Vru (Sub0144) controls expression of proven and putative virulence determinants and alters the ability of Streptococcus uberis to cause disease in dairy cattle

Whole-genome sequencing reveals high genetic diversity of Streptococcus uberis isolated from cows with mastitis

Background

Bovine mastitis is an important cause of economic loss in dairy farms. Streptococcus uberis is among the most frequently isolated bacterial species isolated from cows with mastitis. The aim of this study was to perform an in-depth genetic assessment of S. uberis strains isolated from bovine clinical mastitis (CM) and to perform a phylogenetic analysis to represent the evolutionary relationship among S. uberis sequences.

Results

A total of 159 isolates was genetically characterized using whole genome sequencing. According to the virulence determinants, all strains harbored the hasC, leuS, perR, purH, and purN virulence genes. Thirty-four resistance genes were identified in at least one strain. In terms of acquired genes, we observed that 152 (95.6 %) strains had a resistance gene to lincosamine (lnuD), 48 (30.2 %) to tetracycline (tetM), 4 (2.51 %) to tobramicine (ant6), and 1 to lincosamide (lsa(E)). MLST detected the Sequence Type (ST)797 (n = 23), while 85.5 % of the strains did not match to known STs.

Conclusions

Then, eleven distinct ST were identified after we submitted the new alleles to assign new STs. The other prevalent STs observed were ST1215 (n = 58), ST1219 (n = 35), and ST1213 (n = 15). And it was not possible to identify the MLST of four strains. Phylogenetic lineages indicated a high genomic diversity of S. uberis in our collection, confirming that most strains isolated from bovine mastitis have different reservoirs, typical of environmental pathogens.

Evaluation of Streptococcus uberis Surface Proteins as Vaccine Antigens to Control S. uberis Mastitis in Dairy Cows

There is no effective vaccine against Streptococcus uberis mastitis in dairy cows. Objectives of this study were (1) to extract S. uberis surface proteins (SUSP) and determine immunoreactivity in vitro and (2) immunogenicity and efficacy in vivo. SUSP was extracted from S. uberis, and their immunoreactivity was tested by western blot. In total, 26 Jersey dairy cows were randomly divided into four groups. Groups 1, 2, and 3 were vaccinated subcutaneously with 4 mg, 1 mg, and 100 μg of SUSP, respectively, with Freund’s incomplete adjuvant. Group 4 (control) was injected with placebo. S. uberis UT888 was infused into two contralateral quarters of each cow during early lactation. Somatic cell count (SCC), bacteria count in milk, and mastitis were monitored. Our results show that SUSP contains multiple protein bands, that ranged from 10 to 100 kDa. All vaccinates showed an increased anti-SUSP IgG antibody. The SCC of all experimentally infected quarters increased after challenge but slightly decreased after day 3 with no significant difference among groups. Milk bacterial count was significantly (p < 0.05) reduced in high and medium doses vaccinated groups than low and control groups. In conclusion, SUSP vaccine is immunogenic and showed a promising efficacy to control bovine S. uberis mastitis.

The Role of Streptococcus spp. in Bovine Mastitis

The Streptococcus genus belongs to one of the major pathogen groups inducing bovine mastitis. In the dairy industry, mastitis is the most common and costly disease. It not only negatively impacts economic profit due to milk losses and therapy costs, but it is an important animal health and welfare issue as well. This review describes a classification, reservoirs, and frequencies of the most relevant Streptococcus species inducing bovine mastitis (S. agalactiae, S. dysgalactiae and S. uberis). Host and environmental factors influencing mastitis susceptibility and infection rates will be discussed, because it has been indicated that Streptococcus herd prevalence is much higher than mastitis rates. After infection, we report the sequence of cow immune reactions and differences in virulence factors of the main Streptococcus species. Different mastitis detection techniques together with possible conventional and alternative therapies are described. The standard approach treating streptococcal mastitis is the application of ß-lactam antibiotics. In streptococci, increased antimicrobial resistance rates were identified against enrofloxacin, tetracycline, and erythromycin. At the end, control and prevention measures will be considered, including vaccination, hygiene plan, and further interventions. It is the aim of this review to estimate the contribution and to provide detailed knowledge about the role of the Streptococcus genus in bovine mastitis.

