TYPICAL TRANSMISSION CYCLE OF FOODBORNE DISEASE*
1. A vulnerable food (i.e. one with proper nutrients, moisture and pH that permit growth of the organism) is contaminated with a pathogen through one of the mechanisms previously mentioned.
2. Frequently, the contaminated food is held at temperatures between 20 and 35o which ensures a large infectious dose within a few hours. (This is not always the case.)
3. The pathogen is consumed and circumvents the host’s normal defense mechanisms.
4. Gastrointestinal disease ensues, or in the case of poisonings, other diseases such as botulism may be seen.
*Reference
McKane, L and Kandel, J: Microbiology: essentials and applications, New York, McGraw-Hill, Inc., 1996.
MECHANISMS BY WHICH FOODBORNE PATHOGENS CAUSE DISEASE*
Production of Preformed Toxins
1. True food poisonings.
2. These foodborne diseases follow consumption of food containing preformed toxins, i.e. they do not require multiplication of the microorganism.
3. Toxins that alter the physiology of normal mechanisms of various systems of the body. For example, enterotoxins alter the physiology of the intestinal tract and neurotoxins alter the physiology of the nervous system.
4. Examples of organisms that cause disease by producing preformed toxins in foods are Staphylococcus aureus and Clostridium botulinum.
*Reference
McKane, L and Kandel, J: Microbiology: essentials and applications, New York, McGraw-Hill, Inc., 1996.
MOUTH
Primarily mechanical breakdown:
STOMACH
Three primary functions:
1) stores ingested food until it can be emptied into small intestine at certain rate;
2) secretes hydrochloric acid and gastric juice (with an enzyme) which begin protein digestion;
proteins are degraded into peptides; and
3) food is pulverized to produce chyme which enters small intestine.
PANCREAS
Accessory organ: food does not pass through here but pancreas secretes several enzymes through pancreatic duct into lumen of small intestine.
These enzymes act on proteins, polysaccharides, and fats.
LIVER
Accessory organ: produces bile which is an emulsifying agent that breaks up fat globules into much smaller globules. Bile transported to gall bladder and released into lumen of small intestine.
Note that another important function of the liver is to store glucose as glycogen after eating and break down glycogen to glucose to maintain the glucose concentration of blood between eating.
SMALL INTESTINE
Digestion occurs in lumen and epithelial cells.
Pancreatic enzymes act on food in lumen to reduce fats to glycerol and free fatty acids, proteins into small peptide fragments and some amino acids, and carbohydrates to disaccharides and some monosaccharides.
Final digestion of protein and carbohydrates occurs with small intestine enzymes in epithelial cells of small intestine prior to absorption.
All nutrients (except fats) diffuse into the capillaries and immediately pass through the liver for processing before being distributed to cells throughout the body. Liver removes excess glucose and stores it as glycogen.
Glucose ----------> Glycogen & Water
Between eating periods, when glucose level of blood falls below a certain level, glycogen is broken down to glucose and enters the circulation.
| EXAMPLE | PROTECTIVE ACTION |
|---|---|
| Lysozyme in saliva | Dissolves cell walls of some bacteria |
| Swallowing | Moves organisms into stomach |
| Gastric juice | Stomach acid destroys most organisms |
| Mucous lining of intestine | Prevents attachment of organisms to epithelial cells |
| Normal flora of intestine | Produces substances that inhibit pathogens | Epithelial sloughing of intestine | Sheds epithelial cells to which microorganisms may have attached |
*Reference
McKane, L and Kandel, J: Microbiology: essentials and applications, New York, McGraw-Hill, Inc., 1996.
HOW ORGANISMS THAT CAUSE FOODBORNE DISEASE ARE ABLE TO OVERCOME GASTROINTESTINAL DEFENSES*
1. Ingestion of large doses of pathogens. Infectious dose for most pathogens of digestive tract is 100 million to 1 billion. For organisms that are more virulent, a lower infectious dose is needed.
2. Consistency and composition of foods plays a role. If foods pass rapidly through the stomach, their period of exposure to gastric secretions is shortened, and thus, contaminating organisms may escape destruction. Fluids and semi-solids are transported fastest.
3. Various substances in the food or other substances may decrease the infectious dose. For example, high protein meals may decrease the infectious dose by neutralizing acid while occupying the protein-digesting enzymes. Antacids may also neutralize the stomach acid with sodium bicarbonate.
