Why the cholera bacterium is a returning fugitive

Muhammad Zahir Hassan Nabil

The cholera bacteria evaded many scientific excellences for centuries and survived. Scientists have long wondered how these virulent species managed to hide in the environment to cause the next epidemic.

A new breakthrough review by icddr,b scientists suggests that the cholera bacteria take shelter inside certain types of cyanobacteria (phytoplankton) also known as the blue-green algae.

Blue-green algae that make the ocean look blue. Photo: Maggie Schreiter. CC BY-NC 2.0
The phytoplankton are also abundant in inland freshwater systems like ponds, lakes, rivers and canals. They can act as the inter-epidemic reservoir of Vibrio cholerae (V. cholerae), pathogens that cause cholera, in Bangladesh.

“We propose that climatic conditions such as rain, sunshine, temperature, and other chemical properties like pH, oxygen, carbon dioxide, salinity of the water bodies facilitate the phytoplankton to form bloom during the cholera seasons of the year,” observes Dr Sirajul Islam, icddr,b emeritus scientist and first author of the review. published in Vaccine. It provides the cholera bacteria with the most suitable condition to multiply itself to enable transmission of disease,” he adds.

Based on evidence from laboratories and environmental studies, the review argues in favour of the role of aquatic flora mainly the blue green algae as the reservoir of the cholera bacteria, shares Prof Niyaz Ahmed, senior director of icddr,b laboratory sciences and services and a senior author of the review.

“Studies have shown that the cholera bacteria, V.cholerae can attach itself with A. variabilis (a Cyanobacterium) but not with other algae,” explains Dr Islam.

A number of studies have furnished evidence that the V.cholerae feed on nutrients emitted from the blue-green algae. When exposed to sunshine, the algae produce oxygen which the cholera bacteria would ‘inhale’ and produce carbon dioxide that the algae would need for photosynthesis i.e. producing nutrients using sunshine.

Microscopic image: Authors

“It looks like a perfect match between the two that they supply necessities to each other,” adds Dr Siraj.

But how do the bacteria transmit from surface water bodies to human populations in the community?

The matter may not be very straightforward. Based on insights from different studies, the review went on to argue that during longer periods of sunshine, lower rain, favourable temperature and other chemical conditions, the algae grow due to increased photosynthesis.

At the same time, increased sunshine causes water volume to decrease, raises concentration of the algae, changes water pH condition and triggers additional chemical reactions between the algae, bacteria and surface water bodies. Thus water bodies get vigorously contaminated with cholera bacteria.

Human contact with water carrying cholera bacteria takes place during various activities. Photo: GMB Akash / icddr,b

Activities like bathing, washing utensils and occasional drinking of surface water bring in contact with the pathogens to be infected with cholera.

Once a person is infected, cholera is further transmitted in the community due to lack of knowledge about safe water, sanitation and hygiene, and this is why an outbreak may occur. icddr,b research has already demonstrated this phenomenon by tracking household-level transmissions.

Poor hygiene, lack of safe water and sanitation spread the disease. Photo: GMB Akash / icddr,b

Researchers have observed that these events taking place at the time of increased sunshine hours correspond to seasonal cholera peaks when epidemics are most likely to occur, once in the winter and in the summer.

The role of climate may be crucial in regulating and triggering these outbreaks, indicates existing icddr,b research. “It is, however, regrettable that use of unsafe water in places like Bangladesh is responsible for turning disease transmission into an outbreak,” opines Dr Islam.

Based on long-observations from health and demographic surveillance, icddr,b researchers have already suggested that climatic conditions trigger outbreaks of diseases such as cholera.

Therefore, the researchers observe that remote sensing-assisted cholera prediction efforts are necessary to enable an early warning system that will not only allow scientists to predict potential outbreaks but also help them understand how climatic conditions may be influencing disease outbreaks and how it may be intervened. Dr Sirajul Islam and this team are already part of such initiatives.