Current and pending taxonomy of the Pasteurellaceae

Since the family Pasteurellaceae was established in 1981 the taxonomy of this group of bacteria has undergone significant changes. 16S DNA sequencing showed in the early 1990-ties, that the three original genera were genetically heterogeneous. Presently, the family consists of 13 genetically coherent genera with 38 properly classified species.

In addition, there are 24 misclassified species that have been excluded from the genera Actinobacillus, Pasteurella and Heamophilus and which probably will be allocated to new genera in the future. At the moment 92 different taxa (named species and provisional taxa) have been described within the family.

In the presentation different methods for species delineation will be presented and potential problems discussed in relation to selected examples. It is currently recommended that a new species should be separated from other species by at lest two phenotypic tests. Within genus Mannheimia the separation between M. haemolytica and M. glucosida was based on a polyphasic taxonomic investigation, but there is only one phenotypic character to separate the species and presently no genotypic test.

A group of Mannheimia sp. is genetically distinct but lack common phenotypic characters and can consequently not be formally named. Such problems could be solved by establishing genomospecies. Histophilus somni represent a genotypically well defined species but show great phenotypic variability. Here a PCR test based on the rrn-operon was included in the formal species description. [Haemophilus] parasuis show marked variability in the rnn-operons and illustrates that analysis of 16S rDNA sequences can lead to incorrect conclusions in a non-clonal population.

The separation between Actinobacillus pleuropneumoniae and A. lignieresii is not possible based on the rrn-operons alone but the existence of two separate species is evident based on AFLP-genotyping. Although 16S sequencing is a versatile tool for taxonomic investigations, its discriminative power for species delineation is in a number of cases limited.

DNA-DNA hybridization is still regarded as a golden standard for separating species but the sequencing of housekeeping genes represents a promising alternative. Whole genome sequencing will probably accelerate as less labour intensive methods are developed and prices go down. Data from these genome sequencing projects will make a more detailed study of the genetic affiliation between strains and species feasible.

At the same time the genotypic variation within a species and the description of the “pan-genomes” and “core-genomes” will represent a challenge to our conception of what a species is.