These microorganisms are normally divided into two groups: “infections” (e.g. Salmonella, Campylobacter) and "intoxications" (e.g. Staphylococcus aureus, Clostridium botulinum).
In simple terms, the first group comprises organisms that multiply in the human intestinal tract, while the second group comprises organisms that produce toxins either in the food or during passage through the intestinal tract.
The microorganisms responsible for the majority of food poisoning incidents are limited to a few bacteria species. These include Listeria monocytogenes, Salmonella species, Yersinia enterocolitica, Vibirio parahaemolyticus, Aeromaonas hydrophila, Escherichia coli (enteropathogenic strains), Campylobacter jejuni and coli, Bacillus cereus and Clostridium perfringens.
Other important culprits are toxinogenic organisms, such as Staphylococcus aureus, Clostridium botulinum, Bacillus subtilis and Bacillus licheniformis. Bacteria that are less frequently involved in food poisoning include Shigella species, Enterococcus species, Pseudomonas aeruginosa, Vibirio vulnificus and Clostridium difficile.
The role of viruses as food-poisoning microorganisms is believed to be huge. In the United States and, perhaps, throughout much of world, Norwalk-like viruses (NLVs) are the number one cause of food-poisoning