Nationwide high prevalence of CTX-M and an increase of CTX-M-55 in Escherichia coli isolated from patients with community-onset infections in Chinese county hospitals

Background: In order to investigate the epidemiology, molecular characteristics, and distribution of extended-spectrum beta-lactamase (ESBL)- and AmpC-producing Escherichia coli from community-onset infections in Chinese county hospitals. Methods: E. coli isolates were collected from patients with community-onset infections in 30 county hospitals. ESBL activity was confirmed by double-disc diffusion. Genetic confirmation and molecular typing of ESBL- and AmpC-producing isolates was determined by PCR and DNA sequencing. ESBL-positive isolates were further characterised by multi-locus sequence typing. Results: Of 550 E. coli isolates, 256 (46.5%) carried ESBL genes and all were of the CTX-M type. The prevalence of ESBL-producing strains varied from 30.2% to 57.0% across different regions of China. Overall, 12 bla(CTX-M) subtypes were detected; the most abundant were bla(CTX-M-14) (163/256 isolates, 64.5%), bla(CTX-M-55)(47/256, 18.4%), and bla(CTX-M-15) (31/256, 12.1%). CMY-2-like AmpC beta-lactamases were detected in 11 strains, three of which co-existed with bla(CTX-M). A total of 64 sequence types (STs) were detected in 256 ESBL-producing strains, including nine that were new. ST131 was the most abundant type (27 isolates, 12.7%), followed by ST69 (14 isolates, 6.6%), ST405 (14 isolates, 6.6%), and ST38 (12 isolates, 5.6%). Conclusions: This study revealed that the widespread prevalence of ESBLs among outpatient infections has reached a high level in county hospitals. The CTX-M genotype was most dominant, comprising a variety of subtypes. This is the first time the incidence of CTX-M-55 has exceeded that of CTX-M-15 in China. No predominant ST was detected, suggesting that ESBL-producing E. coli strains originate in different clones.