Instituto de Microbiologia, Instituto de Medicina Molecular, Faculdade de Medicina, Universidade de Lisboa, Av. Prof. Egas Moniz, 1649-028 Lisbon, Portugal.
J Bacteriol. 2010 Mar;192(5):1361-9. doi: 10.1128/JB.01130-09. Epub 2009 Dec 18.
The polysaccharide capsule is a major antigenic factor in Streptococcus agalactiae (Lancefield group B streptococcus [GBS]). Previous observations suggest that exchange of capsular loci is likely to occur rather frequently in GBS, even though GBS is not known to be naturally transformable. We sought to identify and characterize putative capsular switching events, by means of a combination of phenotypic and genotypic methods, including pulsed-field gel electrophoretic profiling, multilocus sequence typing, and surface protein and pilus gene profiling. We show that capsular switching by horizontal gene transfer is not as frequent as previously suggested. Serotyping errors may be the main reason behind the overestimation of capsule switching, since phenotypic techniques are prone to errors of interpretation. The identified putative capsular transformants involved the acquisition of the entire capsular locus and were not restricted to the serotype-specific central genes, the previously suggested main mechanism underlying capsular switching. Our data, while questioning the frequency of capsular switching, provide clear evidence for in vivo capsular transformation in S. agalactiae, which may be of critical importance in planning future vaccination strategies against this pathogen.
多糖荚膜是无乳链球菌（Lancefield 组 B 链球菌[GBS]）的主要抗原性因素。先前的观察结果表明，GBS 中荚膜基因座的交换很可能经常发生，尽管 GBS 并非天然可转化的。我们试图通过表型和基因型方法的组合，包括脉冲场凝胶电泳分析、多位点序列分型以及表面蛋白和菌毛基因分析，来鉴定和描述可能的荚膜转换事件。我们表明，水平基因转移引起的荚膜转换并不像之前所建议的那样频繁。血清型错误可能是荚膜转换高估的主要原因，因为表型技术容易出现解释错误。鉴定的可能荚膜转化体涉及整个荚膜基因座的获得，并且不限于血清型特异性中心基因，这是之前建议的荚膜转换的主要机制。我们的数据虽然对荚膜转换的频率提出了质疑，但为无乳链球菌体内荚膜转化提供了明确的证据，这对于制定针对该病原体的未来疫苗接种策略可能至关重要。
