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Category: Applied and Industrial Microbiology; Food Microbiology
Staphylococcus aureus, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555815912/9781555814076_Chap22-1.gif /docserver/preview/fulltext/10.1128/9781555815912/9781555814076_Chap22-2.gifAbstract:
This chapter addresses primarily staphylococcal food poisoning (SFP); however, in regard to the staphylococcal enterotoxins (SEs), there is significant overlap in the natural histories of both diseases. Hence, toxic shock syndrome (TSS) is also discussed in the chapter in which this overlap is most relevant. The chapter discusses the nomenclature and evolution of the SE family of toxins. SEC expression is affected by glucose through at least two different mechanisms. First, the metabolism of glucose indirectly influences SEC production through agr by reducing pH. Glucose also reduces sec expression in agr mutant strains. This observation suggests the existence of a second glucose-dependent mechanism for reduction of SE expression, independent of agr and apparently not involving pH. The chapter talks about toxic dose and susceptible populations. The study of purified SEs has provided useful comparative information and has important research applications, but its direct relevance to SFP is uncertain because potential stabilization of unpurified SEs by food is an important consideration. The chapter discusses virulence factors and mechanisms of pathogenicity. Although the increased number of identified SEs and putative SEs is beginning to make immunological detection of SEs obsolete, several commercial reagents relying on this technique are still widely used. However, detection based on antigenicity is gradually being replaced by molecular techniques, especially multiplex PCR. Progress has been made toward understanding the molecular aspects relevant to SFP.
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Alignment of primary sequences of mature SEs and SEls according to the current literature. Also shown are the consensus sequences (at the bottom) and dashes to indicate gaps in the sequences made by alignment. Sequence alignment and output were conducted by using the CLUSTAL W program ( 139 ).
Interactions between APCs and T cells facilitated by conventional Ags and SAgs. Following processing by the APC, conventional Ags are presented to highly specific TCRs in association with the Ag-binding groove of the MHCII molecule. SAgs interact with MHCII molecules (without processing) outside the Ag-binding groove. The SAg-MHCII bimolecular complex binds to the TCR through specificity determined only by the variable region of the receptor α- or β-chain.
Tree representation demonstrating molecular relatedness of the currently known members of the SE family and TSST-1. This tree was created with the clustering feature of the PHYLIP program ( 37 ).
General characteristics of the agr locus in S. aureus. This physical map shows the relative locations of genes within the locus and other interacting regulatory genes and gene products (not drawn to scale).
Schematic diagrams of the SEC3 crystal structure illustrating major structural features. Numerical designations defining the locations of select residues and each α-helix and β-strand are shown within the two major domains. Also indicated are the N and C termini. The intramolecular disulfide linkage between Cys residues 93 and 110 (arrows) connects the disulfide loop to the β5-strand containing the conserved residues (see Fig. 22.7 ) potentially important for emesis. The zinc atom bound by SEC3 faces toward the back of the SEC3 molecule between domains 1 and 2 and is coordinated by D83, H118, and H122. In contrast, the high-affinity zinc-binding site in SEA is positioned on the opposite edge of domain 2. The conformational topology of domain 1 is the same as those of the OB domains of several other proteins described in the text.
(A) Schematic diagram of the SEA crystal structure. SEA has two MHCII-binding sites. Relatively low affinity MHCII binding occurs at a generic binding site which is conserved in most SEs. A high-affinity MHCII-binding site is located on the external surface of domain 2. This includes the high-affinity zinc-binding site, formed by His187, His225, and Asp227. The zinc ion mediates cross-linking of SEA with MHCII molecules and is crucial for maximal B- and T-cell activation. (B) Hypothetical model of MHCII-SEC-TCR complex based on the modeling predicted from the crystal structures of the SEC3-HLA-DR1 (low-affinity binding site) and SEB-Vβ complexes.
Comparison of cysteine loop and adjacent sequences for SEs and the analogous regions of the SEls. Evidence suggests that proper positioning of the critical downstream residues by a stable disulfide bond is required for emesis. Toxins designated as emetic are those reported as inducing emesis in the monkey feeding assay ( 7 , 93 , 101 – 103 , 138 ). SEI was reported to be weakly emetic ( 93 ).
General characteristics of selected Staphylococcus species a
Biochemical and functional properties of SEs
Prevalence of S. aureus in several foods