Stability and emetic activity of enterotoxin like X (SElX) with high carrier rate of food poisoning Staphylococcus aureus

In order to analyze and clarify the thermal stability of food poisoning Staphylococcus aureus (S. aureus) enterotoxin-like X (SElX) and the biological characteristics of digestive enzymes, and to evaluate the risk of S. aureus carrying selx gene in food poisoning, the selx gene carrying rates of 165 strains isolated from 95 food poisoning events from 2006 to 2019 were first statistically analyzed. Subsequently, the purified recombinant SElX protein was digested and heated, and the superantigen activity was verified with mouse spleen cells and peripheral blood mononuclear cells of kittens. At the same time, the emetic activity and toxicity of SElX were evaluated using the kitten vomiting animal model, mice toxin model and in vitro cell models. The results showed the selx gene carrying rate of 165 food poisoning S. aureus strains was 90.30 %. SElX had significant resistance to heat treatment and pepsin digestion (pH = 4.0 and pH = 4.5), and had good superantigen activity and emetic activity. However, there is no significant lethal effect on mice and no significant toxicity to cells. Importantly, we found that SElX had an inhibitory effect on acidic mucus of goblet cells in various segments of the small intestine. The present study investigated the stability of SElX, and confirmed the emetic activity of SElX by establishing a kitten vomiting model for the first time, suggesting that SElX is a high risk toxin of food poisoning, which will provide new ideas for the prevention and control of S. aureus food poisoning.

[Cloning and expression of recombinant truncated SElX protein and evaluation on the related emetic activities]

Objective: To analyze the amino acid polymorphism of truncated Staphylococcal enterotoxin-like toxin X (tSElX), and to evaluate its related emetic activities. Methods: Sequence of tselx was compared with both the genome sequence of 145 CC398 strains completed in our research group and the NCBI database. Primers were designed to amplify the target gene of tselx, and the fragment was recombined into pMD18-T vector and sequenced. PCR product was digested with BamHⅠ and EcoRⅠ, and constructed into plasmid pGEX-6P-1 and pET-28a (+). After recombinant plasmid was identified, the protein expression was induced by IPTG. Proteins expressed in the form of inclusion bodies were denatured and renatured, then purified by affinity chromatography and ultrafiltration. Purified tSElX protein was then fed to common marmosets with the dose of 250 μg/kg to observe the vomiting reaction. Results: tselx gene was present in 145 strains of CC398 strains from the different origins (patients, healthy people and animals) in China. Homology of the amino acid sequence of the protein from the Chinese strains appeared 100.0%, while the homology with the four American strains were 97.8%(1) and 98.9%(3), respectively. Through two sets of expression systems and different induction conditions, tSElX was expressed in the form of inclusion bodies. The high purity soluble recombinant tSElX was thus obtained by denaturated and renaturated processes. At the dose of 250 μg/kg, tSElX protein did not cause vomiting in common marmosets. Conclusions: Results of this study showed that the amino acid sequence of tSElX was highly conserved and was universally present in a particular clone group. We obtained soluble recombinant tSElX protein with high purity. We also noticed that tSElX did not have the animal emetic activity at a dose of 250 μg/kg.