Foodborne Bacterial Pathogens

An understanding of the growth characteristics and sources of bacterial pathogens in foods is essential to conducting a hazard analysis of a food and subsequently controlling the identified hazards. 

The discussion will center on gram-negative rods and gram-positive rods and cocci. The pathogens within each group have some similarities in addition to their gram stain. For example, gram negative rods are nonspore formers and tend to have a fecal source. On the other hand, gram positive rods and cocci can be spore formers and are typically associated with environmental sources like soil and sediments.

GRAM-NEGATIVE RODS

Campylobacter

Campylobacter jejuni infection, called Campylobacteriosis, causes diarrhea, which might be watery or sticky and might contain blood. Other symptoms include fever, abdominal pain, nausea, headache, and muscle pain.  Onset of illness occurs 2-5 days after eating contaminated food or water and lasts between 7 and 10 days, with a relapse in 25% of cases. While Campylobacter infections are self-limiting, antibiotics can further limit the amount of time that bacteria are shed in the feces of infected individuals.

The infective dose is considered to be small; human feeding studies suggest that as few as 400-500 bacteria might cause illness. While the pathogenic mechanism is not completely understood, it is an infection. Estimated numbers of cases of campylobacteriosis exceed two to four million per year.  In fact, it is considered the leading cause of human diarrheal illness in the

U.S. and is reported to cause more disease than Shigella and Salmonella spp. combined.  Despite this, death is rare, with one fatality per 1,000 cases. Those most frequently afflicted are children under 5 years and young adults ranging from 15 to 29.

Raw and undercooked chicken, raw and improperly pasteurized milk, raw clams, and non-chlorinated water have been implicated in campylobacteriosis. The organism has been isolated from crabmeat. It is carried by healthy chickens and cows and can be isolated from flies, cats, and puppies.

Campylobacter is unique because of its special oxygen requirements. It is microaerophilic, which means it requires reduced levels of oxygen to grow -- about 3-15% oxygen (conditions similar to the intestinal tract). Also, it will not grow at temperatures below 86°F or at salt levels above 1.5%. The organismsis considered fragile and sensitive to environmental stresses like drying, heating, disinfection, acid, and air which is 21% oxygen. It requires a high water activity and fairly neutral pH for growth.

The controls include  proper cooking and pasteurization, proper hygienic practices by food handlers to prevent recontamination, and adequate water treatment.

Yersinia -- Yersinia spp: Y. entercolitica; Y. pseudotuberculosis; Y. pestis

Of the 11 recognized species of Yersinia, three are known to be pathogenic to humans -- enterocolitica, pseudotuberculosis, and pestis.  Only enterocolitica and pseudotuberculosis are recognized as foodborne pathogens. Y. pestis, the microorganism responsible for the black plague, is not transmitted by food and so is not addressed below.

Onset of illness for yersiniosis is about 3 to 7 days, but periods of up to 11 days have been reported. The illness usually lasts 1 to 3 days, but in some cases it might persist for 5 to 14 days or longer. Yersiniosis is often characterized by gastroenteritis with diarrhea and/or vomiting, but fever and abdominal pain are the hallmark symptoms. Yersinia infections mimic appendicitis, which has led to unnecessary appendectomies. Both enterocolitica and pseudotuberculosis have been associated with reactive arthritis, which might occur even in the absence of fever and abdominal pain. Another complication is septicemia, an infection of the blood system. This is rare.  Fatalities are also rare.

The infective dose of Yersinia has not been determined. Yersiniosis is rare in U.S.; CDC estimates that only 17,000 cases or so occur annually in the U.S.. Yersiniosis is a far more common problem in Northern Europe, Scandinavia, and Japan. As usual, the most susceptible populations -- both for the condition itself and for possible complications ­- are the very young, the debilitated, the very old, and those undergoing immunosuppressive therapy.

Yersinia can be found in raw vegetables, milk, ice cream, cakes, pork, soy products, salads, oysters, clams, and shrimp. They are found in the environment, such as in lakes, streams, soil, and vegetation. They have isolated from the feces of dogs, cats, goats, cattle, chinchillas, mink, and primates; and in the estuarine environment.  Many birds, including waterfowl and seagulls, might be carriers.

