Molecular characterization of ESBL-producing Shigella sonnei isolates from patients with bacilliary dysentery in Lebanon.
BACKGROUND
Emergence of extended-spectrum beta-lactamases (ESBLs) in Shigella species imparting resistance to third-generation cephalosporins is a growing concern worldwide. The aim of this study is to molecularly characterize the newly emerging beta-lactam resistant Shigella sonnei, specifically ESBLs in Lebanon, and compare them to beta-lactam sensitive isolates.


METHODS
We compared five beta-lactam-resistant S. sonnei isolates to six isolates susceptible to beta-lactams. Presence of ESBLs was established by the combination disk method. PCR amplification and sequence analysis of the beta-lactamase-encoding genes, along with other antimicrobial resistance genes, were performed. The localization of beta-lactamase genes was established by conjugation experiments. Beta-lactamase gene transcription levels were determined by real-time RT-PCR. Molecular typing was performed by pulsed-field gel electrophoresis (PFGE).


RESULTS
Four of five beta-lactam resistant isolates were extended spectrum beta-lactamase producers. These harbored the bla-CTX-M-15 gene borne on a 70 Kb plasmid and class 2 integron genes on their chromosomes. The bla-CTX-M-15 gene was flanked by an insertion element ISEcp1. A chromosomal bla-TEM-1 gene was detected in one beta-lactam resistant Shigella isolate and two of the ESBL producing isolates. The bla-CTX-M-15 gene transcription levels were increased in EBSL isolates exposed to subinhibitory concentrations of ceftazidime. PFGE analysis revealed that the four bla-CTX-M-15 positive isolates were nonclonal but two of them shared genotypes with -lactam susceptible isolates.


CONCLUSIONS
Dissemination of broad-spectrum beta-lactam resistance in Shigella sonnei is mediated by bla-CTX-M-15 through horizontal plasmid transfer rather than by clonal spread of the resistant isolates. Expression of this gene is further induced in the presence of ceftazidime.
Sabra
Ahmad H
AH
Department of Microbiology and Immunology, Faculty of Medicine, American University of Beirut, Beirut, Lebanon.
Araj
George F
GF
Kattar
Mireille M
MM
Abi-Rached
Roland Y
RY
Khairallah
Marie-Therese
MT
Klena
John D
JD
Matar
Ghassan M
GM
D000328:Adult; D001426:Bacterial Proteins / Q000096:biosynthesis / Q000235:genetics; D015373:Bacterial Typing Techniques; D002648:Child; D002675:Child, Preschool; D016000:Cluster Analysis; D003227:Conjugation, Genetic; D016172:DNA Fingerprinting; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics; D004405:Dysentery, Bacillary / Q000382:microbiology*; D005260:Female; D022761:Gene Transfer, Horizontal; D005838:Genotype; D006801:Humans; D007223:Infant; D007861:Lebanon; D008297:Male; D008826:Microbial Sensitivity Tests / Q000379:methods; D010957:Plasmids; D016133:Polymerase Chain Reaction; D017422:Sequence Analysis, DNA; D012764:Shigella sonnei / Q000201:enzymology* / Q000302:isolation & purification*; D055815:Young Adult; D001618:beta-Lactamases / Q000096:biosynthesis* / Q000235:genetics
D001426:Bacterial Proteins; D004269:DNA, Bacterial; D001618:beta-Lactamases