Cholera research provides lessons about how bacteria cause pandemics

Infectious diseases remain the most significant cause of mortality for children worldwide. For those 5 years old and younger, diarrheal disease is one of the most prominent causes of morbidity and mortality. One such disease, cholera, remains a major scourge in countries with limited access to clean drinking water and sanitation, which continue to struggle with the bacteria that causes the infectious disease. 

The cholera pathogen Vibrio cholerae is one of the two bacteria that can cause pandemics and has done so for centuries. Thus, understanding its pandemic potential is a significant factor in protecting children globally. Surprisingly, only one group of V. cholerae has historically caused pandemics — the aptly named pandemic cholera group. Why only this bacterial group has caused cholera pandemics has remained a mystery. Cholera bacteria are capable of sharing mobile genetic elements — DNA sections that can move or copy themselves within or between genomes. That means the pandemic group can spread the genes that source its virulence, potentially making other innocuous bacteria virulent, but such events have never been observed.

Salvador Almagro-Moreno, PhD, St. Jude Department of Host-Microbe Interactions, studies these mobile genetic elements to better understand why other groups of cholera bacteria have not acquired the virulence genes that would give them the ability to colonize the human gut. In research published in Proceedings of the National Academy of Sciences, his team sheds light on the question. 

“We found that not all mobile genetic elements are created equal,” said Almagro-Moreno. “Cholera must have the right series of allelic variations in its virulence genes on the right genomic background to become virulent. It’s like a genetic slot machine; the pathogenic variants of these allelic variations all need to be present to lead to the jackpot of pandemic potential.”

Filling out cholera’s family tree to find pandemic fingerprints

To find what made the pandemic cholera group different, the scientists compared over 1,840 V. cholerae genomes, including virulent and nonvirulent strains. “We built the largest phylogeny of the species with state-of-the-art computational analyses,” Almagro-Moreno explained. “By including environmental strains of V. cholerae, we could uncover the variants in their mobile elements that didn’t match with those in the ones that can cause disease in humans.”

After identifying these genetic differences, the researchers sought to find if they contributed to the capability of the pandemic cholera group to cause disease. To test this, they introduced the variations from the environmental V. cholerae mobile genetic elements into a pandemic cholera strain. In a mouse model, the modified pandemic strain lost its ability to colonize the gut while still maintaining the ability to grow in the environment.

“Having the alternative allelic variations doesn’t cost cholera much in the environment, but it costs a lot in the gut,” Almagro-Moreno said. “Now that we’ve identified that cost, we are curious to understand how the variations found in the pandemic cholera strains allow successful infection in the host.”

Making models that clarify cholera’s colonization potential

In addition to using mouse models, the researchers created three new cholera models: crustaceans, cyanobacteria and mollusks. The former two are considered critical environmental reservoirs of the bacterium. Pandemic strains that had environmental variations introduced to their mobile genetic elements could still infect these species, indicating there is no evolutionary pressure to gain or lose the pandemic-related variants. 

Through their work, the researchers unexpectedly upended conventional wisdom about how cholera infects crustaceans, the vector of the disease. They infected the shrimp with fluorescently tagged cholera, which the scientists expected to colonize the chitinaceous surface (hard shell) of the organisms. Instead, they observed the cholera bacteria in the gut of the shrimp, providing evidence for how the pathogenic bacteria can gain the ability to infect humans.

“We thought that the bacterium attached to the surface because that was the long-held belief in the field,” Almagro-Moreno said. “Instead, we found it colonizes perfectly well in the gut of the shrimp, nearly filling it. It makes a little ‘cholera bomb,’ which explains how drinking water with these small crustaceans present, even in minor amounts, can cause severe disease in humans.”

Preventing bacterial pandemics

Understanding how cholera transmits and develops into human disease enables scientists to guard against the advent of a potential pandemic, an ongoing and urgent need. 

“Our goal is to be able to predict outbreaks and eventually control them,” Almagro-Moreno said. “Cholera, which is still present in much of the world, can cause an adult to lose 20 liters of water daily, which is about a third of their body weight. Imagine that in a child, and you start to understand why cholera is such a large cause of childhood mortality. By understanding the disease better, we can find ways to mitigate it and create new, more effective therapeutics.”

The findings may not be restricted to cholera. Other bacterial species with serious pandemic or outbreak potential, such as Yersinia Pestis, the cause of the Black Plague, are similarly poorly understood. The phylogenetic tree approach, combined with in-depth sequence analysis of mobile elements, provides a new template for examining these diseases to find novel insights.

“We’ve shown our surveillance of these pathogens may need to be much more granular to really understand how they cause disease,” Almagro-Moreno said. “It’s not enough to see if virulence genes are present; we also need to detect the slight variations that rougher approaches may miss, which may give us new places to intervene. With that information, we may be able to protect more children’s lives.”