Why only some bacteria develop multi-drug resistance

The study subjected E. coli to fluctuating and steady environments, noting their evolutionary response

During evolution, the fitness costs experienced by bacteria under constant and fluctuating environments pose a problem that has not be solved. One way of seeing this is through the example of multi-drug resistance. It is not clear why some bacteria evolve multi-drug resistance while others do not. New research from the Population Biology Lab at IISER Pune could hold a key to this and a similar class of puzzles. Multi-drug resistance is a menace in public health, however it is a fascinating problem to an evolutionary biologist who sees it from this angle: possessing multi-drug resistance implies that the bacteria is adept at handling multiple antibiotics simultaneously. This would increase its fitness appreciably. Given that antibiotics exert a very strong selection pressure, it would appear that every bacteria in nature can become multi-drug resistant, which is not the case. “One of the reason given for why that does not happen is fitness cost,” says Sutirth Dey, in whose lab the study was carried out. When bacteria become fit in one environment, they either lose fitness or fail to increase fitness in other environments. “Our study is showing that when the environment is fluctuating, large (but not small) populations can by-pass this effect,” he adds. Yashraj Chavhan, Sarthak Malusare, and Sutirth Dey studied populations of small and large sizes across different constant and fluctuating environments and then subjected the evolved populations to whole-genome, whole-population sequencing analysis. They found that small populations acquire a certain set of mutations which allow them to survive in one environment while paying a cost in others. Large populations also develop these mutations but, in addition, have certain compensatory mutations that together give them fitness to survive in different environments. Thus, population size determines the kind of mutations available to the bacteria, which in turn, leads to the type of fitness costs they evolve.