Staphylococcus Enterotoxin B

Introduction

Fig. 1: Grape-like cluster of Staphylococcal aureus

Staphylococcal Enterotoxin B (SEB) is an exotoxin produced by the bacterium Staphylococcus aureus (Pinchuk et al. 2010). These bacteria (Fig. 1), which generally measure between 0.8 and 1.2 μm, are non-motile, gram-positive, non spore-forming and can be either aerobic or anaerobic. They are normally present on the skin and in the mucous membranes of both humans and animals, but can also be found in the environment and in (contaminated) food.

Besides SEB, S. aureus produces other serotypes (A, C-G), as well as TSST-1, the toxic shock syndrome toxin. Given that a number of new toxin genes have been profiled in recent years, it is more than likely that there are a many more serotypes than previously thought. S. aureus is the most pathogenic of all staphylococci for humans. It causes two types of illness:

The bacterium itself causes purulent, localised surface infections, such as boils, abscesses and infected lacerations. At the same time it can also lead to major systemic infections like blood poisoning, osteomyelitis, pneumonia, internal abscesses and endocarditis). S. aureus is also one of the most common causes of hospital-acquired infections. Antibiotic-resistant – or “multiresistant” – strains of staphylococcus can be found in almost all hospitals worldwide.

Illnesses transmitted by the toxin are caused by the ingestion of enterotoxins (food poisoning), chiefly as a result of food contaminated with S. aureus. SEB is also toxic by inhalation, though such cases are rarely fatal. The risk of death is highest among newborns and those whose health is already compromised.

Structure and properties of SEB

Fig. 2: SEB co-crystallised with sialyllactose (N- to C-terminus blue to red ribbon; modified from 1SE3.pdb)

The superantigen Staphylococcal Enterotoxin B is a protein with a mass of 28.4 kDa. It is considerably more stable in non-physiological conditions than other protein-based toxins such as ricin and botulinum neurotoxins. For example, SEB can survive in boiling water for several minutes. The superantigen SEB deregulates the immune system by stimulating human T-cells, which in turn causes an over-production of cytokines. This seems to delay recognition of the bacteria (Staphylococcus aureus), thus enabling the effective proliferation of the toxin. It is the massive over-production of cytokines which is responsible for toxic shock syndrome (TSS). In vitro studies have found that under certain circumstances these toxins can inhibit the humoral immune response, thereby preventing the production of antibodies.

Toxicity of SEB

Fig 3: As a bifunctional molecule, superantigens bond simultaneously with the receptor sites of Class II MHC molecules in the cell containing antigens and with the T-cell receptors (TCR). Source: http://bioinfo.bact.wisc.edu/ themicrobialworld/staph.html

Three to 12 hours after inhaling the toxin, the onset of flu-like symptoms occurs, such as high fever, dry tickly cough, muscle pain and headaches, possibly accompanied by diarrhoea and vomiting. The inhalation of high concentrations of the toxin is associated with additional symptoms, including breathing difficulties, nausea, weakness and even pulmonary oedema. Such cases are rarely fatal. Shortly after ingesting the toxin (2 to 4 hours), hypersalivation and nausea occur, followed by bouts of vomiting, stomach cramps and diarrhoea. As a general rule, ingestion of the toxin does not cause breathing difficulties or fever. However, high doses can lead to septic shock and death. There is no effective therapy against SEB. Treatment is merely symptomatic. The oral ED₅₀ value (effective dose) of SEB is 0.3 µg/kg (Raj and Bergdoll, 1969). According to the scientific literature, sensitivity to the toxin can vary considerable from one individual to another. The lethal quantity of SEB for monkeys is estimated to be 20 µg/kg i.v. (Gill, 1982).

Detection

The Staphylococcal Enterotoxin B (SEB) protein can be identified quite easily using immunological methods. Highly-sensitive rapid detection systems - or lateral flow assays - are readily available on the market that can identify SEB (limit of detection = < 10 ppb or < 10 ng/ml) in the space of 20 minutes (manufacturer: Tetracore, USA). These types of assay are designed primarily to handle aqueous solutions and swab samples. The ELISA kit, which is also readily available on the market (manufacturer: R-Biopharm, Germany), can screen for the presence of a range of Staphylococcal Enterotoxin serotypes in food. Besides immunological methods such as the ELISA Sandwich (microtiter plate with 96 wells) or multiplex systems (X-Map Luminex Technology, BioPlex 200 by Bio-Rad; Pauly et al., 2009), traditional laboratory analyses also involve the use of other protocols like mass spectrometry (peptide map; Kull et al., 2010).

SEB as a biological warfare agent

In military circles SEB is better known as an "incapacitating agent". Tiny amounts can cause acute poisoning, which completely incapacitates the victim within a short space of time. However, such cases are seldom fatal. The use of this agent can rapidly overwhelm the medical infrastructure if it causes large-scale incapacitation (Ahanotu et al., 2006).

References

Pinchuk, I., Beswick, E., Reyes, V. Staphylococcal Enterotoxins. Toxins 2010, 2(8), 2177-2197.

Raj, H.D. and Bergdoll, M.S., Effect of Enterotoxin B on Human Volunteers; Journal of Bacteriology, May 1969, 833-834

Gill, M., Bacterial Toxins: a Table of Lethal Amounts; Microbiological Reviews, Mar. 1982, 86-94

Pauly, D., Kirchner, S., Stoermann, B., Schreiber, T., Kaulfuss, S., Schade, R., Zbinden, R., Avondet, M.A., Dorner, M.B., Dorner, B.G. (2009). Simultaneous quantification of five bacterial and plant toxins from complex matrices using a multiplexed fluorescent magnetic suspension assay. Analyst 134, 2028–2039.

Kull, S., Kirchner, S., Pauly, D., Stoermann, B., Dorner, M.B., Lasch, P., Naumann, D., Dorner, B.G. (2010). Multiplex detection of microbial and plant toxins by immunoaffinity enrichment and matrix-assisted laser desorption/ionization mass spectrometry. Anal. Chem. 82, 2916-24.

Ahanotu, E., Alvelo-Ceron, D., Ravita, T. and Gaunt, E., Staphylococcal Enterotoxin B as a Biological Weapon: Recognition, Management, and Surveillance of Staphylococcal Enterotoxin; Applied Biosafety, 11(3) 120-126