Effect of Mutations on the Evolution of Extended Spectrum β-lactamases (ESBL)

Bacterial antimicrobial resistance is a great public health threat worldwide, a situation that is much escalated by the rapid propagation of Extended Spectrum β-lactamase (ESBL) enzymes. These can hydrolyze and inactivate a broad range of β-lactams, including third-generation cephalosporins, penicillin, and aztreonam and are known to be associated with various bacterial infections, ranging from uncomplicated urinary tract infections to life-threatening sepsis.Variation is the essential raw material of Darwinian evolution and the accumulation of mutations plays one of the most important roles in it. Sequential acquisition of spontaneous mutations followed by successive rounds of selection can be attributed as one of the major reasons for the rapid diversification of ESBL enzymes. The ESBLs are excellent examples of 'microevolution' that led to 'gain-of-function' with an extended substrate spectrum. However, acquiring newer phenotypes sometimes comes with fitness costs and different mutational pathways interact with each other, triggering both additive and non-additive fitness to generate a rugged fitness landscape, that influences the path a strain must follow to adapt and evolve under selection pressure. Therefore, it is important to understand the role of mutations in the emergence of these enzyme variants. This review focuses on the understanding of different facades of mutational pathways that lead to the adaptive evolution of ESBL phenotype. The structural and mechanistic basis of the extension of the substrate spectrum by mutations are also discussed.