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Playing ‘rock-paper-scissors’ with bacteria

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Kolkata
13 Feb 2019
IISER Kolkata and McMaster University, Canada, studied a model of coexistence among bacteria

We live in a ‘eat or be eaten’ world, where cooperation and conflict form the basis of all coexisting living beings ranging from microscopic bacterial communities to large oceanic ecosystems with some of the largest mammals. This ability to coexist changes a few dynamics of the entire system and can influence how the participating organisms act. In a recent study, researchers from the Indian Institute of Science Education and Research Kolkata (IISER Kolkata) and McMaster University, Canada, have studied one such model of coexistence among bacteria.

One of the growing concerns today is the rise of antibiotic-resistant bacteria that have evolved to haunt us as ‘superbugs’. Hence, it is becoming increasingly difficult to treat infectious diseases like pneumonia, tuberculosis, gonorrhoea and other food-borne illnesses. In this study, published in the journal PLOS One, the researchers have looked into a competitive model of three types of bacteria—antibiotic producers like those from the genus Bacillus, non-producers that do not produce antibiotics like those from the genus Helicobacter, and other sensitive bacterial cells that succumb to antibiotics.

“Our work clarifies the conditions under which multi-species coexistence is possible in more realistic scenarios of microbial competition. Here, the production rate of antibiotics modulates the growth rates of antibiotic producer and sensitive species”, remarks Dr. Supratim Sengupta, Associate Professor at IISER Kolkata, who is one of the senior investigators of this study.

Many types of bacteria, found in the soil, produce antibiotic compounds that kill certain disease-causing bacteria. On the other hand, some of these disease-causing bacteria have evolved to withstand the effects of these antibiotics. In the current study, the researchers have used a computational model to explore how the rate of antibiotic production by some bacteria influences the coexistence of all three species in the microbial ecosystem.

Akin to the well-known game of rock-paper-scissors, where each player outcompetes one of the other two (rock crushes scissors, scissors cut paper and paper covers rock), competition in the bacterial model used in this study arises as a result of different growth rates for the three species. Sensitive cells outcompete the non-producers due to their higher growth rate. Similarly, non- producers outcompete antibiotic producers and 'hitch-hike' on the antibiotic producers ability to reduce growth rate of sensitive cells. Antibiotics reduce the growth rate of the sensitive cells.

Although the producers may seem to be the most powerful in this model, their growth rate is suppressed by the metabolic cost involved in producing the antibiotic. This cost reduces the growth rate on the one hand, and inhibits the growth rate of sensitive cells on the other, leading to the coexistence of the three species. “It shows how an antibiotic-resistant strain, who is a non-producer, can free ride by exploiting the presence of antibiotic producers to survive in the population”, explains Dr. Sengupta.

The researchers believe that understanding the dynamics of such bacterial colonies can provide insights into the conditions in which antibiotic-resistant species thrive. “Such insights will hopefully facilitate more effective action in dealing with the growing scourge of antibiotic-resistant strains in the public health domain”, signs off Dr. Sengupta.