New biomarkers underlying acetic acid tolerance in the probiotic yeast Saccharomyces cerevisiae var. boulardii

Evolutionary engineering experiments, in combination with omics technologies, revealed genetic markers underpinning the molecular mechanisms behind acetic acid stress tolerance in the probiotic yeast Saccharomyces cerevisiae var. boulardii. Here, compared to the ancestral Ent strain, evolved yeast strains could quickly adapt to high acetic acid levels (7 g/L) and displayed a shorter lag phase of growth. Bioinformatic-aided whole-genome sequencing identified genetic changes associated with enhanced strain robustness to acetic acid: a duplicated sequence in the essential endocytotic PAN1 gene, mutations in a cell wall mannoprotein (dan4Thr192del), a lipid and fatty acid transcription factor (oaf1Ser57Pro) and a thiamine biosynthetic enzyme (thi13Thr332Ala). Induction of PAN1 and its associated endocytic complex SLA1 and END3 genes was observed following acetic acid treatment in the evolved-resistant strain when compared to the ancestral strain. Genome-wide transcriptomic analysis of the evolved Ent acid-resistant strain (Ent ev16) also revealed a dramatic rewiring of gene expression among genes associated with cellular transport, metabolism, oxidative stress response, biosynthesis/organization of the cell wall, and cell membrane. Some evolved strains also displayed better growth at high acetic acid concentrations and exhibited adaptive metabolic profiles with altered levels of secreted ethanol (4.0-6.4% decrease), glycerol (31.4-78.5% increase), and acetic acid (53.0-60.3% increase) when compared to the ancestral strain. Overall, duplication/mutations and transcriptional alterations are key mechanisms driving improved acetic acid tolerance in probiotic strains. We successfully used adaptive evolutionary engineering to rapidly and effectively elucidate the molecular mechanisms behind important industrial traits to obtain robust probiotic yeast strains for myriad biotechnological applications. KEY POINTS: •Acetic acid adaptation of evolutionary engineered robust probiotic yeast S. boulardii •Enterol ev16 with altered genetic and transcriptomic profiles survives in up to 7 g/L acetic acid •Improved acetic acid tolerance of S. boulardii ev16 with mutated PAN1, DAN4, OAF1, and THI13 genes.

Activation of unfolded protein response pathway during infectious salmon anemia virus (ISAV) infection in vitro an in vivo

Infectious salmon anemia virus (ISAV) is a salmon pathogen causing serious outbreaks in fish farms world-wide. There is currently no effective commercially available vaccine and there is a need for better understanding of host pathogen interactions with this virus. Various strains can cause both acute and persistent infections and therefore establish a balance with the host immune responses. We have studied host responses to this infection by analyzing the main branches of the unfolded protein response (UPR) in salmon cells in vitro and in tissues from infected fish to gain a better understanding of virus-host interactions. ISAV induce the main symptoms and signaling pathways of UPR (ATF6, PERK and IRE1) without inducing translational attenuation. This may be due to concomitant induction of an important negative feedback loop via the phosphatase regulator GADD34. The host cells can therefore respond with translation of cytokine and antiviral proteins to curb or control infection.