(http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Modeling Neisseria meningitidis metabolism

A genome-scale flux model for primary metabolism of Neisseria meningitidis was constructed; a minimal medium for growth of N. meningitidis was designed using the model and tested successfully in batch and chemostat cultures.

Outer membrane vesicles (OMV) contain immunogenic proteins and contribute to in vivo survival and virulence of bacterial pathogens. The first OMV vaccines successfully stopped Neisseria meningitidis serogroup B outbreaks but required detergent-extraction for endotoxin removal. Current vaccines use attenuated endotoxin, to preserve immunological properties and allow a detergent-free process. The preferred process is based on spontaneously released OMV (sOMV), which are most similar to in vivo vesicles and easier to purify. The release mechanism however is poorly understood resulting in low yield. This study with N. meningitidis demonstrates that an external stimulus, cysteine depletion, can trigger growth arrest and sOMV release in sufficient quantities for vaccine production (±1500 human doses per liter cultivation). Transcriptome analysis suggests that cysteine depletion impairs iron-sulfur protein assembly and causes oxidative stress. Involvement of oxidative stress is confirmed by showing that addition of reactive oxygen species during cysteine-rich growth also triggers vesiculation. The sOMV in this study are similar to vesicles from natural infection, therefore cysteine-dependent vesiculation is likely to be relevant for the in vivo pathogenesis of N. meningitidis.

At present, only vaccines containing outer membrane vesicles (OMV) have successfully stopped Neisseria meningitidis serogroup B epidemics. These vaccines however require detergent-extraction to remove endotoxin, which changes immunogenicity and causes production difficulties. To investigate this in more detail, the protein content of detergent-extracted OMV is compared with two detergent-free alternatives. A novel proteomics strategy has been developed that allows quantitative analysis of many biological replicates despite inherent multiplex restrictions of dimethyl labeling. This enables robust statistical analysis of relative protein abundance. The comparison with detergent-extracted OMV reveales that detergent-free OMV are enriched with membrane (lipo)proteins and contain less cytoplasmic proteins due to a milder purification process. These distinct protein profiles are substantiated with serum blot proteomics, confirming enrichment with immunogenic proteins in both detergent-free alternatives. Therefore, the immunogenic protein content of OMV vaccines depends at least partially on the purification process. This study demonstrates that detergent-free OMV have a preferred composition.

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