Molecular Typing and Antimicrobial Susceptibility Profiles of Streptococcus uberis Isolated From Sheep Milk

Abstract

Intrammary infections are a major problem for dairy sheep farms, and Streptococcus uberis is one of the main etiological agents of ovine mastitis. Surveys on antimicrobial resistance are still limited in sheep and characterization of isolates is important for acquiring information on resistance and for optimizing therapy. In this study, a sampling of 124 S. uberis isolates collected in Sardinia (Italy) from sheep milk was analysed by multilocus-sequence typing (MLST) and pulsed field gel electrophoresis (PFGE) for genetic relatedness. All isolates were also subjected to antimicrobial susceptibility analysis by the disk diffusion test using a panel of 14 antimicrobials. Resistance genes were detected by PCR assays. MLST analysis revealed that the isolates were grouped into 86 sequence types (STs), of which 73 were new genotypes, indicating a highly diverse population of S. uberis. The most frequently detected lineage was the clonal complex (CC)143, although representing only 13.7% of all characterized isolates. A high level of heterogeneity was also observed among the SmaI PFGE profiles, with 121 unique patterns. Almost all (96.8%) isolates were resistant to at least one antimicrobial, while all exhibited phenotypic susceptibility to oxacillin, amoxicillin-clavulanic acid and ceftiofur. Of the antimicrobials tested, the highest resistance rate was found against streptomycin (93.5%), kanamycin (79.8%) and gentamicin (64.5%), followed by novobiocin (25%) and tetracycline-TE (19.3%). Seventy-four (59.7%) isolates were simultaneously resistant to all aminoglycosides tested. Seventeen isolates (13.7%) exhibited multidrug resistance. All aminoglycosides-resistant isolates were PCR negative for aad-6 and aphA-3’ genes. Among the TE-resistant isolates, the tetM gene was predominant, indicating that the resistance mechanism is mainly mediated by the protection of ribosomes and not through the efflux pump. Three isolates were resistant to erythromycin, and two of them harboured the ermB gene. This is the first study reporting a detailed characterization of the S. uberis strains circulating in Sardinian sheep. Further investigations will be needed to understand the relationships between S. uberis genotypes, mastitis severity, and intra-mammary infection dynamics in the flock, as well as to monitor the evolution of antimicrobial resistance.

Evaluation of Streptococcus species isolated from subclinical sheep mastitis by molecular methods and determination of virulence factors and antimicrobial resistance genes

“…The aad-6 gene was detected in S. uberis and S. dysgalactiae species in dairy cows (Kaczorek et al 2017a). Another study reported that aminoglycoside-resistant Streptococcus isolates are negative for aad-6 genes (Rosa et al 2021). Our results indicate that only 6 S. uberis isolates carried the aad-6 gene, aligning with the inherent aminoglycoside resistance of Streptococcus strains.…”

“…The blaZ gene is frequently detected in France for S. uberis, lnuD for S. uberis, and S. dysgalactiae in Poland (Naranjo-Lucena and Slowey 2023). Detection of the blaZ gene has also been reported in sheep milk (Rosa et al 2021). However, we detected the blaZ gene in S. uberis isolates, and the lnu gene in one S. agalactiae isolate (3.1%), which is consistent with its reported low prevalence (Haenni et al 2018).…”

“…In several countries, including France, Brazil, Canada, Portugal, Poland, Egypt, Argentina, and China, S. uberis, S. agalactiae, or S. dysgalactiae have been reported to harbour the gene tetM in bovine mastitis (Naranjo-Lucena and Slowey 2023). Also, it has been reported that the dominance of the tetM gene indicates that the mechanism of resistance is mainly mediated by the protection of the ribosomes and not by the efflux pump (Rosa et al 2021). The dominance of the tetM gene suggests that resistance mechanisms primarily involve ribosome protection rather than efflux pumps (Rosa et al 2021).…”

“…Also, it has been reported that the dominance of the tetM gene indicates that the mechanism of resistance is mainly mediated by the protection of the ribosomes and not by the efflux pump (Rosa et al 2021). The dominance of the tetM gene suggests that resistance mechanisms primarily involve ribosome protection rather than efflux pumps (Rosa et al 2021). We detected resistance to tetracycline in our isolates, with tetM being the most frequently detected resistance gene in S. uberis (62.5%), followed by tetS (50%), ermC (43.7%), and tetL (40.6%).…”

“…Saed and Ibrahim (2020) reported the common presence of the ermB gene in Streptococcus isolates from mastitis in dairy cows, with a prevalence of 40% among erythromycin-resistant isolates. Erythromycinresistant Streptococcus isolates from sheep milk have also been reported to carry the ermB gene (Rosa et al 2021). The ermC gene resistance was found in 11 S. uberis isolates and 3 S. agalactiae isolates.…”