Genomic Comparisons and Shiga Toxin Production among Escherichia coli O157:H7 Isolates from a Day Care Center Outbreak and Sporadic Cases in Southeastern Wisconsin

Contour-clamped homogeneous electric field pulsed-field gel electrophoresis (CHEF-PFGE) was used to compare Wisconsin isolates of Escherichia coli O157:H7, including 39 isolates from a 1994 day care center outbreak, 28 isolates from 18 individuals from the surrounding geographic area with sporadic cases occurring during the 3 months before the outbreak, and 3 isolates, collected in 1995, from patients with hemolytic-uremic syndrome (HUS) who were from eastern Wisconsin counties other than those inhabited by the day care center and sporadic-case individuals. The technique of CHEF-PFGE using XbaI identified seven highly related restriction endonuclease digestion profiles (REDPs) (93 to 98% similarity) among the 39 day care center isolates and nine XbaI REDPs (63 to 93% similarity) among the 28 isolates from sporadic-case individuals, including REDP 33, which was exhibited by both day care and sporadic-case isolates. PFGE analyses of sequential E. coli O157:H7 isolates from symptomatic day care center attendees revealed that the REDPs of 25 isolates from eight patients were indistinguishable whereas the REDPs of 2 of 6 isolates from two patients differed slightly (93 to 95% similarity). The REDPs of the three isolates from 1995 HUS patients were 78 to 83% similar, with REDP 26 being exhibited by one HUS-associated isolate and an isolate from one day care attendee who did not develop HUS. The genes for both Shiga toxins I and II (stx(1) and stx(2), respectively) were detected in all but one isolate (sporadic case), and Shiga toxin production by the day care center isolates was not significantly different from that of the other isolates, including the three HUS-associated isolates. Analyses of E. coli O157:H7 isolates from both the day care center outbreak and sporadic cases by CHEF-PFGE permitted us to define the REDP variability of an outbreak and geographic region and demonstrated that the day care center outbreak and a HUS case in 1995 were caused by E. coli O157:H7 strains endemic to eastern Wisconsin.

本研究采用钳位均匀电场脉冲场凝胶电泳（Contour-clamped homogeneous electric field pulsed-field gel electrophoresis，CHEF-PFGE），对威斯康星州分离得到的大肠杆菌O157:H7（Escherichia coli O157:H7）菌株进行比对分析，其中包括1994年某托儿所暴发疫情中分离的39株菌株、疫情暴发前3个月内从周边地理区域18名散发感染病例体内分离的28株菌株，以及1995年采集自威斯康星州东部既非疫情托儿所所在、也非散发感染病例所属郡县的溶血性尿毒综合征（hemolytic-uremic syndrome，HUS）患者体内分离的3株菌株。采用XbaI限制性内切酶的CHEF-PFGE技术，在39株托儿所暴发株中鉴定出7个高度同源的限制性内切酶酶切图谱（restriction endonuclease digestion profiles，REDPs），菌株间相似性为93%~98%；在28株散发感染株中则鉴定出9个XbaI酶切图谱，相似性为63%~93%，其中REDP 33同时存在于暴发株与散发感染株中。对有症状的托儿所参与者的连续大肠杆菌O157:H7分离株进行脉冲场凝胶电泳（pulsed-field gel electrophoresis，PFGE）分析后发现，8名患者的25株菌株的酶切图谱完全一致，而2名患者的6株菌株中的2株的酶切图谱仅存在微小差异，相似性为93%~95%。1995年分离的3株HUS相关菌株的酶切图谱相似性为78%~83%，其中REDP 26分别出现在1株HUS相关菌株，以及1名未发生HUS的托儿所参与者分离的菌株中。除1株散发感染菌株外，所有菌株均检出志贺毒素I和志贺毒素II（Shiga toxins I and II，stx₁和stx₂）的编码基因；托儿所暴发株的志贺毒素产生水平与其他菌株（包括3株HUS相关菌株）无显著差异。通过CHEF-PFGE对托儿所暴发疫情及散发感染的大肠杆菌O157:H7分离株进行分析，本研究明确了单次暴发疫情及特定地理区域内的REDP变异特征，并证实该托儿所暴发疫情与1995年1例HUS病例均由威斯康星州东部地方性流行的大肠杆菌O157:H7菌株引起。