Complete nucleotide sequence of the IncI1 plasmid pSH4469 encoding CTX-M-15 extended-spectrum ß-lactamase in a clinical isolate of Shigella sonnei from an outbreak in the Republic of Korea.

An outbreak of extended-spectrum ß-lactamase (ESBL)-producing Shigella sonnei infections occurred in a school for disabled children in Gyeongbuk Province, Republic of Korea, in 2008. Five students were affected. Pulsed-field gel electrophoresis (PFGE) analysis revealed that all of the ESBL-producing S. sonnei isolates belonged to the same clone, and nucleotide sequence analysis of ESBL genes revealed that they harboured bla(CTX-M-15). This is the first identification of bla(CTX-M-15) in Shigella spp. in South Korea. In this study, a plasmid carrying the bla(CTX-M-15) gene, designated pSH4469, recovered from a S. sonnei isolate responsible for the outbreak was characterised. Replicon typing and plasmid multilocus sequence typing (pMLST) analysis of plasmids in the outbreak strain identified that the bla(CTX-M-15) gene was located on an IncI1 incompatibility group plasmid of sequence type 16 (ST16). The complete nucleotide sequence of pSH4469 revealed that this plasmid is 91109bp and harbours 119 putative genes, including another antibiotic resistance gene (bla(TEM-1b)) that is often associated with the ISEcp1-bla(CTX-M-15)-orf477delta transposable unit. The plasmid consists of a large backbone with considerable homology to the pEK204 plasmid isolated from Escherichia coli in the UK, except for insertion of an IS66 element found in pEK204. These data demonstrate that IncI1 plasmids are used as a successful platform for efficient horizontal gene transfer, thereby resulting in the dissemination of CTX-M-type ß-lactamases among Enterobacteriaceae.

Cloning, characterization and evaluation of potent inhibitors of Shigella sonnei acetohydroxyacid synthase catalytic subunit.

Acetohydroxyacid synthase (AHAS) is a thiamin diphosphate (ThDP)- and flavin adenine dinucleotide (FAD)-dependent plant and microbial enzyme that catalyzes the first common step in the biosynthesis of essential amino acids such as leucine, isoleucine and valine. To identify strong potent inhibitors against Shigella sonnei (S. sonnei) AHAS, we cloned and characterized the catalytic subunit of S. sonnei AHAS and found two potent chemicals (KHG20612, KHG25240) that inhibit 87-93% S. sonnei AHAS activity at an inhibitor concentration of 100uM. The purified S. sonnei AHAS had a size of 65kDa on SDS-PAGE. The enzyme kinetics revealed that the enzyme has a K(m) of 8.01mM and a specific activity of 0.117U/mg. The cofactor activation constant (K(s)) for ThDP and (K(c)) for Mg(++) were 0.01mM and 0.18mM, respectively. The dissociation constant (K(d)) for ThDP was found to be 0.14mM by tryptophan fluorescence quenching. The inhibition kinetics of inhibitor KHG20612 revealed an un-competitive inhibition mode with a K(ii) of 2.65mM and an IC(50) of 9.3µM, whereas KHG25240 was a non-competitive inhibitor with a K(ii of) 5.2mM, K(is) of 1.62mM and an IC(50) of 12.1µM. Based on the S. sonnei AHAS homology model structure, the docking of inhibitor KHG20612 is predicted to occur through hydrogen bonding with Met 257 at a 1.7Å distance with a low negative binding energy of -9.8kcal/mol. This current study provides an impetus for the development of a novel strong antibacterial agent targeting AHAS based on these potent inhibitor scaffolds.