Detection of a Shiga toxin- and extended-spectrum- -lactamase-producing Escherichia coli O157:H7 human clinical isolate

Sir, Shiga toxin-producing Escherichia coli (STEC) is recognized as an important human pathogen that continues to be one of the major causes of food- and waterborne outbreaks. Human infection with STEC is potentially fatal and may be associated with a wide range of symptoms, from uncomplicated diarrhoea to haemorrhagic colitis and haemolytic uraemic syndrome.1 E. coli isolates resistant to oxyimino cephalosporins due to the production of extended-spectrum β-lactamases (ESBLs) have emerged worldwide and a number of different ESBL genes such as blaSHV, blaTEM and blaCTX-M have been identified from infections in humans as well as in animals.2 Until recently there have only been a few reports of ESBL-producing STEC strains isolated from humans or animals,3–5 including the outbreak in May and June 2011 in which an ESBL-producing enteroaggregative E. coli (EAEC) STEC O104:H4 infected 3793 patients in Germany.6 In Denmark, all STEC isolates from all culture-confirmed infections are sent to Statens Serum Institut where they are O:H serotyped, examined for virulence factors, PFGE typed and checked for resistance to antimicrobial markers using MIC values. Screening for ESBL producers among STECs revealed one STEC O157:H7 isolate with reduced susceptibility to cefalotin (MIC >64 mg/L) and ceftiofur (MIC >8 mg/L) and resistant to ampicillin (MIC >32 mg/L). The isolate originated from a faecal sample from a 2-year-old girl with bloody diarrhoea who was part of a small outbreak from 2004. The outbreak involved five patients in total that had all been to the same visiting farm. All five patients had a STEC O157:H7 infection.7 Four of the five isolates had the same PFGE profile, but the patient with the ESBL-producing STEC had a slightly different profile containing one additional band, approximately the same size as the 104.5 kb band of the global standard for PFGE. The presence of a plasmid was verified by plasmid S1 PFGE profiling of the wild-type isolate and a transformant carrying the single plasmid present in the wild-type isolate. A band of 110 kb was identified in both the wild-type and the transformant. Further plasmid characterization using PCR-based replicon typing8 identified the plasmid as being an IncI1 plasmid. The isolate was found to be positive for both blaTEM and blaCTX-M-1 group genes using the Identibac AMR array system (Identibac, Colindale, UK) and subsequent sequencing revealed the presence of the blaTEM-104 gene as well as the blaCTX-M-28 gene, the latter being able to hydrolyse β-lactams with extended spectrum.