Revised description and classification of atypical isolates of Pasteurella multocida from bovine lungs based on genotypic characterization to include variants previously classified as biovar 2 of Pasteurella canis and Pasteurella avium

Abstract

Strains deviating in key phenotypic characters, mainly isolated from cases of bovine pneumonia in five European countries, were genotyped in order to examine their genotypic relationship withPasteurella multocida. Twenty-two strains ofPasteurella aviumbiovar 2, including variants in indole, xylose and mannitol, 18 strains ofPasteurella canisbiovar 2 and variants of this taxon, five strains ofP. multocidasubsp.septicashowing variations in indole and ornithine decarboxylase, nine strains ofP. multocidasubsp.multocidashowing variation in ornithine decarboxylase and mannitol, and type strains of the subspecies ofP. multocidawere included. Ribotyping was used to examine the relationship of the strains, and 13 types, each containing between one and 20 isolates, were observed. Identical ribotypes were observed in some cases forP. aviumbiovar 2 and eitherP. canisbiovar 2 orP. multocidasubsp.septica. ITS (16S–23S rRNA internal transcribed spacer) fragment-length profiling showed identity of the majority of strains (47 of 52), representing all four taxa, with only five divergent strains. A 16S rRNA sequence comparison of 11 strains representing the main ribotype clusters showed 99·9 % similarity to the type strain ofP. multocidasubsp.multocida, but only 97·4 % similarity was obtained toP. canis(biovar 1) and 93·7 % toP. avium(biovar 1). A species-specific PCR test forP. multocidagave a positive result with biovar 2 variants ofP. aviumandP. canis. DNA–DNA hybridizations between strains ofP. multocida, biovar 2 variants ofP. aviumandP. canis, andP. multocidasubsp.septicaconfirmed similarity at the species level. It is proposed, on the basis of genotypic similarity, thatP. multocidabe reclassified to include the biovar 2 variants ofP. aviumandP. canisand that the existence of the biovar 2 variants ofP. aviumandP. canisis highly questionable. It is concluded that the redefinedP. multocidais genotypically homogeneous, although phenotypically diverse lineages exist with respect to ornithine decarboxylase, indole and mannitol, characters that have been regarded as essential for identification to the species level. A formal reclassification of the species is not possible, however, since too few strains have been found to vary in these key characters. Considering the phenotypic diversity ofP. multocida, identification will have to depend partly on genotypic methods and the source host also seems important for safe diagnosis.