Losee Ling and colleagues detect no bacterial resistance to the new antibiotic molecule teixobactin (L. L. Ling et al. Nature 517, 455–459; 2015), but this could be because the conditions of their test may limit its sensitivity (see J. Ramsayer et al. Evol. Appl. 6, 608–616; 2013). 'Evolutionary rescue' is a more powerful assay for evaluating the probability of resistance to novel antibiotics in large bacterial samples, and therefore for informing decisions about their usage.

Evolutionary rescue assays can distinguish between resistant mutants that are present initially and those that emerge later (H. A. Orr and R. L. Unckless PLoS Genet. 10, e1004551; 2014). This type of assay can also be used to evaluate factors that contribute to the emergence of bacterial resistance, such as 'horizontal' gene transfer from other bacteria or the presence of bacterial 'mutator' strains with vastly increased mutation rates.