Classification of Foodborne Causative Agents
Foodborne illnesses are generally caused by contamination of food with biological agents or pathogens (viruses, bacteria, parasites, prions); chemical agents (toxins, metals); or physical agents (glass fragments, bone chips). Pathogens are the most significant cause of the more than 200 known diseases transmitted through food. With the exception of certain parasites, nearly all foodborne pathogens are microscopic in nature. In increasing order of size, these pathogens include viruses, bacteria, protozoa and other parasites.
Viruses are particulate in nature and multiply only in other living cells. Most are incapable of surviving for long periods outside the host. While more than 100 types of enteric viruses have been shown to cause foodborne illness, the most common foodborne virus pathogens are noroviruses (formerly known as Norwalk-like viruses or NLVs), rotavirus, astrovirus, and hepatitis A.
Bacteria are one-celled microorganisms. Each has a cell wall but no nucleus. They exist in a variety of shapes, types, and properties. Some pathogenic bacteria are capable of spore formation and are thus highly heat-resistant. These include Clostridium botulinum, C. perfringens, Bacillus subtilus, and B. cereus. Others are capable of producing heat-resistant toxins (Staphylococcus aureus). Most pathogens are mesophilic, meaning that their optimal growth temperature range is between 20 to 45oC (68 to 113oF). However, certain foodborne pathogens (termed psychrotrophs) are capable of growth under refrigerated conditions or temperatures less than 10oC (50oF).
The most well-documented psychrotrophic foodborne pathogens are Listeria monocytogenes and Yersinia enterocolitica. Listeria monocytogenes will grow (albeit slowly) at temperatures just of above freezing (approximately 33 to 34oF). Certain strains or serotypes of Bacillus cereus, Clostridium botulinum, Salmonella spp., E. coli O157:H7, and Staphylococcus aureus may also grow slowly under refrigeration conditions. Bacteria also vary in their resistance to high salt, sugar, or total solids level as well as food product acidity. Some pathogens are capable of survival at low pH.
Parasitic protozoa are one-celled microorganisms without a rigid cell wall but with an organized nucleus. They are larger than bacteria. Like viruses, they do not multiply in foods, only in hosts. The transmissible form of these organisms is called a cyst. Protozoa that have been associated with food and waterborne infections include Entamoeba histolytica, Toxoplasma gondii, Giardia lamblia, Cryptosporidium parvum, and Cyclospora cayatenensis.
Multi-cellular parasites are animals that live at the expense of their host. They may occur in foods in the form of eggs, larvae, or other immature forms. Trichinosis has been an important reportable pathogen associated with undercooked pork. Other parasites of concern include flatworms or nematodes (associated with fish), cestodes or tapeworms (usually associated with beef, pork, or fish), and trematodes or flukes (more of a concern outside the United States).