Identification of Streptococcus uberis multilocus sequence types highly associated with mastitis

Abstract

Multilocus sequence typing analysis of Streptococcus uberis has identified a cluster of isolates associated with clinical and subclinical mastitis and a cluster associated with cows with low somatic cell counts in their milk. Specific groups of genotypes (global clonal complex [GCC] sequence type 5s [ST5s] and GCC ST143s) were highly associated (P = 0.006) with clinical and subclinical mastitis and may represent a lineage of virulent isolates, whereas isolates belonging to GCC ST86 were associated with low-cell-count cows. This study has, for the first time, demonstrated the occurrence of identical sequence types (ST60 and ST184) between different continents (Australasia and Europe) and different countries (Australia and New Zealand). The standardized index of association and the empirical estimation of the rate of recombination showed substantial recombination within the S. uberis population in Australia, consistent with previous multilocus sequence type analyses.

FIG. 1.

Dendrogram showing genetic relatedness of the 46 Australian S. uberis as determined by the unweighted pair grouping method using arithmetic means. Clusters are denoted by the bars and Roman numerals. Strains belonging to local clonal complexes are indicated with a bar and by the ST which corresponded to the founder of the LCC. The disease status is indicated as C (clinical mastitis), S (subclinical), and L (low somatic cell count). The global clonal complexes assigned to the isolates are given. The PFGE types and subtypes are represented with Arabic numbers and letters, respectively, with the number of isolates if more than one is given in parentheses. The countries of origin of previously reported STs are listed as United Kingdom (UK) and New Zealand (NZ). The yqiL-negative [yqiL (−)] isolates (which have yet to be assigned a ST by the MLST database) are indicated in the Note column and by their strain identifications in the sequence type column.

FIG. 2.

eBURST analysis of 46 Australian S. uberis isolates. The founder of a clonal complex is situated at the center of the chart. Disease status was indicated as C (clinical mastitis) and SC (subclinical mastitis) with numbers which correspond to ST. Single-locus variants are joined by lines.

FIG. 3.

Snapshot of global population structure of S. uberis generated by eBURST. The three GCCs (ST143, ST5, and ST86) and the Australian cluster are represented by light shaded ovals. The Australian isolates are indicated by numbers corresponding to their ST. The sizes of the dots are in direct proportion to the size of the population. Single-locus variants are joined by lines.

FIG. 4.

Assignment of single-locus variants and clonal complexes by BURST for empirical estimation of rate of recombination to mutation. The founder of a CC is shown in the center circle, and SLVs are in the outer circle. Small circles with a black background indicate that the SLV arose by recombination, whereas a circle with a white background indicates that the SLV arose by point mutation. The numbers in the small circles show the number of nucleotide differences between a variant allele of the SLV and the corresponding allele of the founder.