Oral immunization with Lactococcus lactis secreting attenuated recombinant staphylococcal enterotoxin B induces a protective immune response in a murine model

Background: Staphylococcus aureus is unrestrictedly found in humans and in animal species that maintain thermal homeostasis. Inadequate cleaning of processing equipment or inappropriate handling can contaminate processed food and cause severe food poisoning. Staphylococcal enterotoxin B (SEB), a potent superantigenic exotoxin, is produced by 50% of clinical isolates of S. aureus and is associated with massive food poisoning and with the induction of toxic shock syndrome. Results: A gene sequence encoding a recombinant SEB (rSEB), devoid of superantigenic activity, was successfully cloned and expressed in a cytoplasmic or a secreted form in the food-grade lactic acid bacterium Lactococcus lactis. The recombinant protein detected in the cytoplasm or in the culture medium exhibited the expected molecular mass and was recognized by a SEB-polyclonal antibody. Oral immunization with the recombinant L. lactis strains induced a protective immune response in a murine model of S. aureus infection. Immunized mice survived intraperitoneal challenge with an S. aureus SEB-producer strain. Counts of S. aureus in the spleen of rSEB-immunized mice were significantly reduced. The rSEB-immunized mice showed significant titers of anti-SEB IgA and IgG in stools and serum, respectively. Both recombinant L. lactis strains were able to elicit cellular or systemic immune responses in mice, with no significant difference if rSEB was produced in its cytoplasmic or secreted form. However, recombinant L. lactis expressing the cytoplasmic rSEB increased the survival rate of the challenged mice by 43%. Conclusions: These findings show the vaccine efficacy of L. lactis carrying an attenuated SEB, in a murine model, following lethal S. aureus challenge.