The blaCTX-M-1 gene located in a novel complex class I integron bearing an ISCR1 element in Escherichia coli isolates from Zhenjiang, China

Sir, CTX-M enzymes include more than 60 variants belonging to five different clusters (CTX-M-1, CTX-M-2, CTX-M-8, CTX-M-9 and CTX-M-25) according to their amino acid sequence. In recent years, CTX-M enzymes have become the most prevalent extended-spectrum b-lactamases (ESBLs), both in nosocomial and in community settings. Different genetic elements might be involved in the mobilization of blaCTX-M genes, such as insertion sequences (ISs) ISEcp1, IS26 and IS903. The blaCTX-M genes have also been associated with ISCR1 (ISs common region 1, previously also called orf513), which is embedded in class 1 integrons. Class 1 integrons containing ISCR elements are known as complex class 1 integrons. They contain the classic integron structure and a second copy of the 30CS. Between the two copies of the 30CS is a 2.1 kb ISCR, as has been identified in In6, In7, In117, In34, In35 and In601, – 5 followed by a variable region that contains resistance genes, e.g. dfrA10, catII, blaDHA-1 (pSAL-1), blaCTX-M-9 or blaCTX-M-2, 5 but never blaCTX-M-1 thus far. Between 2005 and 2006, a total of 146 Escherichia coli isolates were obtained from the Affiliated People’s Hospital of Jiangsu University. ESBL production was detected in 21/146 isolates using the standard double-disc synergy test. Eighty-nine of 146 (61%) isolates were found to carry class 1 integrons of different types, and 10 of these carried complex class I integrons according to the PCR analysis. The primers were designed according to the GenBank sequence (GenBank accession numbers AF174129 and AF071413). Five of the isolates carrying a complex class 1 integron also produced an ESBL. In these five isolates, a blaCTX-M-1 gene was detected between orf513 and an IS3000 element (Figure 1). The sequence of the complex class I integron bearing a blaCTX-M-1 gene has been submitted to GenBank under accession no. EU687490. Recently, it has been suggested that ISCR1 elements were members of an extended family of IS91-like elements that can transpose adjacent DNA sequences by a mechanism termed rolling-circle transposition and are responsible for the mobilization of antibiotic resistance genes, including the blaCTX-M genes. However, this is the first report of a CTX-M-1 gene found to be associated with an ISCR element. The ISCR element is proposed to provide a powerful mechanism to mobilize antibiotic resistance genes, and, if this is the case, further spread of blaCTX-M-1 can be expected.