Optimized Fermentation Boosts Streptococcus suis Biomass for Inactivated Vaccine Testing

Researchers have developed a scalable fermentation process for growing Streptococcus suis serotype 31, a clinically important swine pathogen, and used it to produce an inactivated vaccine candidate that showed protective activity in a mouse challenge model. The study, published in BMC Biotechnology, addresses a persistent problem in veterinary vaccine manufacturing: conventional culture methods often yield insufficient bacterial biomass, while aggressive high-density fermentation can perturb the physiological features that determine vaccine quality.

The team used a Response Surface Methodology (RSM) guided strategy that combined single-factor screening with Box-Behnken modeling to optimize the culture microenvironment. This statistical approach identified glucose, organic nitrogen and newborn bovine serum as the dominant growth variables affecting SS31 cultivation.

The optimized formulation contained 2.19 g/L glucose, 43.8 g/L organic nitrogen and 5.89% serum. When the process was scaled to a 14-L bioreactor with residual sugar controlled at 1–3 g/L and pH maintained at 7.2, it achieved 1.51 × 10¹⁰ CFU/mL, approximately seven-fold higher than shake-flask cultivation.

The researchers also examined how the culture medium influenced the bacterium’s virulence-associated phenotype. The LD₅₀ estimate was 2.50 × 10⁶ CFU/mouse for SS31 cultured in the optimized medium, compared with 2.14 × 10⁷ CFU/mouse for SS31 cultured in Martin medium, indicating a difference in virulence-associated phenotype under the experimental conditions. The authors note that these estimates do not demonstrate the conservation or enhancement of vaccine antigens.

Four vaccine formulations were tested in the study. The aluminum-adjuvanted vaccine containing 3 × 10⁹ pre-inactivation CFU equivalents/mL yielded the highest observed survival, protecting 10 of 10 mice after challenge with 2 LD₅₀ of the pathogen.

Organ bacterial-load and histopathological findings were evaluated descriptively, providing supporting observations alongside the survival data rather than formal statistical comparisons.

According to the authors, the findings demonstrate the technical feasibility of the SS31 cultivation process at the 14-L scale and support further evaluation of the resulting inactivated vaccine against S. suis. The work represents an early-stage proof of process scalability rather than a validated commercial vaccine protocol.

The study is published as an open-access article in BMC Biotechnology, with the version shared carrying a permanent DOI and subject to further edits before the final Version of Record.

Subject of Research: Fermentation optimization and inactivated vaccine development for Streptococcus suis serotype 31

Article Title: High-density fermentation of Streptococcus suis serotype 31 for inactivated vaccine production

Article References: Liu, L., Zhou, T., Cui, J., Han, S., Hu, J., Gao, J., & Liu, Z. (2026). High-density fermentation of Streptococcus suis serotype 31 for inactivated vaccine production. BMC Biotechnology. https://doi.org/10.1186/s12896-026-01241-y

Image Credits: AI Generated

DOI: 10.1186/s12896-026-01241-y

Keywords: Streptococcus suis, serotype 31, high-density fermentation, inactivated vaccine, Response Surface Methodology, bioreactor, biomass, LD50, aluminum adjuvant, swine pathogen, vaccine development, Box-Behnken design