Laboratório de Bacteriologia, Complexo Hospital de Clínicas, Universidade Federal do Paraná, Curitiba, Brazil.
Programa de Pós-graduação em Biotecnologia e saúde da Criança e do Adolescente, Faculdades e Instituto de Pesquisa Pelé Pequeno Príncipe, Curitiba, Brazil.
Pediatr Pulmonol. 2020 Jun;55(6):1388-1393. doi: 10.1002/ppul.24730. Epub 2020 Mar 16.
Small-colony variants (SCVs) are a morphologic subtype of Staphylococcus aureus that may occur through several mechanisms including auxotrophism for thymidine, hemin, or menadione. Auxotrophic SCV for thymidine fail to synthesize DNA specifically because of mutations in the thymidylate synthase gene. We isolated S. aureus thymidine-dependent SCVs (TD-SCV) from blood and respiratory samples of a pediatric patient with cystic fibrosis and pulmonary exacerbation.
Nutritional dependence of SCVs on hemin, menadione, and thymidine was evaluated. Antimicrobial susceptibility testing was performed through broth microdilution. Polymerase chain reaction was carried out for mecA, ermA, ermB, ermC, msrA, and msrB resistance genes. DNA sequencing was used to determine mutations in thyA and the multilocus sequence typing to identify genetic relatedness.
Methicillin-sensitive S. aureus with normal and TD-SCV phenotypes were isolated from respiratory samples and a TD-SCV phenotype was isolated from blood culture. Macrolides resistance was attributed to ermC and msrB genes. All isolates belonged to ST398. The thyA gene in S. aureus is 957 nucleotides in length and encodes a protein of 318 amino acids. The TD-SCV isolates carried a -2 nt frameshift mutation (delta 667GC668) in thyA, creating a stop codon at residue 222 close to the predicted binding site for deoxyuridine monophosphate.
The pathogenesis of SCVs is complex and not fully elucidated. Factors inherent to the patient such as physiological conditions, recurrent infections, or coinfection should be considered. Although SCVs are considered less virulent, they showed the ability to invade and cause bacteremia in the patient.
小菌落变异体（SCV）是金黄色葡萄球菌的一种形态亚型，可能通过几种机制发生，包括胸腺嘧啶、血红素或维生素 K 的营养缺陷型。胸腺嘧啶营养缺陷型 SCV 无法合成 DNA，具体原因是胸苷酸合酶基因发生突变。我们从患有囊性纤维化和肺部恶化的儿科患者的血液和呼吸道样本中分离出金黄色葡萄球菌依赖胸腺嘧啶的 SCV（TD-SCV）。
评估 SCV 对血红素、维生素 K 和胸腺嘧啶的营养依赖性。通过肉汤微量稀释法进行抗菌药物敏感性测试。进行 mecA、ermA、ermB、ermC、msrA 和 msrB 耐药基因的聚合酶链反应。使用 DNA 测序确定 thyA 基因突变，并进行多位点序列分型以确定遗传相关性。
从呼吸道样本中分离出表型正常和 TD-SCV 的甲氧西林敏感金黄色葡萄球菌，从血培养中分离出 TD-SCV 表型。大环内酯类耐药归因于 ermC 和 msrB 基因。所有分离株均属于 ST398。金黄色葡萄球菌 thyA 基因长 957 个核苷酸，编码 318 个氨基酸的蛋白质。TD-SCV 分离株在 thyA 中携带-2nt 移码突变（delta 667GC668），在第 222 位残基附近产生一个终止密码子，该位置靠近脱氧尿苷单磷酸的预测结合位点。
SCV 的发病机制复杂，尚未完全阐明。应考虑患者内在的因素，如生理状况、反复感染或合并感染。尽管 SCV 被认为毒力较低，但它们显示出入侵和引起患者菌血症的能力。
