Division of Infectious Diseases, Department of Medicine, Atlanta, Georgia, United States of America.
PLoS One. 2011;6(9):e25124. doi: 10.1371/journal.pone.0025124. Epub 2011 Sep 22.
Streptococcus oralis, Streptococcus mitis, and Streptococcus sanguinis are members of the Mitis group of streptococci and agents of oral biofilm, dental plaque and infective endocarditis, disease processes that involve bacteria-bacteria and bacteria-host interactions. Their close relative, the human pathogen S. pneumoniae uses pilus-islet 2 (PI-2)-encoded pili to facilitate adhesion to eukaryotic cells.
METHODOLOGY/PRINCIPAL FINDINGS: PI-2 pilus-encoding genetic islets were identified in S. oralis, S. mitis, and S. sanguinis, but were absent from other isolates of these species. The PI-2 islets resembled the genetic organization of the PI-2 islet of S. pneumoniae, but differed in the genes encoding the structural pilus proteins PitA and PitB. Two and three variants of pitA (a pseudogene in S. pneumoniae) and pitB, respectively, were identified that showed ≈20% difference in nucleotide as well as corresponding protein sequence. Species-independent combinations of pitA and pitB variants indicated prior intra- and interspecies horizontal gene transfer events. Polyclonal antisera developed against PitA and PitB of S. oralis type strain ATCC35037 revealed that PI-2 pili in oral streptococci were composed of PitA and PitB. Electronmicrographs showed pilus structures radiating >700 nm from the bacterial surface in the wild type strain, but not in an isogenic PI-2 deletion mutant. Anti-PitB-antiserum only reacted with pili containing the same PitB variant, whereas anti-PitA antiserum was cross-reactive with the other PitA variant. Electronic multilocus sequence analysis revealed that all PI-2-encoding oral streptococci were closely-related and cluster with non-PI-2-encoding S. oralis strains.
CONCLUSIONS/SIGNIFICANCE: This is the first identification of PI-2 pili in Mitis group oral streptococci. The findings provide a striking example of intra- and interspecies horizontal gene transfer. The PI-2 pilus diversity provides a possible key to link strain-specific bacterial interactions and/or tissue tropisms with pathogenic traits in the Mitis group streptococci.
口腔链球菌、米氏链球菌和血链球菌是唾液链球菌属的成员，也是口腔生物膜、牙菌斑和感染性心内膜炎的病原体，这些疾病过程涉及细菌-细菌和细菌-宿主相互作用。它们的近亲人类病原体肺炎链球菌使用菌毛岛 2（PI-2）编码的菌毛来促进与真核细胞的黏附。
方法/主要发现：在口腔链球菌、米氏链球菌和血链球菌中鉴定出了 PI-2 菌毛编码的遗传岛，但在这些种的其他分离株中不存在。PI-2 岛类似于肺炎链球菌 PI-2 岛的遗传组织，但在编码结构菌毛蛋白 PitA 和 PitB 的基因上有所不同。分别鉴定出了 pitA（肺炎链球菌中的假基因）和 pitB 的两个和三个变体，其核苷酸和相应的蛋白质序列差异约为 20%。种间独立的 pitA 和 pitB 变体组合表明先前存在种内和种间水平基因转移事件。针对口腔链球菌 ATCC35037 型菌株的 PitA 和 PitB 制备的多克隆抗血清表明，口腔链球菌中的 PI-2 菌毛由 PitA 和 PitB 组成。电镜照片显示，野生型菌株表面辐射出 >700nm 的菌毛结构，但在同源性 PI-2 缺失突变体中则没有。抗 PitB 抗血清仅与含有相同 PitB 变体的菌毛反应，而抗 PitA 抗血清与其他 PitA 变体发生交叉反应。电子多位点序列分析显示，所有编码 PI-2 的口腔链球菌密切相关，并与非编码 PI-2 的口腔链球菌菌株聚类。
结论/意义：这是首次在唾液链球菌属口腔链球菌中鉴定出 PI-2 菌毛。这些发现提供了一个引人注目的例子，说明种内和种间水平基因转移。PI-2 菌毛多样性为将菌株特异性细菌相互作用和/或组织嗜性与米氏链球菌属的致病性特征联系起来提供了一个可能的关键。
