Pichia Pastoris exhibits high viability and a low maintenance energy requirement at near-zero specific growth rates

The yeast Pichia pastoris is a widely used host for recombinant protein production. Understanding its physiology at extremely low growth rates is a first step in the direction of decoupling product formation from cellular growth and therefore of biotechnological relevance. Retentostat cultivation is an excellent tool for studying microbes at extremely low specific growth rates but has so far not been implemented for P. pastoris. Retentostat feeding regimes were based on the maintenance energy requirement (mS) and maximum biomass yield on glucose (Y_X/S ^max) estimated from steady-state glucose-limited chemostat cultures. Aerobic retentostat cultivation enabled reproducible, smooth transitions from a specific growth rate (mu;) of 0.025 h^-1 to near-zero specific growth rates (mu; -1). At these near-zero specific growth rates, viability remained at least 97%. The value of m_S at near-zero growth rates was 3.10± 1 mg glucose per g biomass and h, which was 3-fold lower than themS estimated from fastergrowing chemostat cultures. This difference indicated that P. pastoris reduces its maintenance energy requirement at extremely low , a phenomenon not previously observed in eukaryotes. Intracellular levels of glycogen and trehalose increased, while progressively declined during retentostat cultivation. Transcriptional reprogramming toward zero growth included the upregulation of many transcription factors as well as stress-related genes and the downregulation of cell cycle genes. This study underlines the relevance of comparative analysis of maintenance energy metabolism, which has an important impact on large-scale industrial processes.