4. Medical conditions which decrease production of stomach acid or gastrectomy may predispose to infection.
5. Some protozoa form protective cysts that are resistant to acid destruction and pass through the stomach unharmed but emerge and reproduce in the intestines.
6. Antibiotic therapy may disrupt normal flora and predispose to infection.
7. Preformed toxins or highly virulent organisms.
*Reference
McKane, L., and Kandel, J.: Microbiology: essentials and applications, New York, McGraw-Hill, Inc., 1996.
A QUICK LOOK AT YOUR NERVOUS SYSTEM
Parts of the Nervous System
1. Central Nervous System - composed of brain and spinal cord.
2. Peripheral Nervous System - nerves which extend to and from brain and spinal cord to all parts of body. Consists of sensory nerves and motor nerves (there is a third type of nerves called associative nerves or interneurons located within the CNS but we will not consider them here). Nerves, of course, are responsible for carrying impulses or messages.
Sensory nerves carry messages from sensory receptors of body (both internal and external) to the central nervous system. Sensory receptors in the eye respond to light, for example, those in skin to temperature, touch, pressure, pain, etc. and baroreceptors in the aorta respond to blood pressure. The messages are sent to the CNS which interprets them and then responds through motor neurons. Motor neurons carry the message (in essence our response if necessary) back to the particular area of concern.
Divisions of the Peripheral Nervous System
a) Somatic Nervous System - Includes nerves that serve the musculoskeletal system and the exterior sense organs. Receptors in the exterior sense organs receive environmental stimuli and initiate impulses in sensory nerves to the CNS. The CNS then sends messages back to the skin and those muscles that move the skeleton, such as muscles of hand, trunk, and limbs. The motor neurons of the somatic nervous system control all voluntary movements of the body.
b) Autonomic Nervous System - Controls the involuntary, homeostatic activities of body’s internal organs and blood vessels. Internal sensory receptors respond to various changes in internal organ systems and initiate impulses to the CNS. The CNS then sends messages back via motor nerves to the organs, glands, and smooth muscles of the body (Sympathetic Nervous System and Parasympathetic Nervous System).
Neurons or Nerve Cells
All types of neurons (both sensory and motor) operate by generating electric signals and passing them from one part of the cell to another and by releasing chemical messages to communicate with neighboring nerves on the target organ, e.g. muscle.
Communication of Neurons with Muscles (a target organ)
The axon is the main nerve fiber and it terminates with so-called axonic terminals which are responsible for releasing chemical messages which transfer the new impulse to another nerve or target organ. These chemical messages which are called neurotransmitters must cross a region called a synapse. The membrane from which these messages are released is called the pre- synaptic membrane. After release, these neurotransmitters cross a synaptic cleft to reach the post-synaptic membrane of the neighboring neuron or target organ.
Neurotransmitters
Examples are acetylcholine, norepinephrine, dopamine, and serotonin. Numerous substances exert deleterious effects on the body by affecting these neurotransmitters. Drugs, pesticides, and toxins may exert various effects on the body by affecting these neurotransmitters.
For additional readings on the nervous system, students may consult any of the following texts :
Clostridium is an example of a genus that can form spores. Spores are very resistant dehydrated structures. They can survive for long periods in soil, tolerating temperature extremes and lack of nutrients.
Spores of C. botulinum are widespread in soils and are thus present on the surface of many vegetables used for canning. They are killed only after heating to high temperatures used in proper canning techniques. When food is not adequately heated, however, spores may germinate and the metabolizing forms of the organism start secreting toxin.
Improperly heated home canned food is most commonly incriminated and associated with green beans, mushrooms, peppers, corn, beets and asparagus.
Foodborne botulism follows ingestion of preformed toxin in contaminated food. For an outbreak of foodborne botulism to occur, several conditions must be met. First, food must be contaminated with C. botulinum spores. Second, the food must possess the composition and nutritive properties that allow the spores to germinate and produce toxin. The food must also be inadequately heated, processed or stored.
Following consumption, toxin is absorbed from the intestinal tract and transported via blood and lymph to peripheral nervous system. Incubation period of 12-36 hours reflects time required for toxin to be absorbed from intestinal tract, disseminated through body by blood and attached to peripheral nerves.
Most powerful toxin know - neurotoxin - acts on nerves.
Binds to receptors on nerve ending and causes nerve dysfunction by blocking release of the transmitter acetylcholine from nerve ending.
