Characterization of a Novel Type IV Pilus Locus Encoded on the Large Plasmid of Locus of Enterocyte Effacement-Negative Shiga-Toxigenic Escherichia coli Strains That Are Virulent for Humans

The majority of Shiga-toxigenic Escherichia coli (STEC) strains isolated from humans with gastrointestinal disease carry large (approximately 90-kb) plasmids. We have been analyzing the megaplasmid (designated pO113) from an O113:H21 STEC strain (98NK2). This strain lacks the locus for enterocyte effacement (LEE) and yet was responsible for an outbreak of hemolytic uremic syndrome. In the present study, we demonstrate that pO113 carries a novel type IV pilus biosynthesis locus (pil) related to those of the IncI plasmids R721, R64, and ColIb9. The pO113 pil locus consists of 11 closely linked genes (pilL through pilV) with an additional separately transcribed upstream gene (pilI). It directs the expression of long thin pili on the 98NK2 surface and the hemagglutination of guinea pig erythrocytes. We also demonstrate that pO113 can be transferred by conjugation. However, the type IV pilus encoded by pO113 does not appear to be involved in the adherence of 98NK2 to HEp-2 or Hct-8 cells in vitro. Homologues of the pO113 pil locus were present in several other LEE-negative STEC strains but not in LEE-positive STEC strains belonging to serogroup O26, O111, or O157.

从罹患胃肠道疾病的人类体内分离得到的大多数产志贺毒素大肠杆菌（Shiga-toxigenic Escherichia coli, STEC）菌株，均携带有大型（约90kb）质粒。本研究团队此前针对一株O113:H21型STEC菌株（98NK2）的巨型质粒pO113开展了分析。该菌株缺失肠细胞脱落位点（locus for enterocyte effacement, LEE），却曾引发一起溶血性尿毒症综合征暴发疫情。在本研究中，我们证实pO113携带有一类与IncI型质粒R721、R64及ColIb9的同源位点相关的新型IV型菌毛生物合成位点（pil）。pO113的pil位点包含11个紧密连锁的基因（pilL至pilV），并额外带有一个独立转录的上游基因pilI。该位点可介导98NK2菌株表面表达细长型菌毛，并可引发豚鼠红细胞的血凝反应。我们同时证实，pO113可通过细菌接合方式进行转移。不过，pO113编码的IV型菌毛似乎并未参与98NK2菌株在体外对HEp-2细胞与Hct-8细胞的黏附过程。pO113 pil位点的同源序列存在于其他数株LEE阴性STEC菌株中，但未在隶属于O26、O111或O157血清群的LEE阳性STEC菌株中检出。