Escherichia coli belonging to ST131 rarely transfers blactx-m-15 to fecal Escherichia coli isolates

ESBL producing Escherichia coli causing urinary tract infections often belong to sequence type 131 (ST131), serotype O25, carrying blaCTX-M-15. The main aim of this study was to examine the conjugational frequencies of E. coli with plasmids carrying blaCTX-M-15 to E. coli isolates from the fecal flora of healthy humans to determine whether ST131 is more likely to uptake or donate extended spectrum beta-lactamase (ESBL) resistance compared to other E. coli clones. Donors and recipients were all clinical isolates and did not harbour plasmids with identical incompatibility groups (Inc-groups) based on in silico analyses of Inc-groups and restriction/modification systems (R/M-systems). The in vitro conjugation experiments were performed as filter conjugation with verification of transconjugants by random amplified polymorphic DNA (RAPD) PCR and blaCTX-M-15 PCR. The frequencies of conjugation with blaCTX-M-15-carrying plasmids were found to be very rare with detectable conjugation frequencies in the range of 4x10-9-7x10-7 transconjugants/recipient. Recipients of O25/ST131 type yielded significantly lower conjugation frequencies compared to recipients of other O-types (P=0.004). The applied ST131/O25 donors did not yield detectable levels of transconjugants regardless of the applied recipient. Presence of sub-MIC levels of ampicillin increased plasmid transfer frequencies x100 fold (P=0.07). The results indicate that blaCTX-M-15 is rarely transferred by conjugation to E. coli isolates of the intestinal flora, even though the gene is plasmid-borne.

产超广谱β-内酰胺酶（Extended-Spectrum Beta-Lactamase, ESBL）的大肠埃希菌引发尿路感染时，常归属于序列型131（ST131）、血清型O25，且携带blaCTX-M-15基因。本研究的主要目的是探究携带blaCTX-M-15基因的质粒在大肠埃希菌间的接合频率：供体为携带该基因的大肠埃希菌，受体则取自健康人粪便菌群分离的大肠埃希菌菌株，以此明确ST131是否相较于其他大肠埃希菌克隆株更易获得或传递超广谱β-内酰胺酶（ESBL）耐药性。本研究中供体与受体均为临床分离株，经计算机模拟分析（in silico analyses）不相容群与限制修饰系统（R/M系统）后确认，二者不携带相同不相容群的质粒，且不存在同源的限制修饰系统。体外接合实验采用滤膜接合法开展，接合子的验证通过随机扩增多态性DNA聚合酶链反应（RAPD-PCR）与blaCTX-M-15基因PCR完成。结果显示，携带blaCTX-M-15基因质粒的接合频率极低，可检测到的接合频率范围为4×10^-9 至 7×10^-7 接合子/受体菌。相较于其他血清型的受体菌株，O25/ST131型受体的接合频率显著更低（P=0.004）。而本研究使用的ST131/O25型供体菌株，无论搭配何种受体菌株，均未检测到可观测水平的接合子形成。氨苄西林的亚最低抑菌浓度（sub-MIC）水平可使质粒转移频率提升100倍（P=0.07）。本研究结果表明，即便blaCTX-M-15基因位于质粒上，其通过接合方式向肠道菌群来源的大肠埃希菌分离株转移的概率仍极低。