A number of veterinary clinical pathology laboratories in New Zealand have been reporting emergence of increased minimum in inhibitory concentrations for β-lactams in the common clinical bovine mastitis pathogen Streptococcus uberis. The objective of this study was to determine the genetic basis of this increase in MIC for β-lactams amongst S. uberis. Illumina sequencing and determination of oxacillin MIC was performed on 265 clinical isolates. Published sequences of the five penicillin binding proteins pbp1a, pbp1b, pbp2a, pbp2b, and pbp2x were used to identify, extract and align these sequences from the study isolates. Amino acid substitutions resulting from single nucleotide polymorphisms (SNP) within these genes were analysed for associations with elevated (≥ 0.5 mg/L) oxacillin MIC together with a genome wide association study. The population structure of the study isolates was approximated using a phylogenetic tree generated from an alignment of the core genome. A total of 53 % of isolates had MIC ≥ 0.5 mg/L for oxacillin. A total of 101 substitutions within the five pbp were identified, of which 11 were statistically associated with an MIC ≥ 0.5 mg/L. All 140 isolates which exhibited an increased β-lactam MIC had SNPs leading to pbp2x E381K and Q554E substitutions. The phylogenetic tree indicated that the genotype and phenotype associated with the increased MIC for oxacillin were present in several different lineages suggesting that acquisition of this increased β-lactam MIC had occurred in multiple geographically distinct regions. Reanalysis of the data from the intervention studies from which the isolates were originally drawn found a tendency for the pbp2x E381K substitution to be associated with lower cure rates. It is concluded that there is geographically and genetically widespread presence of pbp substitutions associated with reduced susceptibility to β-lactam antimicrobials. Additionally, presence of pbp substitutions tended to be associated with poorer cure rate outcomes following antimicrobial therapy for clinical mastitis.

中文翻译：

---

牛乳腺炎病例分离的乳房链球菌的β-内酰胺耐药机制。


新西兰的许多兽医临床病理学实验室报告说，临床常见牛乳腺炎病原体乳房链球菌中β-内酰胺的最低抑制浓度出现增加。本研究的目的是确定乳房链球菌中 β-内酰胺 MIC 增加的遗传基础。对 265 个临床分离株进行了 Illumina 测序和苯唑西林 MIC 测定。已发布的五种青霉素结合蛋白 pbp1a、pbp1b、pbp2a、pbp2b 和 pbp2x 的序列用于从研究分离株中识别、提取和比对这些序列。通过全基因组关联研究，分析了这些基因内单核苷酸多态性 (SNP) 引起的氨基酸替换与苯唑西林 MIC 升高 (≥ 0.5 mg/L) 的关联。使用核心基因组比对生成的系统发育树来近似研究分离株的种群结构。总共 53% 的分离株对苯唑西林的 MIC ≥ 0.5 mg/L。 5 个 pbp 内总共鉴定出 101 个取代，其中 11 个与 MIC ≥ 0.5 mg/L 具有统计相关性。所有 140 个表现出增加的 β-内酰胺 MIC 的分离株均具有导致 pbp2x E381K 和 Q554E 取代的 SNP。系统发育树表明，与苯唑西林 MIC 增加相关的基因型和表型存在于几个不同的谱系中，表明这种增加的 β-内酰胺 MIC 的获得发生在多个地理不同的区域。对最初提取分离株的干预研究的数据进行重新分析发现，pbp2x E381K 替代有与较低治愈率相关的趋势。 结论是，pbp 取代在地理和遗传上广泛存在，与降低对 β-内酰胺抗菌剂的敏感性相关。此外，pbp 替代的存在往往与临床乳腺炎抗菌治疗后较差的治愈率结果相关。