Identification and nucleotide sequence determination of the gene responsible for Ogawa serotype specificity of V. cholerae 01

Serotype conversion gene rfbT is directly regulated by histone-like nucleoid structuring protein (H-NS) in V. cholerae O1

Vibrio cholerae serogroup O1 (V. cholerae O1) is closely associated with cholera epidemics and has two main immunologically distinguishable serotypes, Ogawa and Inaba. Isolates serotype as Ogawa if the O-antigen polysaccharide (O-PS) is methylated or as Inaba if the O-PS is not methylated. This methylation is mediated by a methyltransferase encoded by the rfbT gene, and the mutation and low expression of rfbT results in serotype switch from Ogawa to Inaba. Previously, we have shown that cAMP receptor protein (CRP) activates rfbT. In this study, we demonstrated that histone-like nucleoid structuring protein (H-NS) is directly involved in the transcriptional repression of rfbT. This finding is supported by the analyses of rfbT mRNA level, rfbT-lux reporter fusions, electrophoretic mobility shift assay (EMSA), and DNase I footprinting assay. The rfbT mRNA abundances were significantly increased by deleting hns rather than fis which also preferentially associates with AT-rich sequences. A single-copy chromosomal complement of hns partly restored the down-regulation of rfbT. Analysis of rfbT-lux reporter fusions validated the transcriptional repression of hns. Subsequent EMSA and DNase I footprinting assay confirmed the direct binding of H-NS to rfbT promoter and mapped the exact binding site which was further verified by site-directed mutagenesis and promoter functional analysis. Furthermore, we found that in hns deletion mutant, CRP is no longer required for transcriptionally activating rfbT, suggesting that CRP functions as a dedicated transcription factor to relieve H-NS repression at rfbT. Together, this study expanded our understanding of the genetic regulatory mechanism of serotype conversion by global regulators in V. cholerae O1.

Serotype-shifting gene rfbT is a direct transcriptional target of cAMP receptor protein (CRP) in V. cholerae O1

Ogawa and Inaba are two main serotypes of O1 V. cholerae and alternate among cholera epidemics. The rfbT gene encodes a methyltransferase and is required for Ogawa serotype. The Inaba serotype is the consequence of genetic alterations in rfbT gene which results in loss-of-function enzyme product. However, the expression and regulation of rfbT has not been understood yet. Here we demonstrated that a global regulator, cAMP receptor protein (CRP), positively regulates rfbT transcription through a non-canonical CRP binding site (CBS) in its promoter region. This finding is supported by the analyses of rfbT mRNA abundance, rfbT-lacZ fusions and electrophoretic mobility shift assay (EMSA). The analyses of rfbT mRNA level in wild type (WT), Δcrp, and lower or higher level of cAMP backgrounds revealed that CRP is required for rfbT expression in response to intracellular cAMP level. Subsequent rfbT-lacZ fusions and EMSA collectively displayed that cAMP-CRP complex transcriptionally activates rfbT through directly binding to CBS in rfbT promoter region. Consistently, serological microagglutination test showed that crp deletion resulted in at least 4-fold decrease in titer of Ogawa serum compared to its WT. These results expanded our knowledge of understanding the genetic determinants and probable regulatory mechanism of V. cholerae O1 serotype shift between Ogawa and Inaba.

Major Shift of Toxigenic V. cholerae O1 from Ogawa to Inaba Serotype Isolated from Clinical and Environmental Samples in Haiti

In October of 2010, an outbreak of cholera was confirmed in Haiti for the first time in more than a century. A single clone of toxigenic Vibrio cholerae O1 biotype El Tor serotype Ogawa strain was implicated as the cause. Five years after the onset of cholera, in October, 2015, we have discovered a major switch (ranging from 7 to 100%) from Ogawa serotype to Inaba serotype. Furthermore, using wbeT gene sequencing and comparative sequence analysis, we now demonstrate that, among 2013 and 2015 Inaba isolates, the wbeT gene, responsible for switching Ogawa to Inaba serotype, sustained a unique nucleotide mutation not found in isolates obtained from Haiti in 2012. Moreover, we show that, environmental Inaba isolates collected in 2015 have the identical mutations found in the 2015 clinical isolates. Our data indicate that toxigenic V. cholerae O1 serotype Ogawa can rapidly change its serotype to Inaba, and has the potential to cause disease in individuals who have acquired immunity against Ogawa serotype. Our findings highlight the importance of monitoring of toxigenic V. cholerae O1 and cholera in countries with established endemic disease.