Potential factors involved in the early pathogenesis of Streptococcus uberis mastitis: a review

Bovine mastitis is an inflammation of the mammary gland, which could be the result of allergy, physical trauma, or invasion by pathogens as Streptococcus uberis. This pathogen is an environmental pathogen associated with subclinical and clinical intramammary infection (IMI) in both lactating and non-lactating cows, which can persist in the udder and cause a chronic infection in the mammary gland. In spite of the important economic losses and increased prevalence caused by S. uberis mastitis, virulence factors involved in bacterial colonization of mammary glands and the pathogenic mechanisms are not yet clear. In the last 30 years, several studies have defined adherence and internalization of S. uberis as the early stages in IMI. S. uberis adheres to and invades into mammary gland cells, and this ability has been observed in in vitro assays. Until now, these abilities have not been determined in vivo challenges since they have been difficult to study. Bacterial surface proteins are able to bind to extracellular matrix protein components such as fibronectin, collagen and laminin, as well as proteins in milk. These proteins play a role in adhesion to host cells and have been denominated microbial surface components recognizing adhesive matrix molecules (MSCRAMMs). This article aims to summarize our current knowledge on the most relevant properties of the potential factors involved in the early pathogenesis of S. uberis mastitis.

Sequence characterisation and novel insights into bovine mastitis-associated Streptococcus uberis in dairy herds

Streptococcus uberis is one of the most frequent mastitis-causing pathogens isolated from dairy cows. Further understanding of S. uberis genetics may help elucidate the disease pathogenesis. We compared the genomes of S. uberis isolates cultured from dairy cows located in distinctly different geographic regions of Australia. All isolates had novel multi locus sequence types (MLST) indicating a highly diverse population of S. uberis. Global clonal complexes (GCC) were more conserved. GCC ST86 and GCC ST143 represented 30% of the total isolates (n = 27) and were clustered within different geographic regions. Core genome phylogeny revealed low phylogenetic clustering by region, isolation source, and MLST. Identification of putative sortase (srtA) substrates and generation of a custom putative virulence factor database revealed genes which may explain the affinity of S. uberis for mammary tissue, evasion of antimicrobial efforts and disease pathogenesis. Of 27 isolates, four contained antibiotic resistance genes including an antimicrobial resistance cluster containing mel/mef(A), mrsE, vatD, lnuD, and transposon-mediated lnuC was also identified. These are novel genes for S. uberis, which suggests interspecies lateral gene transfer. The presence of resistance genes across the two geographic regions tested within one country supports the need for a careful, tailored, implementation and monitoring of antimicrobial stewardship.

PCR-Based Direct Detection of Streptococcus uberis from Subclinical and Clinical Dairy Cattle Milk Samples

Streptococcus uberis is one of the leading causes worldwide of mastitis in the dairy industry, with the most likely sources of infection attributed to environmental reservoirs such as contaminated bedding materials. Early detection of those cases most likely to progress to clinical disease would lead to improved animal welfare, a critical component of overall health and productivity. A multiplex PCR-based diagnostic test was developed for detection of S. uberis directly from milk and targeting two genes previously identified as important for intramammary colonisation and persistence in dairy cattle. Results indicated the threshold for detection directly from milk was 20,000 CFU/ml and this was achieved without the need for preenrichment. In addition, S. uberis could be identified from milk samples collected during intramammary challenge studies, prior to clinical signs of infection and at much lower detection limits. The PCR test developed for confirmation of the presence of S. uberis directly from infected milk has potential value as a diagnostic test to identify early infection and/or to confirm that antibiotic therapy has been successful.

A novel subtraction diversity array distinguishes between clinical and non-clinical Streptococcus uberis and identifies potential virulence determinants

Streptococcus uberis is an important bovine mastitis pathogen, but not all isolates have equal capacity to cause disease. The aims of this study were to identify possible virulence-associated genes that could be used to identify isolates with enhanced virulence. DNA from a pool of putative commensals was subtracted from a clinical pool resulting in a set of DNA sequences (probes) that were enriched in the clinical mastitis group. The probes were hybridised with DNA from a collection 29 isolates from cases of clinical mastitis and isolates not associated with disease. Hybridization revealed five major clusters. The first cluster (7 isolates) consisted almost entirely of commensals, while the second (7 isolates) was mixed. The remaining three clusters contained 15 S. uberis isolates from cows with clinical mastitis. Twenty-six probes were selected for sequencing based on principal component analysis (PCA) or their presence mainly in clinical isolates. PCA identified five probes with clear differences in intensity between signals from clinical isolates and commensals; these probes could represent novel virulence determinants. Manual inspection of arrays identified genes prominent among clinical isolates that specify carbohydrate and lipid metabolism (possible role in the growth or survival of S. uberis in milk) and genes specifying hypothetical proteins, possibly novel virulence factors. The common occurrence, among clinical isolates, of probes having homology with transposases and insertion sequences suggests recent acquisition of factors that could be associated with virulence. These results suggest the existence of a subset of S. uberis with enhanced virulence, due possession of virulence-associated gene sequences.

