Molecular epidemiology and population structure of bovine Streptococcus uberis

The molecular epidemiology and population structure of 30 bovine subclinical mastitis field isolates of Streptococcus uberis, collected from 6 Portuguese herds (among 12 farms screened) during 2002 and 2003, were examined by using pulsed-field gel electrophoresis (PFGE) for clustering of the isolates and multilocus sequence typing (MLST) to assess the relationship between PFGE patterns and to identify genetic lineages. The 30 isolates were clustered into 18 PFGE types, using a similarity cutoff of 80%, and 3 PFGE types accounted for almost half of the isolates (46.6%). These major types were herd specific, suggesting either cow-to-cow transmission or infection with isolates from the same environmental reservoirs. The remaining unrelated PFGE types of isolates were from different herds strongly suggesting environmental sources of Strep. uberis infection. All 30 isolates were analyzed by MLST and clustered into 14 sequence types (ST). These ST were found to be novel, either with 10 new alleles of 6 housekeeping genes or with different combinations of previously assigned alleles. Five of these ST were clustered into 3 clonal complexes (lineages), ST-143, ST-86, and ST-5, known to include bovine isolates from several geographic locations (Australia, New Zealand, United Kingdom, Sweden, and Denmark) and 9 singletons. To our knowledge, this is the first report that documents molecular typing studies of bovine isolates of Strep. uberis from Portugal, which were shown to represent novel genomic backgrounds of this pathogen.