Pathogenic, commensal and beneficial microorganisms in foods

Abstract

Moulds, yeasts, bacteria, viruses and minute parasites are microorganisms that virtually occur everywhere within the environment. Viruses are the smallest known living organisms (see Figure 1). They do not have a cell wall, a membrane or a nucleus and are defined as obligate intracellular parasites. When they reproduce, they take over the life processes of host cells, which continue to live while producing viral copies. Most cells in food products are dead following processing and therefore simply function as carriers of viral material. Some viruses can be spread by people who handle food and do not follow careful personal hygiene habits. A person may indeed excrete viruses in faeces, urine or even through sneezing; so, if hands are not washed well after using toilet facilities as well as sneezing; any food handled after the event will be contaminated. Foods that are not usually processed thermally after handling - such as bakery products, uncooked oysters or clams, sandwiches, salads and desserts, may therefore carry and hence transmit viral illnesses. Bacteria are single-celled organisms, which multiply and increase in number through cell division given appropriate environmental conditions. Many pathogenic bacteria are facultative anaerobes, so they can grow in either aerobic or anaerobic conditions. Yeasts are also single-celled organisms, which can convert nutrients into alcohol and carbon dioxide via fermentation. Wild yeast spores are permanently floating in the atmosphere and may land on uncovered liquids and foods - hence resulting in contamination. In general, yeast contamination in food generates slime on the surface, bubbles in the bulk and an alcoholic smell or taste. Yeasts can be destroyed by heating to 121°C for 15 min; nevertheless, in industrial food processing, carefully cultured yeasts are used in the production of beer, wine and bread. Moulds are multi-cellular forms of fungi, which can grow on almost any item used as or for food, given suitable conditions. Mould spore casings are present in the environment; when they break, thousands of microscopic mould spores are released, each one capable of germinating and originating a new mould. This is a process that typically occurs in damp, dark environments. As spores on the surface of food ripen, the food develops unpleasant musty odours, which destroy the (normally sought) fresh flavours. Certain moulds may produce poisonous toxins - called mycotoxins. Aflatoxin is one such mycotoxin that is secreted on nuts, corn, wheat and other grains. Aflatoxins may also be found in products made from dry fruits and cereals, such as breads and peanut butter. Ingestion of aflatoxin usually causes low grade fever in humans but can also produce cancer in trout, rats and ducks. Other illnesses thought to worsen via the presence of aflatoxins include Reyes syndrome, cirrhosis and kwashiorkor (Jones, 1992; Jay, 1986). Parasites are organisms that live or feed off other organisms. In general, they are found in raw animal products or seafood. Parasites, which include Trichinella spiralis (a round worm found in wild game or pork) and Anisakis spiralis (commonly referred to as "cod fish worm" or "seal worm" and found in fish), are destroyed by thorough cooking. Most foods harbour a mixture of the aforementioned microorganisms, which play a role in several biological interrelationships ranging from (competitive) amensalism to antagonism and encompass specifically mutualism, commensalism and parasitism (or predation). Mutualism may be understood as a mutual dependency between two microorganisms, in which each microorganism attains some benefit - trivial or vital, from the other. In a commensalism pattern, only one microorganism is able to obtain a benefit from the association, whereas the other is unaffected by it. Finally, in parasitism, pathogenic microorganisms (considered to be disease-causing agents) obtain support from the host at its expense. As previously mentioned, these harmful microorganisms can invade any food and may survive despite aggressive measures at the processing level and storage - or the food may become contaminated during preparation, cooking or serving. When present in the food at or above their infective dose threshold, they will cause illness - sometimes severe and even life-threatening, especially in young children, older adults and persons with compromised immune systems. In pregnant women, foodborne illness may also endanger their unborn babies. The Centers for Disease Control and Prevention (CDC) in the USA, estimate that 76 million people suffer foodborne illnesses each year in that country, hence accounting for 325,000 hospitalizations and more than 5,000 deaths. There are more than 250 known foodborne diseases. Bacteria cause most cases, followed by viruses and parasites. Some diseases are caused by toxins (poisons) from disease-causing organisms, others by host reactions to the organism itself. The most common symptoms of foodborne illness are diarrhoea, abdominal cramps, vomiting, head- or muscle-aches and fever. Symptoms usually appear 12 to 72 hr after eating contaminated food but may occur as early as 30 min or as late as 4 weeks afterwards. © 2007 Springer Science+Business Media, LLC.