First insights into the evolution of Streptococcus uberis: a multilocus sequence typing scheme that enables investigation of its population biology

Abstract

Intramammary infection with Streptococcus uberis is a common cause of bovine mastitis throughout the world. Several procedures to differentiate S. uberis isolates have been proposed. However, all are prone to interlaboratory variation, and none is suitable for the description of the population structure. We describe here the development of a multilocus sequence typing (MLST) scheme for S. uberis to help address these issues. The sequences of seven housekeeping gene fragments from each of 160 United Kingdom milk isolates of S. uberis were determined. Between 5 and 17 alleles were obtained per locus, giving the potential to discriminate between 1.3 x 10(7) sequence types. In this study, 57 sequence types (STs) were identified. Statistical comparisons between the maximum-likelihood trees constructed by using the seven housekeeping gene fragments showed that the congruence was no better than that between each tree and trees of random topology, indicating there had been significant recombination within these loci. The population contained one major lineage (designated the ST-5 complex). This dominated the population, containing 24 STs and representing 112 isolates. The other 33 STs were not assigned to any clonal complex. All of the isolates in the ST-5 lineage carried hasA, a gene that is essential for capsule production. There was no clear association between ST or clonal complex and disease. The S. uberis MLST system offers researchers a valuable tool that allows further investigation of the population biology of this organism and insights into the epidemiology of this disease on a global scale.

FIG. 1.

ML trees of concatenated sequences from combinations of three loci. ST designations are given in Table 3. All horizontal branch lengths are drawn to scale.

FIG. 2.

BURST results: the relationships between the STs in the largest BURST group (group definition of five identical loci). Central STs represent predicted group or subgroup founders. The inner rings contain single locus variants, the outer rings represent double locus variants, and STs outside the rings are triple locus variants.

FIG. 3.

eBURST analysis: the predicted evolutionary descent of STs within the largest e-BURST group (group definition of six identical loci). The predicted founder (ST-5) is in boldface. The area of the circles representing STs correlates with the number of isolates of that ST.