Cholera Reference Genome Gets a Modern Makeover, With Over 96% of Genes Now Annotated

When researchers sequenced the first complete genome of Vibrio cholerae in 2000, the achievement was monumental. The bacterium, which causes the devastating diarrheal disease cholera, carries its genetic blueprint across two nonhomologous chromosomes totaling roughly 4.03 million base pairs. Yet for all its groundbreaking value, that original annotation left a staggering portion of the genome shrouded in mystery. More than 40 percent of the 3,890 genes catalogued at the time were designated as ‘hypothetical protein’ or ‘conserved hypothetical protein,’ meaning their functions were entirely unknown or merely conserved across related species without clear characterization.

A quarter century later, the picture has transformed dramatically. Ravipaul Singh and Jeffrey H. Withey of Wayne State University School of Medicine in Detroit have published a comprehensive comparative analysis in Frontiers in Bacteriology that documents just how far functional annotation of the V. cholerae O1 El Tor strain N16961 reference genome has come. Their findings reveal that the proportion of genes labeled as hypothetical has plummeted from over 40 percent in 2000 to approximately 5.7 percent as of 2026. In other words, the vast majority of the cholera pathogen’s genome now carries a defined functional annotation, a testament to decades of experimental work by the microbiology community.

The strain at the center of this effort, O1 El Tor N16961, holds a special place in cholera research. It was the first complete V. cholerae genome to be sequenced, published by Heidelberg and colleagues in Nature in 2000. That original sequence was revised in 2019 and further refined in 2026 by the Wellcome Sanger Institute in Hinxton, Cambridgeshire, United Kingdom, with considerable improvements to the functional annotations of genes. These updated sequences are publicly available through the National Center for Biotechnology Information, bringing the genomic annotations in line with contemporary knowledge and highlighting the significant increase in annotated genes compared to the original release.

Singh and Withey approached the comparison meticulously, going gene by gene across both versions of the genome. For the 2000 sequence, referred to as Vc (2000), genes on chromosome 1 carry ‘VC’ numbers while those on chromosome 2 carry ‘VC_A’ numbers. The 2026 sequence, Vc (2026), uses either four-letter gene names or ‘FY484_RS’ designations followed by a five-digit number, a classification that applies to both chromosomes. By hovering over any gene in the updated annotation, researchers can ascertain critical information such as the gene name, annotation, length in nucleotides, position on the chromosome, and any relevant comments. The authors evaluated both genomes using gene annotation, gene size, and chromosomal position as comparative benchmarks.

The results of this gene-by-gene audit are striking. The Vc (2000) annotation contains 3,890 genes, of which 2,307 carry a gene name and 1,577 are designated as hypothetical or conserved hypothetical. Six genes annotated in the 2000 genome are not listed in the 2026 version. The Vc (2026) annotation contains 3,524 genes, of which 3,323 carry a gene name and only 201 remain designated as hypothetical. Notably, 366 genes described in the 2000 genome are not listed in the updated sequence. Many of those missing entries appear to have been placeholders, often consisting of fewer than 100 nucleotides, rather than genuine protein-coding genes. Their removal accounts for the overall reduction in total gene count from 3,890 to 3,524.

To push the annotation even further, the researchers conducted protein-protein BLAST comparisons using BLASTP 2.17.0+ against the ClusteredNR database, which provides highly specific matches at a minimum of 90 percent sequence identity and length. Of the 201 genes still designated as hypothetical in Vc (2026), the analysis provided significant sequence identity for 72 genes, facilitating annotation of their putative functions. That represents a positive return of approximately 38.5 percent for the remaining unknowns. When these predicted functions are acknowledged alongside the current annotated genome, the authors report that over 96 percent of the V. cholerae genome now has an attributed functional annotation in 2026.

This progress did not happen automatically. As Singh and Withey note, reference genomes do not spontaneously update their annotations as new protein functions are uncovered in the literature. A genomic annotation once considered groundbreaking becomes obscure over time unless deliberate effort is invested in revision and refinement. The 2019 and 2026 updates to the N16961 reference genome significantly reduced the number of genes assigned to traditional designations of unknown function, but until now there was no defined manner in which anyone could compare the changes to gene annotations over time between the reference genomes for this strain.

To address that gap, the authors present their analysis as an easily usable resource that allows researchers to search for and compare changes to the functional annotations of the over 3,500 protein-coding genes present in the V. cholerae O1 El Tor str. N16961 genome. The complete list of genes with side-by-side comparisons between Vc (2000) and Vc (2026), along with the BLASTP prediction analysis for genes of unknown function in the 2026 genome, is provided as Excel files in the supplementary data. This resource enables any user to search for a V. cholerae gene using either the historical VC and VC_A numbers or the modern gene names and RS designations, while simultaneously providing a direct comparison of every gene annotation between the two reference genomes.

The implications extend well beyond cholera. Reference genomes serve as primary resources for genomic data across bacterial species, and the N16961 sequence has served and will continue to serve as a cornerstone for V. cholerae research. Advancements in genomic sequencing techniques have made the task of maintaining high-quality annotations significantly easier, but the authors emphasize that constant maintenance of reference genome annotation remains critically important. They thank the V. cholerae research community for its efforts in uncovering the unknown and establishing a well-annotated genome, while underscoring that continued vigilance is necessary to keep the resource current.

The work was supported by the National Institute of Allergy and Infectious Diseases grant R21AI171072. The authors declared no commercial or financial relationships that could be construed as a potential conflict of interest, though Withey noted he was an editorial board member of Frontiers at the time of submission, which had no impact on the peer review process. The study was received in March 2026, accepted in April 2026, and published on May 19, 2026, in Volume 5 of Frontiers in Bacteriology under the editorial guidance of Ansel Hsiao of the University of California, Riverside. For a pathogen that continues to claim lives through contaminated food and water, a genome that is over 96 percent functionally annotated represents a powerful new foundation for understanding and ultimately combating one of humanity’s oldest scourges.

Subject of Research: Updated functional annotation of the Vibrio cholerae O1 El Tor str. N16961 reference genome

Article Title: Updated functional annotation of the Vibrio cholerae O1 El Tor str. N16961 reference genome

Article References: Singh, R., & Withey, J. H. (2026). Updated functional annotation of the Vibrio cholerae O1 El Tor str. N16961 reference genome. Frontiers in Bacteriology, 5, Article 1833038. https://doi.org/10.3389/fbrio.2026.1833038

Image Credits: AI Generated

DOI: 10.3389/fbrio.2026.1833038

Keywords: Updated, functional, annotation, Vibrio, cholerae, N16961, reference, genome, scientific research