Identification and characterization of staphylococcal enterotoxin types G and I from Staphylococcus aureus

Staphylococcal enterotoxins are exotoxins produced by Staphylococcus aureus that possess emetic and superantigenic properties. Prior to this research there were six characterized enterotoxins, staphylococcal enterotoxin types A to E and H (referred to as SEA to SEE and SEH). Two new staphylococcal enterotoxin genes have been identified and designated seg and sei (staphylococcal enterotoxin types G and I, respectively). seg and sei consist of 777 and 729 nucleotides, respectively, encoding precursor proteins of 258 (SEG) and 242 (SEI) deduced amino acids. SEG and SEI have typical bacterial signal sequences that are cleaved to form toxins with 233 (SEG) and 218 (SEI, predicted) amino acids, corresponding to mature proteins of 27,043 Da (SEG) and 24,928 Da (SEI). Biological activities for SEG and SEI were determined with recombinant S. aureus strains. SEG and SEI elicited emetic responses in rhesus monkeys upon nasogastric administration and stimulated murine T-cell proliferation with the concomitant production of interleukin 2 (IL-2) and gamma interferon (IFN-gamma), as measured by cytokine enzyme-linked immunoassays. SEG and SEI are related to other enterotoxins of S. aureus and to streptococcal pyrogenic exotoxin A (SpeA) and streptococcal superantigen (SSA) of Streptococcus pyogenes. Phylogenetic analysis and comparisons of amino acid and nucleotide sequence identities were performed on related staphylococcal and streptococcal protein toxins to group SEG and SEI among the characterized toxins. SEG is most similar to SpeA, SEB, SEC, and SSA (38 to 42% amino acid identity), while SEI is most similar to SEA, SEE, and SED (26 to 28% amino acid identity). Polyclonal antiserum was generated against purified histidine-tagged SEG and SEI (HisSEG and HisSEI). Immunoblot analysis of the enterotoxins, toxic-shock syndrome toxin 1, and SpeA with antiserum prepared against HisSEG and HisSEI revealed that SEG shares some epitopes with SEC1 while SEI does not.