Genotyping and study of adherence-related genes of Streptococcus uberis isolates from bovine mastitis

The aim of this study was to determine the presence, conservation, and distribution of 6 potential adherence genes and their relationship with diverse molecular types in 34 S. uberis isolated from bovine mastitis in Argentina. Pulsed-field gel electrophoresis (PFGE) typing with SmaI was performed. The PCR for the detection of each gene, scpA, acdA, fbp, lbp, lmb, and sua was standardized. Samples of the amplification products were purified and sequenced. The PFGE patterns revealed the high level of heterogeneity of S. uberis, with 26 types of PFGE patterns. A high prevalence of scpA, fbp, lbp, lmb and acdA genes (100%-97%) was detected, whereas 79.41% of S. uberis harbored the sua gene. A high degree of similarity in the nucleotide and amino acid sequences of the 6 genes was observed. Our results showed that all genes are conserved and are present in most S. uberis isolates despite the wide clonal heterogeneity detected. This is the first study reporting an analysis of prevalence, and nucleotides and amino acids sequences of the potential adherence genes scpA, acdA, fbp, lbp, and lmb from S. uberis strains versus reported GenBank sequences, S. uberis 0140J and S. uberis NZ01.

Role of Streptococcus uberis adhesion molecule in the pathogenesis of Streptococcus uberis mastitis

Adherence to and internalization into mammary epithelial cells are central mechanisms in the pathogenesis of S. uberis mastitis. Through these pathogenic strategies, S. uberis reaches an intracellular environment where humoral host defenses and antimicrobials in milk are essentially ineffective, thus allowing persistence of this pathogen in the mammary gland. We reported that S. uberis expresses a surface adhesion molecule (SUAM) that has affinity for lactoferrin (LF) and a central role adherence to and internalization of S. uberis into bovine mammary epithelial cells. To define the role of SUAM in the pathogenesis of S. uberis mastitis, we created a sua gene deletion mutant clone of S. uberis UT888 (Δsua S. uberis UT888) unable to express SUAM. When tested in vitro, Δsua S. uberis UT888 was defective in adherence to and internalization into bovine mammary epithelial cells. To prove that the absence of SUAM reduces bacterial attachment, subsequent colonization and infection of bovine mammary glands, the wild type S. uberis UT888 and its isogenic Δsua S. uberis UT888 were infused into mammary quarters of dairy cows. Results showed that fewer mammary glands infused with Δsua S. uberis UT888 become infected than those infused with the isogenic parental strain. Furthermore, mammary glands infused with Δsua S. uberis UT888 had less severe clinical symptoms as compared to those infused with the isogenic parental strain. These results suggest that the SUAM mutant clone was less virulent than the isogenic parental strain which further substantiates the role of SUAM in the pathogenesis of S. uberis mastitis.

Virulence related sequences; insights provided by comparative genomics of Streptococcus uberis of differing virulence

Background

Streptococcus uberis, a Gram-positive, catalase-negative member of the family Streptococcaceae is an important environmental pathogen responsible for a significant proportion of subclinical and clinical bovine intramammary infections. Currently, the genome of only a single reference strain (0140J) has been described. Here we present a comparative analysis of complete draft genome sequences of an additional twelve S. uberis strains.

Results

Pan and core genome analysis revealed the core genome common to all strains to be 1,550 genes in 1,509 orthologous clusters, complemented by 115-246 accessory genes present in one or more S. uberis strains but absent in the reference strain 0140J. Most of the previously predicted virulent genes were present in the core genome of all 13 strains but gene gain/loss was observed between the isolates in CDS associated with clustered regularly interspaced short palindromic repeats (CRISPRs), prophage and bacteriocin production. Experimental challenge experiments confirmed strain EF20 as non-virulent; only able to infect in a transient manner that did not result in clinical mastitis. Comparison of the genome sequence of EF20 with the validated virulent strain 0140J identified genes associated with virulence, however these did not relate clearly with clinical/non-clinical status of infection.

Conclusion

The gain/loss of mobile genetic elements such as CRISPRs and prophage are a potential driving force for evolutionary change. This first “whole-genome” comparison of strains isolated from clinical vs non-clinical intramammary infections including the type virulent vs non-virulent strains did not identify simple gene gain/loss rules that readily explain, or be confidently associated with, differences in virulence. This suggests that a more complex dynamic determines infection potential and clinical outcome not simply gene content.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1512-6) contains supplementary material, which is available to authorized users.