Subspecies distribution and virulence factors of Streptococcus uberis
Creators: name: Tomita, Takehiro
Publisher: name: RMIT University
Year: 2024

description: Mastitis is the single most important cause of financial loss to the dairy industry worldwide with production losses amounting to more than $US 2 billion annually. In Australia, losses as a result of mastitis are estimated to be in excess of $A 150 million per year, mainly due to decreased milk output and reduced milk quality payments. Although the introduction of mastitis control schemes has been successful in reducing the incidence of contagious mastitis caused by <i>Streptococcus agalactiae </i>and <i>Staphylococcus aureus </i>in dairy herds, these measures have had little impact on environmental pathogens, notably <i>Streptococcus uberis </i>and <i>Escherichia coli</i>.<br><br>In this study, approximately 300 individual quarter milk samples from cases comprising both clinical and subclinical cases of mastitis and from cows with low somatic cell counts in their milk were screened. In total, 46 isolates were confirmed to be S. uberis and used in all subsequent experiments.<br><br>Analyses of the Australian isolates by pulsed-field gel electrophoresis (PFGE) demonstrated the occurrence of a genetically diverse population, with the predomination of no particular clone or PFGE type. The possible transmission of specific <i>S. uberis </i>PFGE type[s] between individual cows or within quarters of the same cow was observed in four instances but such events were uncommon.<br><br>In contrast to the genetically diverse population identified by PFGE analyses, multilocus sequence typing (MLST) analysis of the Australian isolates identified a cluster of MLST types associated with clinical and subclinical mastitis and one associated with cows with low somatic cell counts in their milk. Specifically, genotypes belonging to the global clonal complex (GCC) sequence type 5s (ST5s) and GCC ST143s were highly associated (<i>P &lt; </i>0.006) with clinical and subclinical mastitis and may represent a lineage of virulent isolates, whereas isolates belonging to GCC ST86 were associated only with cows with low somatic cell counts in their milk.<br><br>In this study, the highly characterized and clinically diverse Australian S. uberis isolates facilitated the investigation of S. uberis virulence factors utilizing a subtractive hybridization approach. Subtractive hybridization, using genomic DNA from an isolate from a cow with a low cell count to enrich for sequences from genomic DNA from a <i>S. uberis </i>isolate from a case of clinical mastitis, facilitated the identification and characterization of two putative virulence-associated sequences (T3-06 and T4-19) that were highly associated with clinical / subclinical mastitis and GCC type.<br><br>The subtractive hybridization clone T4-19 encoded a transposase-like gene sequence highly homologous to a sequence identified adjacent to the C5a peptidase virulence factor gene in <i>S. agalactiae</i>. Whilst this transposase-like gene alone is unlikely to represent a <i>S. uberis </i>virulence factor, the T4-19 transposase-like sequence was nevertheless a useful virulence marker exhibiting a high association with both disease status and GCC type in the Australian isolates.<br><br>By analogy with known collagen-like virulence factors in S. aureus and <i>S. pyogenes</i>, the putative collagen-like surface protein encoded by the <i>S. uberis </i>subtractive hybridization clone T3 06 may represent a new <i>S. uberis </i>virulence factor.