A total of 88 uropathogenic Escherichia coli isolates, including 68 isolates from urine and 20 isolates from blood, were characterized by multilocus sequence typing (MLST). MLST has identified an important genetic lineage of E. coli, designated sequence type 131 (ST-131), represented by 52 of these isolates, 51 of which were resistant to extended-spectrum cephalosporins. ST-131 appears to be a drug-resistant uropathogenic strain of E. coli responsible for causing urinary tract infections and bacteremia and is widely disseminated among both community and hospital patients from different geographical areas in the northwest of England. Application of MLST has helped to define the population biology which may underpin the epidemiology of pathogenic E. coli strains. The portability of MLST allows the accurate monitoring of this antibiotic-resistant uropathogenic strain of E. coli and will enhance surveillance for this important group of organisms.Urinary tract infection (UTI) is the most common laboratory-confirmed bacterial infection encountered in medical practice in Europe and North America (9). Escherichia coli is a major cause of UTI, and it accounts for approximately 70 to 95% of community-acquired cases and 50% of all hospitalacquired infections (9). These organisms are responsible for significant social and economic costs for both communities and public health resources. It has been estimated that 150 million cases of UTI occur on a global basis per year and cost about 6 billion dollars for national health resources (6).UTI due to E. coli can progress to bacteremia, which is associated with significant mortality. E. coli is recognized as the one of the two most common causes of bacteremia in England and Wales, and the Health Protection Agency reported 32.5 bacteremia cases per 100,000 subjects in 2005 (7). There is no comprehensive surveillance for community-acquired UTIs in England, and currently, there is only voluntary national surveillance for bacteremia; therefore, it is very difficult to estimate the true incidence of UTI infections and monitor the spread of these organisms (12).In recent years, there has been an increase in the occurrence cephalosporin-resistant strains of E. coli causing UTI and invasive infections (10). These organisms, which are often also resistant to other widely used antibiotics such as fluoroquinolones and trimethoprim, have particular clinical significance due to the limited therapeutic options that are available. Furthermore, the prevention and control of the spread of uropathogenic E. coli infections are hampered by a poor understanding of the population biology of these pathogens. The potential of particular lineages of antibiotic-resistant or uropathogenic E. coli to disseminate and cause disease is unknown. Improved strain characterization and phylogenetic analysis would improve our understanding of the epidemiology of this pathogen and will allow the development of a rapid assay for monitoring of uropathogenic E. coli.Multilocus sequence typing (MLST) is a DNA sequencebas...

Objectives: Uropathogenic and invasive Escherichia coli O25:H4-ST131 isolates producing CTX-M-15 extended-spectrum b-lactamase (ESBL) enzymes have recently been shown to be disseminated across the globe. In the UK, many CTX-M-15 ESBL-producing E. coli strains have been previously defined as belonging to the epidemic strains A -E, as determined by PFGE. The present study was carried out to define the relationship between these two groups of pathogenic E. coli.Methods: Multilocus sequence typing and PFGE were used for molecular characterization of a collection of 61 ESBL-producing E. coli isolates from across the UK.Results: Strains A to E all belonged to the ST131 clone, further underscoring the epidemiological importance of this lineage. Conclusions:The future spread of the ST131 clone, and its UK variants, should be monitored closely and the pathogenic mechanisms explaining their success should be investigated.Keywords: multilocus sequence typing, MLST, molecular epidemiology, uropathogenic Escherichia coli, UPEC, extended-spectrum b-lactamases, PFGE IntroductionEscherichia coli is one of the two most common causes of bacteraemia in England and Wales 1 and causes the majority of urinary tract infections (UTIs) worldwide. The widespread use of antibiotics has selected for drug-resistant strains, with current concern centred on those with fluoroquinolone resistance and extended-spectrum b-lactamases (ESBLs). ESBL-producing E. coli-principally with CTX-M-type enzymes, particularly CTX-M-15-increasingly cause UTIs in the community and longterm care and hospital settings in the UK.2,3 Five major epidemic E. coli strains (A-E) with CTX-M-15 b-lactamase were identified in the UK, based on their PFGE banding patterns, along with many clonally diverse producers of the enzyme. Epidemic strain A is nationally distributed, being particularly dominant in parts of Lancashire, Shropshire, Hampshire and Ulster. Most strain A-E isolates are resistant to oxyiminocephalosporins and, through co-production of OXA-1 b-lactamase, also to b-lactamase inhibitor combinations; in addition, most isolates are resistant to fluoroquinolones, trimethoprim, tetracyclines and amikacin. Strain A is susceptible to gentamicin, the others are generally resistant. A few strain A isolates in the north-west (NW) of England possess plasmid-mediated AmpC b-lactamase and consequently have enhanced resistance to cephamycins and cephalosporin/b-lactamase inhibitor combinations. 4 This rise in E. coli with CTX-M enzymes is not unique to the UK and several recent studies using multilocus sequence typing (MLST) have identified a globally disseminated O25:H4-ST131

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