3 exotoxins and types of food poisoning nutritionally

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Unformatted text preview: poisoning Nutritionally, all bacteria that cause food bourne illness are heterotrophs, organisms that use organic molecules as sources of energy, electrons and carbon. Some examples of bacteria and the diseases they cause are given in Table 9-1. Types of food poisoning In many instances, the damaging effects of a pathogenic bacterium is due to the production an exotoxin released from the organism. There are two types of foodpoisoning: food borne intoxication and food borne infection (Figure 9-1) Food poisoning Food bourne infection Bacteria consumed in food Bacteria grow in body producing an exotoxin Ilness Figure 9-1: Types of food-poisoning Food bourne intoxication Bacteria grow in food producing an exotoxin Exotoxin in food consumed Ilness FOOD-285 Table 9-2: Some examples of exotoxins. Note: ETEC” = Enterotoxigenic E. coli is not the same as the normal “docile” E. coli in our intestines. ETEC has a plasmid that carries exotoxin genes. ETEC is not the same as “hamburger disease” E. coli O157: H7 a.k.a “EHEC” = Enterohemoragic E. coli (Chapter 1). Bacterium Exotoxin gene(s) Exotoxin denatured by typical cooking temperatures? Clostridium botulinum Prophage Yes Bacillus cereus Plasmid No Staphylococcus aureus Chromosome No ETEC E. coli Plasmid: has genes for 2 exotoxins One toxin is heat stable One toxin is heat labile In food borne intoxication the exotoxin is already present in the food due to previous bacterial growth. The food containing the exotoxin (produced prior to cooking, after cooking, during serving or storage) is eaten and results in food poisoning. In food borne infection the exotoxin is produced by bacteria after ingestion of the food containing the bacteria, that is, bacteria present in food are eaten and then grow in the intestine, producing an exotoxin. As will become evident below, the potential for exotoxin production by a bacterium is an important consideration because a treatment which kills a bacterium may not necessarily inactivate an exotoxin already produced by the same bacterium. Exotoxins Chemical nature of exotoxins Most exotoxins are types of extracellular proteins, proteins made in the bacterial cytoplasm and secreted into the environment (Chapter 2). Action of exotoxins Neurotoxins Neurotoxins affect nerve cells. Consider, for example, the botulism toxin. It inhibits synaptic vesicle fusion in the terminus of peripheral motor neurons Synaptic inhibition prevents activation of muscle cells, causing flaccid (relaxation) paralysis and death. Similarly the S. aureus and B. cereus toxins are neurotoxins but they work in a different way. These toxins bind to nerve cells in stomach causing an emetic (vomiting) response. Enterotoxins Enterotoxins affect intestinal cells. For example ETEC (E. coli) makes two enterotoxins. Both bind to membrane proteins of intestinal cells causing them to secrete a large amount of water; this results in diarrhea. Exotoxin genes The genes specifying the exotoxin proteins are often not part of the bacterial chromosome. Rather they are associated with a selfish gen...
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