 Abstract, the evolution of antibiotic resistance in bacteria is of global health concern. This paper examines the role of within-host diversity in the emergence of resistance in Pseudomonas originosa, a common opportunistic pathogen. It was found that mixed strain populations are more likely to develop resistance than single strain populations, which suggests that within-host diversity can play a significant role in the evolution of resistance. Furthermore, it was observed that when multiple strains were present, there was a trade-off between resistance and growth rate, while this trade-off did not exist in single strain populations. These findings suggest that within-host diversity can influence the evolution of resistance in the absence of antibiotics. This article was authored by Julio Diaz Caballero, Rachel M. Wheatley, Natalia Capell, and others.