigate, if extended and consolidated phenotypic characterization is still valid for the diagnosis and outlining of new taxa, a review of my taxa and their final classification is given below.