Kusuma Caroline, Jadanova Anna, Chanturiya Tanya, Kokai-Kun John F
Biosynexus Incorporated, 9119 Gaither Rd., Gaithersburg, MD 20877, USA.
Antimicrob Agents Chemother. 2007 Feb;51(2):475-82. doi: 10.1128/AAC.00786-06. Epub 2006 Nov 13.
Lysostaphin is under development as a therapy for serious staphylococcal infections. During preclinical development, lysostaphin-resistant Staphylococcus aureus variants have occasionally been reported in vitro and in vivo. The acquisition of resistance to this drug, however, leads to a significant increase in beta-lactam antibiotic susceptibility, rendering methicillin-resistant S. aureus (MRSA) strains functionally methicillin susceptible. In this study, we have demonstrated that the development of lysostaphin resistance by two strains of MRSA also led to a loss of fitness in the variants. Consistent with the mutations found in previously reported lysostaphin-resistant S. aureus variants, these two variants had mutations in their femA genes, resulting in nonfunctional FemA proteins and, thus, monoglycine cross bridges in the peptidoglycan. The diminished fitness of the lysostaphin-resistant variants was reflected by (i) a reduced logarithmic growth rate, with the variants being outcompeted in cocultures by their wild-type parental strains; (ii) increased susceptibility to elevated temperatures; and (iii) at least fivefold less virulence of the lysostaphin-resistant variants than their wild-type strains in a mouse kidney infection model, with the lysostaphin-resistant variants being outcompeted in coinfections with their wild-type parental strains. During a 14-day serial passage without selective pressure, the lysostaphin-resistant variants failed to develop compensatory mutations which restored their fitness. These results suggest that should lysostaphin resistance due to an alteration in the FemA function emerge in S. aureus during therapy with lysostaphin, the resistant variants would be less fit and less virulent, and, in addition, infections with these strains would be easily treatable with beta-lactam antibiotics.
溶葡萄球菌酶正作为一种治疗严重葡萄球菌感染的疗法进行研发。在临床前研发过程中，偶尔会在体外和体内报告对溶葡萄球菌酶耐药的金黄色葡萄球菌变体。然而，对这种药物产生耐药性会导致β-内酰胺类抗生素敏感性显著增加，使耐甲氧西林金黄色葡萄球菌（MRSA）菌株在功能上对甲氧西林敏感。在本研究中，我们证明了两株MRSA对溶葡萄球菌酶产生耐药性的同时，其变体的适应性也有所下降。与先前报道的耐溶葡萄球菌酶金黄色葡萄球菌变体中发现的突变一致，这两个变体的femA基因发生了突变，导致FemA蛋白无功能，从而使肽聚糖中的单甘氨酸交联桥无法形成。耐溶葡萄球菌酶变体适应性降低体现在以下几个方面：（i）对数生长速率降低，在共培养中其野生型亲本菌株比变体更具竞争力；（ii）对高温的敏感性增加；（iii）在小鼠肾脏感染模型中，耐溶葡萄球菌酶变体的毒力比其野生型菌株至少低五倍，在与野生型亲本菌株的混合感染中，耐溶葡萄球菌酶变体更易被淘汰。在无选择压力的情况下连续传代14天，耐溶葡萄球菌酶变体未能产生恢复其适应性的补偿性突变。这些结果表明，如果在使用溶葡萄球菌酶治疗期间，金黄色葡萄球菌因FemA功能改变而产生对溶葡萄球菌酶的耐药性，那么耐药变体会适应性更差、毒力更低，此外，用β-内酰胺类抗生素很容易治疗这些菌株引起的感染。
