Bovine Mycoplasmas: Classification of Groups Related to Mycoplasma Mycoides

Abstract

Mycoplasmas which had in various ways proved to be related to Mycoplasma mycoides were studied with a view to classification and nomenclature. Representative strains of M. mycoides — subspecies mycoides (PG1) and subspecies capri (PG3) — and of the as yet unclassified bovine serogroups 7 (PG50) and L (B144P) were compared serologically and by DNA-DNA hybridization. The serological methods used (metabolism inhibition, growth inhibition and immunofluorescence) are usually considered to be species-specific.

• (1) The two subspecies of M. mycoides can be clearly separated by all three serological methods, whereas the hybridization studies revealed great similarity between them, the relatedness value being 0.70.

• (2) The two serogroups (7 and L) cross-reacted with each other in all three serological tests, and their relatedness value was 0.83. The relatedness value between group 7 and M. mycoides subsp. mycoides was found to be 0.72, and only limited cross-reactions were demonstrated by immunofluorescence and growth inhibition.

• (3) Although groups 7 and L were found only to a slight degree to be genetically related to M. bovigenitalium with a DNA homology of 20 % or less, a serological cross-reaction between group 7 and M. bovigenitalium has been demonstrated by growth inhibition in previous studies.

Conclusions drawn on the basis of the premises just mentioned give rise to a dilemma, because three different situations exist. In (1), significant serological differences were disclosed, but the DNA homology was great. In (2), the serogroups 7 and L showed conformity between hybridization and serological findings, whereas, in (3), hybridization showed a distinct separation, but a serological cross-reaction does occur. If the results of hybridization are regarded as decisive, the two subspecies of M. mycoides can be preserved, but this is at variance with the general perception that the serological methods used here are species-specific. If, on the other hand, the serological results are chosen to be decisive, PG1 and PG3 must be regarded as two separate species, and consequently Group L/7 may also be a separate species.

As regards the relation between group L/7 and M. bovigenitalium, the results of hybridization clearly show that they are two different species. As a consequence of this, it must be accepted that cross-reactions may occur — also in growth inhibition — between different species which, genetically, are even far removed from each other.

On account of the fundamental nature of these problems, the authors prefer to leave the decisions — also as to the nomenclature — to the Subcommittee on the Taxonomy of Mycoplasmatales.

Sammendrag

Mykoplasmer, der på forskellige måder har vist sig at være re- laterede til Mycoplasma mycoides, er blevet undersøgt med henblik på klassifikation og nomenklatúr. Repræsentative stammer for Μ. mycoides – subspecies mycoides (PG1) og subspecies capri (PG3) – samt for de endnu ikke klassificerede bovine serogrupper 7 (PG50) og L (R144P), er blevet sammenholdt serologisk og ved DNA-DNA hy- bridisering. De anvendte serologiske metoder (metabolisme inhibition, væksthæmning og immunofluorescens) anses sædvanligvis for at være speciesspecifikke.

• (1) De 2 underarter af Μ. mycoides kan klart adskilles ved alle 3 serologiske metoder, men hybridiseringsundersøgelserne gav til resultat, at ligheden er stor, idet der fandtes en “relatedness value” på 0,70.

• (2) De 2 serogrupper (7 og L) krydsreagerede med hinanden ved alle 3 serologiske metoder, og der fandtes en “relatedness value” på 0,83. Der fandtes en “relatedness value” på 0,72 mellem gruppe 7 og Μ. mycoides subsp. mycoides, og der påvistes her nogen krydsreaktion ved immunofluorescens og væksthæmning.

• (3) Gruppe 7 og L fandtes i ringe grad at være genetisk relateret til Μ. bovigenitalium, idet der fandtes en DNA-homologi på 20 % og derunder. I serologisk henseende er der i tidligere arbejder ved væksthæmning påvist krydsreaktion meliem gruppe 7 og Μ. bovigenitalium.

I henseende til konklusioner på grundlag af de foranstående præ- misser kommer man i et dilemma, idet der foreligger 3 forskellige si- tuationer. Ved (1) fandtes signifikante serologiske forskelle, men DNA- homologien var stor. Ved serogrupperne 7 og L (2) var der overens- stemmelse meliem hybridisering og serologiske fund. I (3) var der klar adskillelse ved hybridisering; serologisk krydsreaktion er tidli- gere påvist. Vælger man at lade hybridiseringsresultaterne være af- gørende, kan man bibeholde de 2 underarter af Μ. mycoides, men man kommer herved i modstrid med hovedreglen, at de benyttede serologiske metoder er speciesspecifikke. Vælger man at lade Serologien være mest afgørende, bliver PG1 og PG3 at betragte som 2 selvstændige arter, og gruppe L/7 må som en konsekvens også blive en selvstændig art.

Med hensyn til relationen meliem sidstnævnte gruppe og Μ. bovi- genitalium viser hybridiseringsresultaterne klart, at det drejer sig om 2 forskellige arter. Som en konsekvens heraf må man acceptere, at der kan være krydsreaktioner, også ved væksthæmning, mellem forskellige arter, der genetisk set står hinanden endog meget fjernt.

På grundlag af problemernes principielle natur foretrækker for- fatterne at overlade afgørelserne – og dermed navngivningen – til The Subcommittee on the Taxonomy of Mycoplasmatales.