Epidemiology of CTX-M type extended-spectrum beta-lactamase-producing escherichia coli among blood culture isolates in Hong Kong

Abstract

Dissemination of CTX-M type extended-spectrum beta-lactamase

(ESBL)-producing Escherichia coli (E. coli) is a serious health issue in Hong Kong.

However, research knowledge concerning its dissemination mechanism and plasmid

characteristics over time in health care setting is still limited.

This study was conducted to characterize ESBL-producing E. coli from blood

culture isolates and the epidemiology of the plasmids harboring CTX-M-14 collected

from a healthcare region in Hong Kong from two periods of time.

A total of 167 ESBL-producing E. coli in blood culture specimens were retrieved

from period 1 (1996 - 1999, n = 50) and period 2 (2007 - 2008, n = 117).

Antimicrobial susceptibilities were defined by disk diffusion method according to

CLSI. Phylogenetic groups and CTX-M enzymes were detected among all the

ESBL-producers. Clonal relatedness of the hosts was analyzed by pulsed-field gel

electrophoresis and multi-locus sequence typing. A subset of 65

CTX-M-14-producing isolates was undergone for further plasmid characterization.

Conjugation, PCR-based replicon typing, S1-PFGE, southern-blot hybridization, and

genetic environment PCRs were performed. Plasmid PCR-restriction fragment length

polymorphism (pRFLP), F-allele replicon sequence typing and variable region PCRs

were studied in 54 F-plasmids obtained.

Results showed that over half of the ESBL-positive isolates were non-susceptible

to ciprofloxacin, cotrimoxazole and gentamicin. A surprisingly high number of

CTX-M-type ESBL was carried by 98.2% (164/167) of the isolates. CTX-M-9 group

(89.8%, 150/167) and CTX-M-14 (103/109) were predominantly found among both

periods. Overall, nearly half (41.3%, 69/167) of the isolates belonged to 5 major

clones. Clonal types undetermined-ST68 (n = 18) and O102-ST405 (n = 15) were

dominant in period 1 while clonal types O25b-ST131 (n = 30), O15-ST69 (n = 5) and

O12-ST12 (n = 3) emerged in period 2.

Among a subset of 65 CTX-M-14 plasmids, most of them were transferable

(84.6%, 55/65) with high frequency, similar plasmid sizes and genetic environment

ISEcp1-blaCTX-M-14-IS903 (90.8%, 59/65). Replicon types of the CTX-M-14 encoding

plasmids were FII (n = 48) or FII ± FIA/FIB types (n = 6), I1-I (n = 3), B/O (n = 2),

K (n = 1) and undetermined (n = 4). Subtyping of 54 IncF plasmids by replicon

sequence typing, pRFLP and PCR for marker genes (yac, malB, eitA, eitC and parAB)

showed that 79.6% (43/54) of the plasmid subset exhibited identical or highly similar

results with the completely sequenced plasmid, pHK01 (E. coli isolated from urine

sample of a patient in Hong Kong, 2004). These 43 plasmids were originated from

both period 1 (n = 11) and period 2 (n = 32). These pHK01-like plasmids were found

to have spread to the major clones (ST68, ST405 and ST131) and multiple singleton

isolates of all four phylogenetic groups.

In conclusion, this study demonstrated the widespread dissemination of

pHK01-like CTX-M-14 encoding plasmids among isolates of diverse genetic lineages

over a decade. The dissemination was probably due to both clonal expansion and

horizontal gene transfer of pHK01-like IncF plasmid.