Staphylococcus Aureus Enterotoxins: A Review
Correspondence Address :
Dr. Aruna Bhatia, Professor, Dept. Of Biotechnology, Punjabi Univ., Patiala, Punjab, India. Phone Number: 0175-2283088, 09815913588. Fax Number: 0175-282881, 0175-283073. E-mail: firstname.lastname@example.org
Food-borne diseases are of major concern worldwide. To date, 250 different food-borne diseases have been described and bacteria are the causative agents of two thirds of food-borne disease outbreaks. Food poisoning is a term used to express any type of disease, illness or malaffect after consuming food. The most serious type of food poisoning is bacterial food poisoning, which may be due to bacterial infection or food intoxication. Among the predominant bacteria involved in these diseases, Staphylococcus aureus is a leading cause of gastroenteritis resulting from the consumption of a food in which enterotoxigenic staphylococci have grown and produced toxins. As these toxins are excreted from the organism, they are referred to as exotoxins; however, they normally exert their effects on the gastrointestinal tract and therefore are called enterotoxins. While not considered a highly lethal agent due to the low mortality associated with the illness, staphylococcal enterotoxins are considered a potential biological threat because of their stability at high temperatures (100Â°C for 1 h) and ability to incapacitate individuals for several days to two weeks. Here, a brief review on Staphylococcal enterotoxins is given.
Staphylococcus aureus, Food poisoning, Enterotoxins
Staphylococcus aureus is one of the most common causes of food borne infections in most of the countries of the world (1),(2) Especially in India, rate of infection is still higher because of warm and humid climate. Staphylococcus aureus is a facultative anaerobic gram-positive coccus, non-motile, catalase and coagulase positive of the micrococcaceae family. In 1884, Rosenbach described the two pigmented colony types of staphylococci and proposed the appropriate nomenclature: Staphylococcus aureus (yellow) and Staphylococcus albus (white). The latter species is now named Staphylococcus epidermidis. S. aureus colonizes mainly the nasal passages, but it may be found regularly in most other anatomical locales. S epidermidis is an inhabitant of the skin. In humans S. aureus is present on external sites, such as the nostrils (3),(4) or the skin (5) and also transiently in the oropharynx(6)and faeces(7). Up to 30-50% of the human populations are carriers. Staphylococcus aureus is able to grow in a wide range of temperatures (7Â° to 48.5Â°C with an optimum of 30 to 37Â°C), pH (4.2 to 9.3, with an optimum of 7 to 7.5) and sodium chloride concentrations up to 15% NaCl (8). The staphylococcal cell wall is resistant to lysozyme and sensitive to lysostaphin, which specifically cleaves the pentaglycin bridges of Staphylococcus spp. The various isolates of S. aureus show different characteristics features in their susceptibility/resistance towards antibiotics. Some S. aureus strains are able to produce staphylococcal enterotoxins (SEs) and are the causative agents of staphylococcal food poisonings. Staphylococcus aureus is able to grow in a wide range of temperatures (7Â° to 48.5Â°C with an optimum of 30 to 37Â°C), pH (4.2 to 9.3, with an optimum of 7 to 7.5) and sodium chloride concentrations up to 15% NaCl (8). These characteristics enable S. aureus to grow in a wide variety of foods. Staphylococcus aureus strains can be classified into biotypes according to their human or animal origin like human, non-hemolytic human, avian, bovine, ovine, and nonspecific(9).
Pathogenesis of S. aureus infections
The pathogenicity of Staphylococcus aureus is due to the toxins, invasiveness and antibiotic resistance. S.aureus is major cause of nosocomial and community acquired infections (10),(11),(12). It is present as a normal flora of human beings and colonizes skin, but may become pathogenic and result in minor skin infections and abscesses, to life-threatening diseases such as pneumonia, meningitis, endocarditis, toxic shock syndrome (TSS), septicemia, mastitis, phlebitis, urinary tract infections, osteomyelitis and endocarditis.
Adherence to host cell proteins
S. aureus cells attach thought surface proteins to host cellâ€™s surface proteins e.g., laminin and fibronectin. In addition, most strains express a fibrin/fibrinogen binding protein (clumping factor) which promotes attachment to blood clots and traumatized tissue. Most strains of S. aureus express both fibronectin, fibrinogen-binding proteins and collagen binding proteins on surface.
The invasion of host tissues by staphylococci apparently involves the production of a huge array of extra cellular, proteins, toxins, enzymes, proteins having affinity for Igs (13).
Alpha, Beta,Delta and leukocidins are the common toxins found in S.aureus. Alpha toxin most potent membrane-damaging toxin of S. aureus is a monomer and binds to the membrane of susceptible cells through receptors. Subunits then oligomerize to form heptameric rings with a central pore through whic
Staphylococcal food poisoining is of major concern in public health programs worldwide. Predictive models for S. aureus growth and SE production would be powerful tools for microbial risk assessment in food industries. However, many factors affect S. aureus growth and SE production in
foodstuffs and further studies are still necessary in order to develop such predictive tools. Proper storage of food is an important part of reducing the risk of food poisoning. Foods must be stored in the refrigerator and eaten within a short period of time, other foods, such as flour, pulses, canned foods and many others last much longer and can be stored at room temperature. Dried foods too have limited shelf life. The microbes on our food that can cause poisoning are usually temperature controlled by heating (cooking) and/or chilling (refrigerating) the food. While reusing the refrigerated food, one should heat it properly if need to be restored or eaten after sometime. Because simply warming at temperature not very high (20 o C- 45 o C) activates the spores which start reproducing and reuse of the same food gets contaminated with active bacteria. Not only this, the quantity of bacteria also rises due to bacterial multiplication. Though bacteria are limited to the foods but given the chance they can easily spread around the kitchen â€“ via our hands, chopping boards, cloths, knives and other utensils and even through fomites also. They may cross-contaminate other foods â€“ especially cooked and ready-to-eat foods. Good kitchen and personal hygiene practices as well cooked foods are important to help control the consumption of contaminated foods and hence food poisoning. Considerable research effort is still required for better understanding of the interactions between S. aureus and the food matrix and of the mechanisms of SE production in foodstuffs. Research is also needed for the identification of new SEs and of new enterotoxigenic staphylococci. Much effort is being applied towards the development of new and more sensitive methods for SE detection in foodstuffs. Taken together, these studies should lead to better control and a subsequent reduction of staphylococcal food poisoning outbreaks.
Conflict of Interest: None declared
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