Characterisation of North American Brucella isolates from marine mammals

Abstract

Extension of known ecological niches of Brucella has included the description of two novel species from marine mammals. Brucella pinnipedialis is associated predominantly with seals, while two major Brucella ceti clades, most commonly associated with porpoises or dolphins respectively, have been identified. To date there has been limited characterisation of Brucella isolates obtained from marine mammals outside Northern European waters, including North American waters. To address this gap, and extend knowledge of the global population structure and host associations of these Brucella species, 61 isolates from marine mammals inhabiting North American waters were subject to molecular and phenotypic characterisation enabling comparison with existing European isolates. The majority of isolates represent genotypes previously described in Europe although novel genotypes were identified in both B. ceti clades. Harp seals were found to carry B. pinnipedialis genotypes previously confined to hooded seals among a diverse repertoire of sequence types (STs) associated with this species. For the first time Brucella isolates were characterised from beluga whales and found to represent a number of distinct B. pinnipedialis genotypes. In addition the known host range of ST27 was extended with the identification of this ST from California sea lion samples. Finally the performance of the frequently used diagnostic tool Bruce-ladder, in differentiating B. ceti and B. pinnipedialis, was critically assessed based on improved knowledge of the global population structure of Brucella associated with marine mammals.

Fig 1. Phylogenetic relationships of BruMLSA21 STs based on concatenated sequence data.

Bar = substitutions per nucleotide position.

Fig 2. Minimum spanning tree of available BruMLSA21 profiles from European and North American sampling illustrating the relationship between ST and host species.

Each circle denotes a particular ST type with the size of the circle illustrating the number of isolates of that particular type. Thick solid lines denote single locus variants (SLV), while thinner and thinnest solid lines represent STs that vary at 2 or 3 loci respectively. Dashed lines types that vary at 4 or more loci. The halos surrounding groupings represent clusters defined in Bionumerics created if neighbours differed in no more than 3 of 21 loci.

Fig 3. Bruce-ladder profiles of representatives of each of the major marine mammal Brucella BruMLSA21 STs based on the Mayer-Scholl protocol [32].

Lane 1. Ladder. Lane 2. ST24 F8/08-32. Lane 3. ST25 F8/08-2. Lane 4. ST26 F8/08-50. Lane 5. ST27 F8/08-1. Lane 6. ST52 F8/08-12. Lane 7. ST53 F8/08-13. Lane 8. ST54 F8/08-11. Lane 9. ST102 F8/08-46. Lane 10. ST103 F8/08-23. Lane 11. ST23 VLA04/72. Lane 12. ST50 UK4/06. Lane 13. ST51 55/94. Lane 14. ST55 UK1/2000. Lane 15. ST56 VLA06/1. Lane 16. PCR negative. Lane 17. Ladder. Arrow = 794bp band.