The researchers used a combination of genomic analyses and antibiotic challenge tests to quantify within-patient bacterial diversity and antibiotic resistance. Most patients in the study (approximately two thirds) were infected by a single Pseudomonas strain. Crucially, resistance increased by about 20% more when patients with mixed strain infections were treated with antibiotics, compared to patients with single strain infections. When samples from single strain and mixed strain patients were cultured in the absence of antibiotics, the AMR strains grew more slowly compared with non-AMR strains. The study ‘Mixed strain pathogen populations accelerate the evolution of antibiotic resistance in patients’ has been published in Nature Communications.