For the first time in 100 years, in October 2010, cholera caused by toxigenic strains of V. cholerae was introduced in Haiti. Conventional and genetic analysis revealed that a single clone of V. cholerae O1 biotype El Tor, serotype Ogawa strain was brought to Haiti by Nepalese Peace-Keeping troops. These troops arrived in January of that year to provide humanitarian assistance following Haiti’s deadliest earthquake. Subsequently our team has monitored the cholera epidemic by acquiring clinical and environmental samples to assess whether or not the pathogen was able to establish its environmental reservoirs within the country, and whether V. cholerae could undergo evolutionary changes in order to adapt to the stressors imparted by human gastrointestinal tract and environmental aquatic reservoirs. In this study we show that over the past 5 years the initially-introduced and circulating V. cholerae Ogawa serotype has significantly shifted to Inaba serotype by sustaining multiple mutations in wbeT gene, the gene known to mediate serotype shift. Our findings suggest that the new Inaba serotype may be replacing the Ogawa serotype in order to evade Ogawa-induced host immunity and thereby causing cholera potentially among individuals who may previously have suffered from cholera. Our study highlights the importance of ongoing monitoring of cholera in Haiti.

Sequence polymorphisms of rfbT among the Vibrio cholerae O1 strains in the Ogawa and Inaba serotype shifts

Background

Vibrio cholerae serogroup O1 has two major serotypes, Ogawa and Inaba, which may alternate among cholera epidemics. The rfbT gene is responsible for the conversion between the two serotypes. In this study, we surveyed the sequence variance of rfbT in the Ogawa and Inaba strains in China over a 48-year (1961-2008) period in which serotype shifts occurred among epidemic years.

Results

Various mutation events including single nucleotide, short fragment insertions/deletions and transposases insertions, were found in the rfbT gene of the Inaba strains. Ectopically introducing an intact rfbT could overcome the mutations by converting the Inaba serotype to the Ogawa serotype, suggesting the effects of these mutations on the function of RfbT. Characteristic rfbT mutations were recognized in the Inaba strains among Inaba serotype dominant epidemic years which were separate from the Ogawa dominant epidemics. Three distinguishable mutation sites in rfbT between the classical and the El Tor biotype strains were identified and could serve as biotype-specific biomarkers.

Conclusions

Our results provide a comprehensive picture of the rfbT gene mutations among the V. cholerae O1 strains in different epidemic periods, which could be further used as the tracing markers in clonality analysis and dissemination surveillance of the epidemic strains.

Lipopolysaccharides of Escherichia coli K12 strains that express cloned genes for the Ogawa and Inaba antigens of Vibrio cholerae O1: identification of O-antigenic factors

Structural and serological studies were performed with the lipopolysaccharide (LPS) expressed by Escherichia coli K12 strains No. 30 and No. 64, into which cosmid clones derived from Vibrio cholerae O1 NIH 41 (Ogawa) and NIH 35A3 (Inaba) had been introduced, respectively. The two recombinant strains, No. 30 (Ogawa) and No. 64 (Inaba), produced LPS that included, in common, the O-polysaccharide chain composed of an alpha (1-->2)-linked N-(3-deoxy-L-glycero-tetronyl)-D-perosamine (4-amino-4,6-dideoxy-D-manno-pyranose) homopolymer attached to the core oligosaccharide of the LPS of E. coli K12. Structural analysis revealed the presence of N-(3-deoxy-L-glycero-tetronyl)-2-O-methyl-D-perosamine at the non-reducing terminus of the O-polysaccharide chain of LPS from No. 30 (Ogawa) but not from No. 64 (Inaba). Serological analysis revealed that No. 30 (Ogawa) and No. 64 (Inaba) LPS were found to share the group antigen factor A of V. cholerae O1. They were distinguished by presence of the Ogawa antigen factor B [co-existing with relatively small amounts of the Inaba antigen factor (c)] in the former LPS and the Inaba antigen factor C in the latter LPS. It appears, therefore, that No. 30 (Ogawa) and No. 64 (Inaba) have O-antigenic structures that are fully consistent with the AB(c) structure for the Ogawa and the AC structure for the Inaba O-forms of V. cholerae O1, respectively. Thus, the present study clearly confirmed our previous finding that the Ogawa antigenic factor B is substantially related to the 2-O-methyl group at the non-reducing terminus of the alpha (1-->2)-linked N-(3-deoxy-L-glycero-tetronyl)-D-perosamine homopolymer that forms the O-polysaccharide chain of LPS of V. cholerae O1 (Ogawa).

Mutations in the rfbT gene are responsible for the Ogawa to inaba serotype conversion in Vibrio cholerae O1

In cultures of Vibrio cholerae strains of Ogawa serotype, variant strains which had undergone serotype conversion from Ogawa to Inaba were identified. The rfbT genes cloned from the parent strains were found to produce a 31-kDa protein in the maxicell system, and to cause serotype conversion when introduced into E. coli cells expressing Inaba serotype specificity. On the other hand, rfbT genes cloned from the variant strains neither produced the 31-kDa protein nor caused serotype conversion. Nucleotide sequence of these rfbT genes as well as those of two clinical Vibrio cholerae strains of Inaba serotype revealed that mutations causing premature termination of their rfbT genes were invariably present in strains expressing Inaba serotype specificity. The results strongly suggested that genetic alteration of the rfbT gene is responsible for serotype conversion of Vibrio cholerae O1.