Department of Microbiology, Monash University, Clayton, Victoria, Australia.
PLoS Pathog. 2010 Jun 10;6(6):e1000944. doi: 10.1371/journal.ppat.1000944.
Staphylococcus aureus frequently invades the human bloodstream, leading to life threatening bacteremia and often secondary foci of infection. Failure of antibiotic therapy to eradicate infection is frequently described; in some cases associated with altered S. aureus antimicrobial resistance or the small colony variant (SCV) phenotype. Newer antimicrobials, such as linezolid, remain the last available therapy for some patients with multi-resistant S. aureus infections. Using comparative and functional genomics we investigated the molecular determinants of resistance and SCV formation in sequential S. aureus isolates from a patient who had a persistent and recurrent S. aureus infection, after failed therapy with multiple antimicrobials, including linezolid. Two point mutations in key staphylococcal genes dramatically affected clinical behaviour of the bacterium, altering virulence and antimicrobial resistance. Most strikingly, a single nucleotide substitution in relA (SACOL1689) reduced RelA hydrolase activity and caused accumulation of the intracellular signalling molecule guanosine 3', 5'-bis(diphosphate) (ppGpp) and permanent activation of the stringent response, which has not previously been reported in S. aureus. Using the clinical isolate and a defined mutant with an identical relA mutation, we demonstrate for the first time the impact of an active stringent response in S. aureus, which was associated with reduced growth, and attenuated virulence in the Galleria mellonella model. In addition, a mutation in rlmN (SACOL1230), encoding a ribosomal methyltransferase that methylates 23S rRNA at position A2503, caused a reduction in linezolid susceptibility. These results reinforce the exquisite adaptability of S. aureus and show how subtle molecular changes cause major alterations in bacterial behaviour, as well as highlighting potential weaknesses of current antibiotic treatment regimens.
金黄色葡萄球菌经常侵入人体血液，导致危及生命的菌血症，并且经常继发感染病灶。抗生素治疗未能消除感染的情况经常被描述；在某些情况下，这与金黄色葡萄球菌抗菌药物耐药性的改变或小菌落变异（SCV）表型有关。对于一些耐多药金黄色葡萄球菌感染的患者，新型抗生素，如利奈唑胺，仍然是最后的可用治疗方法。我们使用比较和功能基因组学研究了一位患者连续分离的金黄色葡萄球菌分离株的耐药性和 SCV 形成的分子决定因素，该患者在经历了多种抗生素（包括利奈唑胺）治疗失败后，持续且反复发生金黄色葡萄球菌感染。关键葡萄球菌基因中的两个点突变极大地影响了细菌的临床行为，改变了毒力和抗菌药物耐药性。最引人注目的是，RelA 中的单个核苷酸取代（SACOL1689）降低了 RelA 水解酶的活性，并导致细胞内信号分子鸟苷 3'，5'-双（二磷酸）（ppGpp）的积累和严格反应的永久激活，这在金黄色葡萄球菌中以前没有报道过。我们使用临床分离株和具有相同 relA 突变的定义突变体，首次证明了金黄色葡萄球菌中严格反应的活性的影响，这与生长减少和在大蜡螟模型中减弱的毒力相关。此外，编码核糖体甲基转移酶的 rlmN（SACOL1230）中的突变导致利奈唑胺敏感性降低，该酶在 23S rRNA 的 A2503 位置甲基化。这些结果强化了金黄色葡萄球菌的高度适应性，并展示了微小的分子变化如何导致细菌行为的重大改变，并突出了当前抗生素治疗方案的潜在弱点。
