Toxic cholera bacteria flourish after heavy rains and persist at other times, researchers suggest

Women retrieving water from environmental source
Women retrieving water from environmental source in Haiti.

EPI investigator Afsar Ali led a study characterizing how the pathogen that causes cholera persists in wild, low-nutrient aquatic environments for decades.

new study published in “Frontiers in Microbiology” by EPI investigator Afsar Ali found that after long-term persistence in nutrient-poor aquatic conditions, toxigenic V. cholerae O1 bacteria sustained a mutation that rendered them unable to move independently while also supporting connection to other bacteria in the form of a biofilm. The study’s authors suggest that this biofilm allows the bacteria to survive, and when heavy rainfall brings an influx of necessary nutrients into the water, the bacteria can grow and reproduce more extensively. They propose that the increase in cholera transmission after monsoons, hurricanes, and other intense weather phenomena is likely due to runoff from agricultural lands into nearby lakes and streams.

With rainy season gearing up in Haiti, understanding the connection between weather, cholera persistence, and spikes in transmission is particularly important.

Shrestha Sinha Ray, Ph.D., pipetting in the Emerging Pathogen Institute’s BSL-2 lab.

“In countries where cholera-causing bacteria persist in rivers, lakes, and other aquatic reservoirs, fertilizers and other types of agricultural runoff help these bacteria increase from small doses to infectious doses,” said Dr. Shrestha Sinha-Ray, a research scholar working with Dr. Afsar Ali in the College of Public Health and Health Professions and the primary author of the study. Ali is the senior author on the study, and a professor in both the department of environmental and global health and the Emerging Pathogens Institute.

In nutrient-rich conditions such as the human body, V. cholerae bacteria are motile – using their flagella to move independently toward nutrients.

Sinha-Ray found that when chitin, phosphate and other necessary nutrients were lacking in filter-sterilized lake water, however, a mutation within the cholera bacteria caused them to lose their flagellar motility after persisting in the water for 700 days.

Given the rapid changes V. cholerae faces in dynamic aquatic reservoirs, the authors assume this process also takes place under natural conditions, allowing the bacteria to better persist in aquatic reservoirs.

Sinha-Ray and Ali found out that the mutation of the flagellar regulatory gene, flrA, which led to the loss of flagellar motility in V. cholerae, enhanced biofilm formation only in nutrient-poor lake water conditions. The integrity of the biofilm was maintained by mannose sensitive hemagglutinin pili, or MSHA pili. These pili are small, hair-like appendages found on the surface of many bacteria that help them attach both to surfaces and to each other.

By deleting the mshA gene responsible for creating MSHA pili in the now non-flagellated bacteria, Sinha-Ray rendered the bacteria incapable of producing biofilm; this change resulted in very few cells surviving in her water sample.

“Not all bacteria can survive in water,” Sinha-Ray said. The experiment compared cholera bacteria with and without flagella, finding that those without flagella were able to produce much more biofilm, helping them stay alive in the water while nutrients were low. 

The authors also observed that the 700-day-old starving cholera bacteria were better capable of moving towards nutrients than the wild-type strain. How the bacteria move without flagella is still unclear. In future studies, Sinha-Ray hopes to learn more about how these biofilms are able to move as a unit.