Twelve suckling mice were used for the repeat of attachment assay. Assay of CT production by GM1-ELISA CT production in culture supernatants was estimated in V. cholerae strains (N16961, N169-dtatABC, and N169-dtatABC-cp) incubated with AKI media (containing 1.5% Bacto peptone, 0.4% yeast extract-Difco, check details 0.5% NaCl and 0.3% NaHCO3), cultured at 37°C for 4 h in a stationary test tube and then for 16 h in a shaken flask, and measured by GM1-ELISA [30]. In our study, the medium used for all cultures was AKI with 0.3% sodium

bicarbonate. To determine CT production, the strains incubated under static conditions for 4 h at 37°C were poured into flasks with a 20-fold greater volume than the tubes to continue growth at 37°C for 18 h with shaking at 220 rpm. All culture supernatants were concentrated 10-fold with PEG6000. A standard curve was generated using the purified B subunit of CT. As a second antibody, the monoclonal antibody against the B subunit of CT was added. Color intensity was measured at 492 nm in an ELISA reader (Bio-Rad). Three independent triplicate repeats were done for each strain. Transcription analysis of the ctxB gene in N16961 and N169-dtatABC cells The overnight cultures

of N16961 and N169-dtatABC cells were re-cultured to OD600 1.0 with fresh LB, and then 1:100 dilutions were transferred PR 171 into AKI medium. The medium used for all cultures was AKI complemented with 0.3% sodium bicarbonate. To determine the ctxB transcription levels, the strains incubated under still conditions for 4 h at 37°C find more were poured into flasks with a 20-fold greater volume than the tubes to continue growth at 37°C for 18 h with shaking at 220 rpm. The RNeasy Mini Kit (Qiagen) was used to extract total RNA from 1 ml of bacterial cultures. The RNase-free DNase set Kit (Qiagen) was used to eliminate

DNA. RNA samples were diluted to 1 ng/μl in order to obtain the template for RT-PCR after quantification. Primers 5′-CGC ATG AGG CGT TTT ATT ATT C-3′ and 5′-AAA GCG ATT GAA AGG ATG AAG G-3′ were used to amplify ctxB gene. The housekeeping gene thyA (primers 5′-ACA TGG GAC GCG TGT ATG G-3′ and 5′-ATA TGA CCA CCA TCA GGC TTA GC-3′) and 16S-rDNA (primers 5′-TTG ACA TCC AGA GAA TCT AGC GG-3′ and 5′-TTA ACC CAA CAT TTC ACA ACA CGA-3′) were selected as the references. RNA extraction and RT-PCR quantification were done in triplicate for each strain. 2-ΔΔCt method was used to calculate the Ct difference of ctxB between N16961 and N169-dtatABC, with the SB202190 existence of the control genes.