Vibrio cholera and Vibrio metecus: The Batman and Robin of diarrheal diseases

The definition of what constitutes a species in the microbial world is exceedingly controversial. Prokaryotic genomes can be extremely variable: a single milliliter of seawater can contain over a thousand genotypes of a single classically defined species of the marine heterotrophic genera Vibrio. These genotypes can differ significantly from each other, by up to 20% in genome size. This observation makes it likely that there are many more bacterial species than we previously thought. High-resolution typing methods used to characterize hundreds of environmental strains have allowed us to identify a novel sister species to the well-known causative agent of the human diarrheal disease Cholera, Vibrio cholera. The new species, Vibrio metecus, co-occurs in the environment with V. cholera but has clearly different niche. We have also found V. metecus to be responsible for a rising number of human infections in North America, suggesting it could become an emerging pathogen. This is likely not an isolated case and many other cryptic bacterial species could be discovered by combining molecular techniques offering high phylogenetic resolution with large sample sizes.