Since the first report of ESBLs in 2002 (Chanawong

et al., 2002), blaCTX-M has been predominant in mainland (Yu et al., 2007; Liu et al., 2009). In this multicentre study, the prevalence of ESBL production in K. pneumoniae has been demonstrated to be about 40%. Of 158 ESBL-producers, the isolates harboring ESBL genes and blaCTX-M-14 were 94.3% and 49.4%, respectively, and were shown to increase 10% and 9% to those in another large-scale study (Yu et al., 2007), respectively. The proportion of blaCTX-M increased 12% compared to the percentage (72.3%) described in a report of southern China three years ago (Liu et al., 2009) and doubled the percentage reported nine years ago (Li et al., 2003). Because the usage of plasmid-based amplification method in this study and the potential Deforolimus false-negative products

owing to the unbinding on some novel bla, the detection of β-lactamase genes see more may have been underestimated. Although there are some differences in the source of the isolates in our study as compared to the studies mentioned above, our results clearly suggest the increasing prevalence of blaCTX-M in K. pneumoniae in China. CTX-M-type ESBLs exhibit powerful activity against cefotaxime and ceftriaxone but generally not against ceftazidime, and several variants with enhanced ceftazidimase activity have been reported (Poirel et al., 2002; Bonnet et al., 2003; most Rossolini et al., 2008). In this study, it was observed that the isolates harboring CTX-M-15 or CTX-M-27 alone exhibited higher resistance rates to ceftazidime and aztreonam than that in subgroup CTX-M-14 without other ESBLs

(Table 2). Further, a high percentage of isolates harboring blaCTX-M-27 demonstrated the MDR phenotype. To our knowledge, this is the first study about the high prevalence of CTX-M-27 in Enterobacteriaceae in China. This warrants for an active surveillance to monitor these resistant bacteria. The overall resistance rates to the tested β-lactam antimicrobial agents were over 30% except for cefepime, piperacillin/tazobactam, and cefotetan in this study. As shown in Table 2, only 9.3% isolates harboring CTX-M-14 alone showed resistance to cefepime, but 50% isolates harboring CTX-M-15 exhibited resistance (P < 0.01), and a 100% resistance rate when CTX-M-15 coexisted with other ESBLs. Nevertheless, piperacillin/tazobactam show only 10.1% resistance rate in vitro, although the proportion increased to 26.7% when the isolates contained two types of ESBLs(blaCTX-M + blaSHV)(Table 1). Several clinical intervention studies also supported that piperacillin/tazobactam may contribute to preventing the ESBL-producing K. pneumoniae outbreaks (Lee et al., 2007; Tangden et al., 2011). These properties highlight the value of piperacillin/tazobactam as empirical therapy for infections by suspected organisms possessing a single ESBL (especially the blaCTX-M).