Divergence of acquired stress resistance among yeast species and establishing a flow cytometry-based assay for post-stress survival quantification

Many organisms display a phenomenon called Acquired Stress Resistance (ASR), where exposure to a primary, non-lethal stress enhances survival against a secondary stress that may otherwise be severe or even lethal. ASR is widely observed across domains of life and is believed to provide survival advantages in environments with predictable changes. Our lab previously identified a divergent ASR response in two related yeast species: in an opportunistic yeast pathogen, a primary, non-lethal phosphate starvation treatment enhanced the survival of the cells against a lethal dose of hydrogen peroxide, while the same primary stress provides little to no protection in the related low-pathogenic potential relative, the baker’s yeast.

We asked several questions regarding the ASR response. First, what was the ancestral state of ASR? To answer this question, we looked at ASR in a very distantly related species. We found that the ancestral species has a moderate ASR response. The magnitude of the ASR is not the same as the opportunistic pathogen or the baker’s yeast. We then asked whether ASR is correlated with pathogenesis. We answered the question by measuring ASR in a distantly related pathogen, as well as yeast strains from very different environments. The distantly related pathogen has a very mild ASR. The strains from different environments are all similar in their ASR magnitude. We think that ASR has been modified from the ancestral state, but the modification is not highly correlated with pathogenesis.

We separately asked whether the primary phosphate starvation stress is specific to the oxidative secondary stress by hydrogen peroxide. We tested various oxidants as the secondary stressor. We found ASR to be specific to certain oxidants. This indicates that ASR is likely an anticipatory stress response that evolved during an organism's adaptation to the environment.

While studying ASR, we found that the traditional plate growth survival assessment is slow and time consuming. A high-throughput method for accessing yeasts cell death is currently lacking. We optimized and justified a flow cytometry-based yeast survival assay, applied it in diverse species, and compared it to the traditional method. We demonstrated the survival rate measured by the flow cytometry-based method correlates with survival rate determined by the traditional method. The new method can be applied to quantify ASR survival. These results suggest that the new method may serve as an alternative to the traditional method for quickly assessing yeast survival. The efficient nature of the flow cytometry-based assay makes it useful in diverse screening scenarios.