Salinity induced apoptosis in food spoilage yeast Zygosaccharomyces bisporus

Abstract

Background: Food spoilage is one of the most serious challenges in agriculture, and food and beverage industry, which can lead to worldwide food economic loss. The crucial organoleptic species, Zygosaccharomyces bisporus, is a highly resistant yeast fungus that can escape industrial quality check. They survive high salt environments by undergoing immediate programmed cell death (PCD), which plays an important role in mediating adaptive responses to adverse environmental conditions. Production of reactive oxygen species (ROS) prompted by salt stress is an early event in apoptosis, which in later stage is associated with prime genomic degradation.

Methodology: In this study, the tolerance mechanism to salt of Z. bisporus MTCC 4801 cells was investigated by serial dilution of exponential growth phase of the cells in 1.0M sodium chloride (NaCl) as salt stressor, and spotting on Yeast Peptone Dextrose Agar (YPDA) plates with incubation at 28oC for growth assessment and colony count. Transmission electron microscopy (TEM) was used to demonstrate characteristic ultrastructural hallmark features of apoptosis on Z. bisporus cells exposed to 1.0M NaCl at three different stress interval periods; 60, 90, and 120 minutes.

Results: Growth of Z. bisporus cells on the YPDA plates was observed after 16 hours incubation period. Comparing the growths, Z. bisporus tolerated salt concentration below 1.0M NaCl but no growth was observed at 1.0M NaCl concentration indicating 1.0M NaCl to be limiting concentration for Z. bisporus growth. TEM analyses showed that treatment of Z. bisporus with 1.0M NaCl resulted in nuclear and cytoplasmic condensation, membrane blabbing, cytoskeletal distortion, and formation of apoptotic bodies. However, on prolonged stress span (90 and 120 minutes), the fungal cells were able to osmoadapt and repaired the damaged cells, resulting in lowering of the apoptotic ratio.

Conclusion: These qualitative analyses contribute more insights regarding stress adaptive mechanisms in moderately halotolerant food spoilage yeast.

Key words: Apoptosis; salt stress; food spoilage yeast; TEM; cell survival