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Bacteria may be classified as either gram negative or gram positive, both of which are a concern in food-borne illness. Gram positive bacteria involved in food-borne illness typically produce a number of toxins, such as the bacteria Clostridium perfringens, Bacillus cereus, Staphylococcus Aureus and Clostridium Botulinum. Listeria monocytogenes is another gram positive species that differs in the fact that it replicates within the cytosol of pathogens that it invades and does not produce toxins. Gram negative bacteria are significantly different and more complex than gram- positive. Types of gram-negative bacteria include Campylobacter jejuni, Salmonella and E.coli (Jay, M., et.al., 2005). Gram-positive Staphylococcus aureus and Clostridium botulinum will now be further described.
Clostridium Botulinum is a large, anaerobic, motile, heat-resistant-endospore forming rod-shaped bacterium that produces one of the most potent natural toxins known to affect humans (Todar, K. 2008). "One milligram of pure botulinum toxin is enough to kill more than 1 million guinea pigs" (Tortora, G.J. et. al., 2007).There are 3 types of toxin known to affect humans; type A which is the most virulent, and types B and E. Each type being produced by different proteolytic and non-proteolytic strains of the organism (Tortora, G.J. et. al., 2007).
Due to the soil and water origin of C.botulinum, its spores may contaminate raw foods during harvest or slaughter. Therefore if contaminated foods are improperly processed and cooked at temperatures below that of a pressure cooker (Evans, A. S. et.al., 1982), clostridial spores will survive and produce their toxin in food. These toxins can be inactived if heated at 80ËšC for 10minutes (Brock, T. D. et.al., 1991). Quite often signs of C. botulinum affected food containers are bulging or an off odour due to gas formation as proteins are broken down into amines (Todar, K. 2008).
Botulism is an uncommon form of food poisoning that occurs following consumption of food containing botulinum toxin. Most reported cases of botulism are in home canned-foods which encourage growth through the sealed anaerobic environment (Evans, A. S. et.al., 1982). C.botulinum cannot generally survive in foods that are low in acid (<4.6) (Kendall, P. 2006). Once consumed, the botulinum toxin is absorbed in the upper GI tract and travels in the blood stream to peripheral neuromuscular synapses where it binds to presynaptic terminals. It's binding blocks acetycholine release, which is a neurotransmitter required for muscle contraction. Symptoms typically begin 18-24 hours after ingestion and include weakness, dizziness, dryness of the mouth and nausea. Neurologic features soon develop as a consequence of neurotransmitter blockage. These include blurred vision, difficulty swallowing and speaking, and a progressive flaccid paralysis that results in death from cardiac and/or respiratory failure in 1/3 of patients (Ivana, S. et.al., 2009).
Infant botulism is another form of botulism that occurs in infants below the age of one. It occurs because their normal gut flora are not established enough to compete with the ingestion of clostridial spores, allowing the spores to produce the fatal toxin. Most reported cases have been due to honey consumption (Brock, T. D. et.al., 1991).
Diagnosis of botulism is through inoculation of mice with samples of patient stool, serum or vomitus, and the determination of the toxin type by inoculation of their respective antitoxins. Results are based on whether the mouse dies or not (Tortora, G.J. et. al., 2007).Treatment of botulism relies on immediate intravenous administration of antitoxins to patients, they are used to neutralise circulating unbound toxins. Often respiratory assistance using a ventilator is needed. Non-immune conferring recovery ensues if nerve endings regenerate (Evans, A. S. et.al., 1982).
Staphylococcus aureus, unlike Clostridium Botulinum, is a non-spore-forming, non-motile bacterium and under the microscope can be viewed as grape-like clusters of cocci (Kayser, F. H., et al., 2005). It is known to cause many infections and toxinoses in humans (todar) and is the causative agent for one toxinose in particular, staphylococcal food poisoning (Loir, Y.L., et al., 2003). Staphylococcal food poisoning is initiated by the ingestion of heat-resistant enterotoxins that are produced by some strains of S.aureus. These enterotoxins have super antigen activity and are categorised into numerous serologically distinct types. (Kayser, F. H., et al., 2005). The superantigen activity of S.aureus enterotoxin functions by targeting the interaction between T-cells and antigen presenting cells so that it can escape detection by the immune system and thus invade the host (Ulrich, R.G. 2000).
S.aureus normally inhabits the nasal cavity and skin of approximately 30-50% of healthy humans (an even higher percentage for those in hospitals), thus the primary means of transmission to food is via food handlers. Animal skin, esp. of food animals, may also serve as a reservoir of S.aureus. Therefore, staphylococcal food poisoning most commonly occurs after consumption of foods that involve hand preparation (such as salad) (Evans, A. S. et.al., 1982). However, since S.aureus can grow at a variety of temperatures (7-48.5ËšC), ph (4.2-9.3) and salt concentrations (up to 15%) it can be found in numerous foods (Loir, Y.L., et al., 2003).
Given a suitable environment to multiply, S.aureus will produce enterotoxins during the logarithmic phase of growth (Agrudin, M.A., 2010). Thorough cooking destroys the bacteria, but not the toxin.Toxin laden food may appear palatable, however ingestion of less than 1µg of enterotoxin is enough to cause Staphylococcal food poisoning (Labbé, R. G. et. al. 2001). Symptoms are induced by the enterotoxin's A-B structure, which is comprised of a B subunit that binds to epithelial cells and an A subunit that causes fluid and electrolyte secretion. Intestinal motility is also affected, this leads to severe diarrhea that may be accompanied by nausea and vomiting (Tortora, G.J. et. al., 2007).The onset of symptoms is usually within hours of ingestion and recovery within a few days because of the self-limiting nature of S.aureus (US Food and Drug Administration, 2009).
The aforementioned symptoms are used in diagnosis of Staphylococcal poisoning. Usually S.aureus is not the predominant bacterium in patient samples taken for analysis in laboratories, for that reason selective inhibitory media, such as mannitol agar, should be used for isolation and enumeration. S.aureus needs to then be distinguished from other bacteria and other staphylococcus species. This is commonly achieved through gram-staining, positive catalase, negative oxidase and positive cogulase tests, and by detecting the presence of S.aureus clumping factor A using commercially prepared latex agglutination beads (Labbé, R. G. et. al. 2001).