The muscle response to a nerve impulse is then blocked because no neurotransmitter is released to excite the muscle. The result is paralysis, i.e. muscle cannot receive signals from nerve telling it to contract.
Nausea, vomiting and diarrhea are the first clinical signs.
Classic neurologic symptoms usually appear 12-36 hours after consumption of contaminated food. Primary feature is the symmetric descending paralysis that begins with the ocular muscles (disturbances of vision) and progresses rapidly to the pharyngeal muscles (difficulty in swallowing) and the muscles of the neck, trunk, and limbs. Paralysis of the respiratory muscles usually results in death.
Therapy includes intensive respiratory and supportive therapy and administration of antitoxin; antitoxin neutralizes circulating toxin.
Salmonella enteritidis(SE)
Responsible for foodborne disease in raw or undercooked eggs and poultry.
Persons can decrease their risk for egg-associated infection caused by S. enteritidis by not eating raw or undercooked eggs. Nursing homes, hospitals and commercial kitchens should use only pasteurized egg products.
The problem is that the bacteria can grow in hens’ reproductive tracts so that the egg is contaminated before it is even laid. External contamination can occur with chicken feces or other matter from the environment.
External contamination of commercially purchased poultry eggs has been found to be as high as 9% and the internal contamination may occur in 0.1% to 0.6% of all poultry shell eggs.
The problem with turkeys is being undercooked.
The introduction to the HAACP program has resulted in a decrease in the prevalence of Salmonella in poultry.
E. coli O157:H7
On January 13, 1993, a physician in Washington state reported several children with hemolytic uremic syndrome. Also noted was a dramatic increase in emergency room visits for bloody diarrhea in patients of all ages. These initial events were quickly followed by a multistate outbreak in which more than 500 children and adults became ill. Four children died. The culprit was traced to hamburger patties contaminated with the bacterial pathogen, E. coli O157:H7. This outbreak eventually traced back to the national fast food chain, Jack-in-the-Box, and 93 of its restaurants were implicated.
While the specific meat was traced to one processing plant, exactly how the meat got contaminated is not known. During the slaughter process, however, intestinal fluid or feces of infected cattle can drip onto the surface of the meat, contaminating it. It is theorized that the harmful bacteria on the surface of the raw meat became mixed throughout the meat during the grinding process. One hamburger patty can contain the meat from as as many as 20 cows.
E. coli.O157:H7 has been found in beef, raw milk, unpasteurized apple cider, and other sources. Person-to-person transmission can occur because of the low infectious dose, especially in daycare centers and geriatric facilities. More recently, the organism has been associated with a few outbreaks in lakes or swimming pools. The cause of these outbreaks is being investigated.
E. coli.O157:H7 produces a Shiga toxin which damages the cells of the intestinal lining and allows blood to pass into the stool. From 2-7% of the illnesses progress to hemolytic uremic syndrome where mediators that the toxin induces the production of enter into the bloodstream through the damaged intestinal wall and travels to the arteries that supply the kidneys and damages the vessels. The result is kidney failure and 3-5% of these cases are fatal.
The organism has been found in the feces of between 0.8 and 5% of cattle in regional and national studies and research is underway to determine the factors on cattle farms that lead to the infection of cattle. Additional research is investigating various methods for reducing the organisms on beef carcasses and processing equipment.
References for E. coli.O157:H7
1. Ackman, D., et al.: Swimming associated hemorrhagic colitis due to Escherichia coli.O157:H7 infection: evidence of prolonged contamination of a fresh water lake. Epidemiol. Infect. 119: 1-8, 1997.
2. Castillo, A. et al.: Comparison of water wash, trimming, and combined hot water and lactic acid treatments for reducing bacteria of fecal origin of beef carcasses. Journal of Food Protection 61: 823-828, 1998.
3. Delazari, I., et al.: Decontaminating beef for Escherichia coli.O157:H7, Journal of Food Protection 61: 547-550, 1998.
4. E. coli Alert, Newsweek Magazine, September 1, 1997.
5. Farrell, B.L., Ronner, A.B., and Wong, A.C.L. : Attachment of Escherichia coli.O157:H7 in ground beef to meat grinders and survival after sanitation with chlorine and peroxyacetic acid, Journal of Food Protection, 61: 817-822, 1998.
6. Herriott, D.E., et al.: Association of herd management factors with contamination of dairy cattle by Shiga toxin-positive Escherichia coli.O157:H7, Journal of Food Protection, 61: 802-807, 1998.
