Previous work on the subclones within Escherichia coli ST131 predominantly involved isolates from Western countries. This study assessed the prevalence and antimicrobial resistance attributed to this clonal group. A total of 340 consecutive, non-duplicated urinary E. coli isolates originating from four clinical laboratories in Hong Kong in 2013 were tested. ST131 prevalence among the total isolates was 18.5 % (63/340) and was higher among inpatient isolates (23.0 %) than outpatient isolates (11.8 %, P,0.001), and higher among isolates from patients aged ¢65 years than from patients aged 18-50 years and 51-64 years (25.4 vs 3.4 and 4.0 %, respectively, P,0.001). Of the 63 ST131 isolates, 43 (68.3 %) isolates belonged to the H30 subclone, whereas the remaining isolates belonged to H41 (n517), H54 (n52) and H22 (n51). All H30 isolates were ciprofloxacin-resistant, of which 18.6 % (8/43) belonged to the H30-Rx subclone. Twenty-six (41.3 %) ST131 isolates were ESBL-producers, of which 19 had bla CTX-M-14 (12 non-H30-Rx, two H30-Rx and five H41), six had bla CTX-M-15 (five non-H30-Rx and one H30-Rx) and one was bla CTX-M -negative (H30). In conclusion, ST131 accounts for a large share of the antimicrobial-resistant E. coli isolates from geriatric patients. Unlike previous reports, ESBLproducing ST131 strains mainly belonged to non-H30-Rx rather than the H30-Rx subclone, with bla CTX-M-14 as the dominant enzyme type. INTRODUCTIONThe incidence of infections due to antimicrobial-resistant Escherichia coli is increasing worldwide (Barber et al., 2013). Resistance rates for cotrimoxazole, fluoroquinolones and third generation cephalosporins, which are often used for empirical therapy, are now above the 15-20 % threshold recommended for choosing first-line antimicrobial agents for empirical treatment in Hong Kong (Ho et al., 2007b;. Discordant therapy may cause treatment failure, persistence and recurrence of infection, leading to more patient morbidity and mortality (Barber et al., 2013;Shin et al., 2012). Emerging resistance in E. coli involves acquisition of resistance determinants by susceptible strains and the expansion of pre-existing resistant clones (Naseer & Sundsfjord, 2011). ST131 is a highly successful E. coli clone which has received considerable attention due to its wide geographical distribution, ability to cause a wide range of extra-intestinal infections and association with CTX-M b-lactamases and multidrug resistance (NicolasChanoine et al., 2014). ST131 can be divided into different subclones by other typing methods, of which sequencing the type 1 fimbrial adhesin gene fimH is one widely used approach (Weissman et al., 2012). H30, designated according to the fimH30 variant, is currently the most prevalent subclone of ST131 (Nicolas-Chanoine et al., 2014). Two studies involving whole genome sequencing of ST131 isolates collected from multiple countries came to the same conclusion that fluoroquinolone resistance within ST131 was confined almost entirely to the H30 subclone and that CTX-M-15 producers cluste...