Culture conditions shape metabolism and exopolysaccharide production in Piscirickettsia salmonis: implications for vaccine development

Abstract Piscirickettsia salmonis is the primary bacterial pathogen affecting Chilean salmon farming, resulting in annual economic losses of approximately US$700 million. The limited effectiveness of antibiotics and current vaccines underscores the need to enhance upstream cultivation strategies for producing biomass suitable for vaccine production. In this study, two shake flask designs—baffled (BF) and standard non-baffled (NF)—were compared to evaluate how differences in mass and momentum transfer impact the metabolic activity and physiological behavior of P. salmonis under axenic, submerged batch conditions. Despite a 2.5-fold increase in oxygen transfer in BF flasks, the results indicated that this configuration had a negative impact on bacterial performance. Specific growth rates were reduced by 22–39%, and volumetric productivity decreased by 23% compared to NF. Glutamate was fully consumed in NF but remained partially unused in BF, indicating a reduced uptake under shear or oxidative stress. Respiratory activity also differed: NF cultures exhibited distinct peaks in oxygen transfer rate during growth, while BF cultures showed flatter, more constant profiles. Importantly, exopolysaccharide (EPS) production, a trait associated with virulence and immunogenicity, was over fivefold higher in NF (1.48 g/L) than in BF (0.27 g/L). These findings demonstrate that the physical conditions imposed by flask design have a significant influence on P. salmonis growth, respiration, and EPS output. Adjusting cultivation conditions at this early stage can enhance the consistency and quality of biomass used in bacterin-based vaccines. This approach provides a practical tool to improve upstream standardization and supports the development of more effective control strategies for piscirickettsiosis in the aquaculture industry.