7. Keene, W.E., et al.: A prolonged outbreak of Escherichia coli O157:H7 infection caused by commercially distributed raw milk, Journal of Infectious Diseases, 176: 815-818, 1997.
8. Koutkia, P., Mylonakis, E., and Flanigan, T.: Enterohemorrhagic Escherichia coliO157:H7 - an emerging pathogen. American Family Physician 56: 853-856, 1997.
9. Kudva, I., Blanch, K., and Hovde, C.J.: Analysis of Escherichia coliO157:H7 survival in ovine or bovine manure and manure slurry. Appl. Environ, Microbiol. 64: 3166 - 3174, 1998.
10. Su, C., and Brandt, L.J.: Escherichia coliO157:H7 infection in humans, Ann. Intern. Med. 123: 698-714, 1995.
11. Threatsfrom the Food We Eat, Medical Laboratory Observer, April, 1996.
1. What foods are primarily responsible for outbreaks of disease due to Salmonella enteriditis?
2. Why has the contamination of eggs with Salmonella enteriditis become a big problem?
3. What food/other sources have been associated with outbreaks of disease due to E. coliO157:H7?
4. Explain how hamburger meat may become contaminated with E. coli O157:H7 during the slaughtering process.
5. What is hemolytic uremic syndrome?
Foodborne illness has become one of the fastest growing community-health problems in the United States. Many new, sometimes deadly, disease causing agents capable of contaminating food have emerged, and microorganisms not previously thought to be troublemakers in foods are cropping up as causes of foodborne infections.
Why are foodborne infectious diseases a growing problem?
What are the main causes of foodborne disease threats?
Foodborne infectious diseases are caused by an increasingly large collection of bacteria, including Salmonella, Staphylococcus, Shigella and E. coli. Protozoa and viruses can also cause foodborne disease. These microorganisms can contaminate many foods we eat every day: meat, poultry, milk and other dairy products, raw seafood, egg products, and fresh fruits and vegetables, to name a few.
How are foodborne infections spread?
Many people get foodborne infectious disease from eating food prepared in such a way that it becomes contaminated. Food contaminated with harmful microorganisms may taste and smell fine. After the organisms or their poisonous byproducts are eaten, a delay from a few hours to several weeks can go by before symptoms appear.
However, it is often not what you eat but how you, your family, and restaurant staff prepare food that sets the stage for the spread of infection. Hot or cold foods left standing too long at room temperature provide an ideal climate for bacteria to grow. Improper cooking plays a role. Food can be contaminated at home or in a restaurant when cutting boards and kitchen utensils have been used to prepare raw meat, fish or poultry and then are used for another food before being cleaned.
How can I reduce my risk from foodborne disease threats?
Some simple, common sense practices can reduce your risk from foodborne threats.
Safe food shopping tips
Safe food handling and cooking tips
Safe food storage tips
Egg safety tips
REDUCE THE RISK FROM INFECTIOUS DISEASES ACQUIRED THROUGH TRAVEL
Traveling abroad can put you at risk for infectious diseases that are not widespread in the United States. If you become ill in a country where infectious disease treatment is limited or unavailable, your health can be further jeopardized. Thus, the best defense against known and emerging disease threats during international travel is knowing what the potential hazards are and knowing how to reduce your personal as well as global health risks.
neurotoxin --- exotoxin that damages nerve tissue
lysozyme --- enzyme found in body secretions such as saliva that destroys bacterial cell walls
infection dose --- number of organisms needed to initiate infection in a host
protozoa --- unicellular, non-photosynthetic eukaryotes
cysts --- protective, dormant structure formed by some protozoa
central nervous system (CNS) --- brain and spinal cord
peripheral nervous system --- nerves which extend to and from brain and spinal cord to all parts of the body
sensory nerves (neurons) --- carry messages from sensory receptors of body to CNS
motor nerves (neurons) --- carry messages from CNS to a target organ, i.e., muscles
somatic nervous system --- controls the voluntary movements of the body
autonomic nervous system --- controls the involuntary, homeostatic activities of body’s internal organs and blood vessels
neurotransmitters --- chemical messengers that transfer impulses among nerves or from nerve to a target organ
spores --- reproductive structures produced by some bacteria and fungi which allow the organism to resist adverse environmental conditions
enterotoxin --- exotoxin that affects the intestinal mucosa
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