Enterotoxigenic Staphylococcus aureus in food - Culture; Molecular diagnosis (PCR) of the enterotoxins producers for A (SEA), B (SEB), C (SEC), D (SED), E (SEE), G (SEG), H (SEH), I (SEI) and J (SEJ) enterotoxins.

Información 03-09-2018.

            Staphylococcus aureus is a Gram-positive anaerobic facultative, round-shaped bacterium that is frequently found on the skin and nasal mucosa and in the respiratory tract of humans and animals. The introduction of S. aureus in processed foods is mainly due to the contamination provided by food handlers due to the application of improper handling practices, which would facilitate the presence and development of the pathogen in food. Staphylococcal poisoning occurs as a result of eating foods that contain sufficient amounts of enterotoxins produced by some strains of S. aureus.

            S. aureus produces several virulence factors, including enterotoxins (SE), exfoliative toxin (ET), toxic shock syndrome toxin (TSST-1), thermonuclease, hyaluronidase, hemolysins, lipases and coagulase. Staphylococcal enterotoxins (SE) have been classified into five classic serological types (SEA, SEB, SEC, SED and SEE). It has been reported that these classic SEs are responsible for 95% outbreaks of staphylococcal poisoning. Among enterotoxins, SEA is the most common cause of food poisoning related to staphylococci worldwide, followed by SED and SEB. Several SEs are designated as toxins similar to SE (SEl) toxins because they lack emetic properties or these have not yet been tested. Therefore, to date, a total of 21 SE toxins and SEl toxins have been reported. In addition to the five well-characterized classic staphylococcal enterotoxins SEA, SEB, SEC, SED and SEE, 16 new SE types have been reported (SEG, SEH, SEI, SER, SES, SET) and SEls (SElJ, SElK, SElL, SElM , SElN, SElO, SElP, SElQ, SElU and SEVL).

            The distribution of genes encoding enterotoxins in strains of S. aureus are highly variable, with some stable regions of the chromosome (e.g., group of enterotoxigenic genes-EGC) associated with particular lineages, and others carried to mobile genetic elements. Mobile genetic elements are segments of DNA that encode enzymes and other proteins that confer their ability to move horizontally between bacterial cells. In S. aureus, the main mobile genetic elements are bacteriophages, pathogenicity islands of S. aureus (SaPI), plasmids, transposons and chromosomes of staphylococcal cassettes (SCC). It has been reported that all of them carry SE genes, except for SCCs that generally carry antibiotic resistance genes. Most mobile genetic elements can move at high frequency between isolates of S. aureus, even during the course of infection.

            The detection of enterotoxins of Staphylococcus aureus in a food can be performed by immunodiffusion in gels, ELISA, ELFA, ...,). However, frequently the levels in which these enterotoxins are synthesized are below the limits of detection of the methodologies used (immunoassays in their different variants) or their expression is directly inhibited by the conditions of the medium in which the microorganism is found.

            The use of polymerase chain reaction (PCR) techniques has overcome these difficulties, being an effective, simple, reliable and fast to establish the presence of enterotoxin codifying genes in strains of S. aureus. In IVAMI, we carried out a multiple PCR to detect simultaneously the most known enterotoxin codifying genes of Staphylococcus aureus (the genes producing enterotoxins SEA to SElF), simultaneously, in isolated strains of food linked to outbreaks of food poisoning. Subsequently, the sequencing of the amplified gens can determine the type of enterotoxin present.

Tests carried out in IVAMI:

  • Enrichment culture for 24 hours.
  • Isolation of colonies in solid medium.
  • Molecular diagnosis of the enterotoxins of Staphylococcus aureus A (SEA), B (SEB), C (SEC), D (SED), E (SEE), G (SEG), H (SEH), I (SEI) and J (SEJ). 

Recommended sample:

  • Supposedly contaminated food. Food sample: 25 to 100 g. 

Conservation and shipment of the sample:

  • Refrigerated for less than 2 days and shipped inside white cork box with frozen pack. 

Delivery of results:

  • Enrichment culture, colony isolation and molecular detection: 72 to 96 hours (working days). 

Cost of the test:

  • Enrichment culture, isolation of colonies and molecular detection of the enterotoxin type of Staphylococcus aureus: Consult ivami@ivami.com