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Antibiotics create a powerful selective environment that influence how bacteria evolve. In sterile settings like hospitals, wher antibiotics are heavily used to treat infections and maintain cleanliness, the conditions favor bacteria that have reisistane mechanisms. These resistant bacteria survive while others die off, leading to less competition and more room for the resistant strains to grow and spread
Selective pressure from antibiotic use promotes the survival of resistant bacteria. Even low-dose exposure can create “windows of selection” where only mutants survive, allowing resistance to develop and spread. The figure below shows how different concentrations select for specific genotypes (Baquero, Coque, & Cantón, 2021).
This selective pressure is especially dangerous in environments with immunocompromised patients, where infections are more common and harder to treat. When antibiotics are overused or misused, they wipe out the susceptible bacteria but allow resistant strtains like MRSA, C-Diff, and E.coli to dominate. (Marciniak, Tyczewska, & Grzywacz, 2024). Over time, this constant pressure helps bacteria adapt and pass on resistant genes. Bacteria evolve in responce to this pressure in a few key ways:
Horizontal gene transfer, where they swap resistance genes with other bacteria.
Mutations, which can happen quickly and allow bacteria to survive antibiotic attacks.
Reduced competition, giving resistant strains more space and resources to thrive.
Understanding selective pressures helps us realize why it is so important to use antibiotics carefully and only when truly necessary. It also highlights the need for better infection control, especially in hospital settings, to prevent the rise and spread of antibiotic-resistant bacteria.
This short video helps explain how antibiotic-resisant bacteria proliferate and how it relates to evolution.
Walter Jahn. (2017, December 22). natural selection of antibiotic resistance in bacteria. YouTube. https://www.youtube.com/watch?v=Iw0fcft4v9E
This figure shows how antibiotic concentration gradients create zones of selective pressure. As concentrations increases, different resistant variants are favored depending on their ability to survive in specific segments of the gradient. This stepwise selection contributes to the progressive evolution of high-level resistance (Baquero, Coque, & Cantón, 2021).
The figure shows the evolutionary pathways and gene exchange involving mobile genetic elements (MGEs) and antibiotic resistance genes within phylogenetic trees. the grey and red trees repressnt different phylogenetic lineages while the blue arrows and branches represent MGEs carrying antibiotics.
(A) transferes from the grey tree into the red tree through horizontal gene transfer.
(B) the light blue MGE recombines with the yellow MGE in the red tree creating a new green MGE variant.
(C) the dark blue variant of the light blue MGE arises in the grey tree through mutation or intermal recombination.
(D) the new dark blue MGE segregrates into a new branch showing the evolution of MGEs and their divergence.
Reference:
Baquero, F., Martínez, J. L., Lanza, V. F., Rodríguez-Beltrán, J., Galán, J. C., San Millán, Á., Cantón, R., & Coque, T. M. (2021). Evolutionary pathways and trajectories in antibiotic resistance. Clinical Microbiology Reviews, 34(4), e00050-19. https://doi.org/10.1128/CMR.00050-19
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