Identification of a bacteriophage containing a silent staphylococcal variant enterotoxin gene (sezA+)

Molecular Characteristics and Pathogenicity of Staphylococcus aureus Exotoxins

Staphylococcus aureus stands as one of the most pervasive pathogens given its morbidity and mortality worldwide due to its roles as an infectious agent that causes a wide variety of diseases ranging from moderately severe skin infections to fatal pneumonia and sepsis. S. aureus produces a variety of exotoxins that serve as important virulence factors in S. aureus-related infectious diseases and food poisoning in both humans and animals. For example, staphylococcal enterotoxins (SEs) produced by S. aureus induce staphylococcal foodborne poisoning; toxic shock syndrome toxin-1 (TSST-1), as a typical superantigen, induces toxic shock syndrome; hemolysins induce cell damage in erythrocytes and leukocytes; and exfoliative toxin induces staphylococcal skin scalded syndrome. Recently, Panton-Valentine leucocidin, a cytotoxin produced by community-associated methicillin-resistant S. aureus (CA-MRSA), has been reported, and new types of SEs and staphylococcal enterotoxin-like toxins (SEls) were discovered and reported successively. This review addresses the progress of and novel insights into the molecular structure, biological activities, and pathogenicity of both the classic and the newly identified exotoxins produced by S. aureus.

Prophage-Encoded Staphylococcal Enterotoxin A: Regulation of Production in Staphylococcus aureus Strains Representing Different Sea Regions

The present study investigates the nature of the link between the staphylococcal enterotoxin A (SEA) gene and the lifecycle of Siphoviridae bacteriophages, including the origin of strain variation regarding SEA production after prophage induction. Five strains representing three different genetic lines of the sea region were studied under optimal and prophage-induced growth conditions and the Siphoviridae lifecycle was followed through the phage replicative form copies and transcripts of the lysogenic repressor, cro. The role of SOS response on prophage induction was addressed through recA transcription in a recA-disruption mutant. Prophage induction was found to increase the abundance of the phage replicative form, the sea gene copies and transcripts and enhance SEA production. Sequence analysis of the sea regions revealed that observed strain variances were related to strain capacity for prophage induction, rather than sequence differences in the sea region. The impact of SOS response activation on the phage lifecycle was demonstrated by the absence of phage replicative form copies in the recA-disruption mutant after prophage induction. From this study it emerges that all aspects of SEA-producing strain, the Siphoviridae phage and the food environment must be considered when evaluating SEA-related hazards.

Identification of Staphylococcus aureus enterotoxins A and B genes by PCR-ELISA

We developed PCR-enzyme linked immunosorbent (ELISA) assays to detect Staphylococcus aureus enterotoxins A and B genes. The assays use internal biotin-labelled oligonucleotides as capture probes for immobilizing and subsequently detecting target sequences on microtiter plates. The detection limits of the PCR-ELISAs were approximately 250 gene copies, versus 2500 gene copies by agarose gel analysis. The sensitivity of the assays, as determined from a reference panel of 46 coded samples that included DNA purified from 31 different bacterial species and strains, SEA and SEB plasmid controls, and no-template controls was 100%. No cross-reactivity was observed with DNA from non-staphylococcal species. Using 27 clinical isolates of S. aureus, the SEA PCR-ELISA identified the enterotoxin A (sea) gene in 26 samples, and the SEB PCR-ELISA identified the enterotoxin B (seb) gene in all 27 samples. Compared with conventional antigen capture ELISAs for SEA and SEB toxins, the PCR-ELISAs showed overall superior detection limits. The sensitivity and specificity levels of the SEA PCR-ELISA and the SEA toxin ELISA were comparable within their respective detection thresholds, but the sensitivity and specificity of the SEB PCR-ELISA was much greater than that of SEB toxin ELISA.

Phage-associated differences in staphylococcal enterotoxin A gene (sea) expression correlate with sea allele class

Staphylococcus aureus strains which produced either high or low levels of staphylococcal enterotoxin A (SEA) with a minimal eightfold difference between the two groups were identified. For FRI100 and FRI281A (prototypes for each group), strain differences in the expression of the SEA-encoding gene (sea) were found to occur at the level of sea mRNA concentration, and part of the difference in expression was associated with the sea-containing phages. Southern blot analysis revealed that this phage-associated difference was not due to differences in the copy number of sea. Nucleotide sequence analysis of sea from FRI281A revealed a new allele of sea, with the majority of the sequence differences occurring in the upstream promoter region. Although a strict correlation was observed between the level of SEA production and sea allele class for several strains, the sequence differences observed in the upstream region were not sufficient in themselves to alter the expression level of sea.

Potential problems in the use of oligonucleotide probes for staphylococcal enterotoxin genes

Oligonucleotide probes unique to the five major enterotoxin genes of Staphylococcus aureus were synthesized and used to detect DNA sequences homologous to these genes in 27 non-clinical isolates of Staph. aureus isolated from nasal swabs of 74 healthy human volunteers. Genomic DNA from all 27 isolates reacted with at least one of the probes. In a phenotypic assay for toxin production by a reverse passive latex agglutination test however, only 15 of the 27 isolates produced enterotoxin in culture. The results raise the possibility that a number of Staph. aureus isolates harbour DNA sequences that are apparently silent or mutant copies of the enterotoxin genes. This complicates the identification of enterotoxin producers by tests which depend on oligonucleotide or DNA hybridization.

Synthetic DNA probes for detection of genes for enterotoxins A, B, C, D, E and for TSST-1 in staphylococcal strains

A dot blot hybridization technique with oligonucleotide probes was developed for the specific detection of the TSST-1 gene and the staphylococcal enterotoxin (SE) genes A, B, C, D and E. For each toxin gene a probe sequence was chosen from the previously determined sequence. A total of 145 staphylococcal strains (133 Staphylococcus aureus and 12 coagulase-negative staphylococci (CNS) were studied by this genotypic method and by two phenotypic assays (gel immunodiffusion and ELISA). An excellent correlation (96%) was observed between the genotypic and phenotypic assays. DNA from two CNS strains hybridized with a probe without detection of the corresponding toxin (SEB for one strain and SEC for the other strain). One Staph. aureus strain was shown to be an SEC producer, but was not detected by the corresponding probe. Gene probe and immunological assays seem to be complementary methods for studies of staphylococcal strains producing (or potentially producing) TSST-1 or enterotoxins.

Cryptic alpha-toxin gene in toxic shock syndrome and septicaemia strains of Staphylococcus aureus

The majority of clinical isolates of Staphylococcus aureus that produce toxic shock syndrome toxin-1 (TSST-1) fail to express alpha-toxin, despite having a copy of the hla gene in the chromosome. The hla gene was cloned from an Hla- TSST-1+ strain, Todd 555, which had been isolated from a case of toxic shock syndrome in the USA. Of the 630 bases of the Todd 555 gene sequenced, 46 differed from the hla gene sequence of strain Wood 46. The defect in alpha-toxin expression was shown to be due to a nonsense mutation which converted a CAG glutamine codon in the equivalent position in the functional Wood 46 sequence to a TAG stop codon. The same mutation was present in the hla gene cloned from a human septicaemia strain (V37) isolated in Dublin. The nonsense mutation of Todd 555 was suppressed by the supE44 mutation in Escherichia coli resulting in haemolytic activity in cell lysates. Hybrid hla genes were formed by splicing fragments of hla from Todd 555 and Wood 46. Expression of one such chimaeric hla gene in S. aureus demonstrated that the Todd 555 hla gene has a functional agr-regulated promoter. The silent hla gene may be a cryptic gene in S. aureus.