The Molecular Epidemiology of the Highly Virulent ST93 Australian Community <em>Staphylococcus aureus</em> Strain

In Australia the PVL - positive ST93-IV [2B], colloquially known as “Queensland CA-MRSA” has become the dominant CA-MRSA clone. First described in the early 2000s, ST93-IV [2B] is associated with skin and severe invasive infections including necrotizing pneumonia. A singleton by multilocus sequence typing (MLST) eBURST analysis ST93 is distinct from other S aureus clones. To determine if the increased prevalence of ST93-IV [2B] is due to the widespread transmission of a single strain of ST93-IV [2B] the genetic relatedness of 58 S. aureus ST93 isolated throughout Australia over an extended period were studied in detail using a variety of molecular methods including pulsed-field gel electrophoresis, spa typing, MLST, microarray DNA, SCCmec typing and dru typing. Identification of the phage harbouring the lukS-PV/lukF-PV Panton Valentine leucocidin genes, detection of allelic variations in lukS-PV/lukF-PV, and quantification of LukF-PV expression was also performed. Although ST93-IV [2B] is known to have an apparent enhanced clinical virulence, the isolates harboured few known virulence determinants. All PVL-positive isolates carried the PVL-encoding phage ΦSa2USA and the lukS-PV/lukF-PV genes had the same R variant SNP profile. The isolates produced similar expression levels of LukF-PV. Although multiple rearrangements of the spa sequence have occurred, the core genome in ST93 is very stable. The emergence of ST93-MRSA is due to independent acquisitions of different dru-defined type IV and type V SCCmec elements in several spa-defined ST93-MSSA backgrounds. Rearrangement of the spa sequence in ST93-MRSA has subsequently occurred in some of these strains. Although multiple ST93-MRSA strains were characterised, little genetic diversity was identified for most isolates, with PVL-positive ST93-IVa [2B]-t202-dt10 predominant across Australia. Whether ST93-IVa [2B] t202-dt10 arose from one PVL-positive ST93-MSSA-t202, or by independent acquisitions of SCCmec-IVa [2B]-dt10 into multiple PVL-positive ST93-MSSA-t202 strains is not known.在澳大利亚地区，杀白细胞素（Panton-Valentine Leukocidin, PVL）阳性ST93-IV [2B]菌株，俗称“昆士兰社区获得性耐甲氧西林金黄色葡萄球菌（Queensland CA-MRSA）”，已成为占主导地位的社区获得性耐甲氧西林金黄色葡萄球菌（Community-associated Methicillin-resistant Staphylococcus aureus, CA-MRSA）克隆株。该菌株于21世纪初首次被报道，可引发皮肤感染及包括坏死性肺炎在内的重症侵袭性感染。经多位点序列分型（multilocus sequence typing, MLST）的eBURST分析证实，ST93为单序列型菌株，与其他金黄色葡萄球菌（Staphylococcus aureus, S. aureus）克隆株存在显著差异。 为明确ST93-IV [2B]的流行率升高是否源于单一菌株的广泛传播，本研究对澳大利亚境内长期分离得到的58株ST93型金黄色葡萄球菌展开了详细的遗传相关性分析，采用的分子生物学方法包括脉冲场凝胶电泳（pulsed-field gel electrophoresis, PFGE）、spa分型、多位点序列分型（MLST）、DNA芯片检测、葡萄球菌染色体mec盒分型（staphylococcal cassette chromosome mec typing, SCCmec typing）及dru分型。本研究同时完成了携带杀白细胞素（PVL）编码基因lukS-PV/lukF-PV的噬菌体鉴定、lukS-PV/lukF-PV等位基因变异检测以及LukF-PV蛋白表达量定量分析。 尽管已有研究表明ST93-IV [2B]具有显著增强的临床致病力，但本次研究分离得到的菌株仅携带少量已知的毒力决定因子。所有PVL阳性菌株均携带PVL编码噬菌体ΦSa2USA，且lukS-PV/lukF-PV基因均携带相同的R变异型单核苷酸多态性（single nucleotide polymorphism, SNP）谱型，各菌株的LukF-PV蛋白表达水平相近。尽管spa基因序列已发生多次重排，但ST93的核心基因组仍保持高度稳定。 ST93型耐甲氧西林金黄色葡萄球菌（ST93-MRSA）的出现，源于多个spa分型的ST93型甲氧西林敏感金黄色葡萄球菌（ST93-MSSA）背景菌株独立获得了不同dru分型的IV型和V型葡萄球菌染色体mec盒（SCCmec）元件，部分此类菌株后续又发生了spa基因序列的重排。尽管已对多株ST93-MRSA菌株进行了分型鉴定，但绝大多数分离株的遗传多样性极低，其中PVL阳性ST93-IVa [2B]-t202-dt10菌株在澳大利亚境内占主导地位。目前尚不清楚ST93-IVa [2B]-t202-dt10菌株究竟起源于单一株PVL阳性ST93-MSSA-t202，还是由多株PVL阳性ST93-MSSA-t202菌株独立获得SCCmec-IVa [2B]-dt10元件演化而来。