Amplification of DNA fragments from dnaE, lap, recA, gyrB, cat, ompU, ctxAB, and tcpA was performed with a HotStar Taq MasterMix kit (Qiagen, Westburg b.v., Leusden, The Netherlands). The primers used were previously described by Teh et al. [21]. The ompU genes from 9 isolates (including three epidemic strains (080025/EZ [O1 Ogawa], FFIVC130 [O139], and FFIVC129 [O1 Hikojima]), six environmental isolates (FFIVC114, 080025/FE, 080025/FI, 080025/FL, 17/110/2006, and 2/110/2006) were amplified

using the primers ompU-fw (5′-ACCTATTTCGATTGACGTGGC-3′) and ompU-rv (5′-ACATCCACCAAGAAACGTTGC-3′), which anneal approximately 80 bp up- and downstream of the ompU open reading frames. The PCR products were bidirectionally sequenced. DNA sequencing was

performed by BaseClear B.V. (Leiden, The Netherlands). Sample preparation for MALDI-TOF MS analysis V. cholerae ABT-263 cell line isolates were grown for 16 h at 35°C on blood agar plates. Sample preparation for MALDI-TOF MS analysis of whole cell lysates was performed as previously described [11]. Each isolate sample was spotted eight times on the MALDI target. Four spots were overlaid with 0.5 μl of 10 mg/ml α-cyano-4-hydroxycinnamic acid (HCCA, Bruker Daltonics) in an acetonitrile/water solution (1:1) with 2.5% trifluoroacetic acid (Fluka/Aldrich, Stenheim, Germany). Four spots were overlaid with 0.5 μl of a matrix solution containing 12.5 mg/ml ferulic acid (Sigma-Aldrich), 17% formic acid selleck products and 33% acetonitrile (LC-MS grade, Fluka/Aldrich, Stenheim, Germany), Tangeritin hereafter referred to as FA+ [16, 17]. Spots were dried at room temperature. Mass spectra acquisition The mass spectra were acquired automatically on a Bruker Autoflex III smartbeam instrument (Bruker Daltonics) in linear mode. Spots overlaid with HCCA matrix were analyzed using the following parameters: 50% laser intensity, positive polarity, 350 ns PIE delay, acceleration voltage of 20 kV (source 1) and 18.7 kV (source 2), lens voltage of 8 kV, linear detector voltage of 1.522 kV,

and 500 Da detector gating. Composite mass spectra were generated from 10 different positions per spot using, in total, 2,000 laser shots at each spot generated by a 200-Hz smartbeam laser (355 nm). The mass spectra were recorded in a mass/charge (m/z) range of 2,000 – 20,000. The parameters used for analysis of the spots overlaid with the FA+ matrix were: 80% laser intensity, positive polarity, 350 ns PIE delay, acceleration voltage of 20 kV (source 1) and 18.7 kV (source 2), lens voltage of 2.8 kV, linear detector voltage of 1.522 kV, and 4000 Da detector gating. Composite mass spectra were generated from 10 different positions per spot using, in total, 2,000 laser shots at each spot generated by a 200-Hz smartbeam laser (355 nm). The mass spectra were recorded in a m/z range of 4,000 – 80,000.