Vibrio cholerae O1 El Tor Variant Isolated from Clinical and Environmental Sources in Northeastern Thailand

Abstract

Vibrio cholerae can cause severe diarrhea because of cholera enterotoxin. Within the V. cholerae serogroup O1, two biotypes are recognized: classical (CL) and El Tor (ET). The El Tor biotype is further categorized into 3 waves, based on the CTX phage genes present and the analysis of single-nucleotide polymorphisms (SNPs) in the genome. Wave 1 (CTX-1 or prototype El Tor) harbors ctxBET and rstRET, wave 2 (CTX-2) harbors ctxBCL and rstRCL, wave 3 (CTX-3) harbors ctxBCL and rstRET and variants of wave 3 (CTX-3b, 4, 5, 6, 6b) have specific SNPs in rstA and rstB genes. The presence of variant wave 3 has not been investigated in Thailand. The aim of this study was to investigate the presence of V. cholerae atypical El Tor strains (CTX-2, 3, 3b, 4, 5, 6) from clinical and environmental sources in Northeast Thailand between 2003 and 2012 by using PCR assays for genetic screening of waves 1- 3. Sequencing of rstA, rstB and ctxB genes was used to identify the presence of variant wave 3. All V. cholerae O1 isolates from clinical sources (2007-2010) belonged to variant wave 3 (CTX-6). A similar result was obtained from two of three environmental isolates, while the third showed a CTX-2 variant strain because of the absence of rstB. Most (60%) DNAs extracted directly from 80 water samples were negative for CTX phage genes; 27.5% yielded unclassified CTX genes; 3.7% contained CTX-6, 3.7% were hybrid El Tor, 2.5% were wave 2 and 2.5% were unidentified wave 3. Our findings indicate that most V. cholerae O1 in clinical and environmental sources in Northeast Thailand are CTX-6. The prevalence of atypical El Tor strains found in environmental samples suggested that the aquatic environment might be an important source of the evolution of the outbreak strains.