The foodborne nature of yersiniosis is well established, and numerous outbreaks have occurred worldwide. Two outbreaks in Quebec, Canada, in the mid-1970s affected 138 children and were traced to raw milk.  In the U.S., an outbreak in New York in 1976 affected 217 students.  In this case, pasteurized chocolate milk was implicated. A 1980 outbreak in Washington affected 87 people. The source of contamination was traced to unchlorinated spring water used in packaging tofu.  More outbreaks occurred in 1983 in a tri-state area in the southeast, and again pasteurized milk was implicated.

Yersinia are facultative anaerobes. They are psychrophilic organisms, with a minimum growth temperature of 30°F. Yersinia love cold and can withstand repeated freezing and thawing. Other than that, Yersinia are pretty typical for gram negative bacteria. They have a high water activity and relatively neutral pH requirement, along with a low salt tolerance. Yersinia can be eliminated through pasteurization or the use of commercial sanitizers.

Yersinia are controlled by proper cooking or pasteurization, proper food handling to prevent recontamination, adequate water treatment, and preventing time-temperature abuse. Proper use of sanitizers is also an effective control.

Staphylococcus. aureus

There are four syndromes of human salmonellosis -- Salmonella gastroenteritis, typhoid fever, non-typhoidal Salmonella septicemia, and asymptomatic carrier.  Salmonella gastroenteritis might be caused by any of the Salmonella species other than Salmonella Typhi and is usually a mild, prolonged diarrhea. 

True typhoid fever is caused by infection with Salmonella Typhi. While fatality rates might exceed 10% in untreated patients, they are less than 1% in patients who receive proper medical treatment. Survivors might become chronic asymptomatic carriers of Salmonella bacteria.  Asymptomatic carriers show no symptoms of illness yet are capable of passing the organisms to others.

Non-typhoidal Salmonella septicemia might result from infection with any of the Salmonella species and can affect virtually all organ systems, sometimes leading to death. Survivors might become chronic asymptomatic carriers of Salmonella bacteria. In this manual the discussion will be limited to Salmonella gastroenteritis because it is the most common form in the U.S..

Although symptoms might appear a few hours after eating contaminated food, it might take one or more days. Symptoms include nausea, vomiting, abdominal cramps, diarrhea, fever, chills, and headache. In some cases, reactive arthritis or Reiter’s syndrome can occur up to three weeks after the other symptoms. Depending on the host, the dose, and the strain characteristics, symptoms might last 1-2 days, or longer. For particularly susceptible individuals, the infectious dose might be as few as 15-20 organisms. Salmonella cause between 40,000 and 60,000 reported cases per year, and perhaps as many as 3 million unreported cases. Fatalities range from 1% to 4%. While healthy people can get sick from Salmonella, the population most at risk is the young, the old, and the sick.

Salmonella often live in animals – especially poultry and swine – as well as in a number of environmental sources. Salmonella has been found in water, soil and insects, on food-contact surfaces, and in animal feces. They can also survive in a variety of foods, including raw meats and poultry, dairy products and eggs, fish, shrimp and frog legs, yeast, coconut, sauces and salad dressing, cake mixes, cream-filled desserts and toppings, dried gelatin, peanut butter, orange juice, cocoa and chocolate.

In 1985, a salmonellosis outbreak involving 16,000 confirmed cases in six states was traced to milk from one Chicago dairy, where FDA inspectors found a cross-connection between raw and pasteurized milk. An outbreak involving ice cream was the result of transporting ice cream mix in trucks which had previously hauled raw eggs.  An orange juice outbreak proved to be an example of Salmonella adapting to an acidic environment.

In 1985, S. enteritidis was blamed for at least 71 illnesses in Maryland, and scrambled eggs from a breakfast bar were implicated. CDC estimates that 75% of S. enteritidis outbreaks are associated with eating raw or inadequately cooked eggs. The U.S.DA published regulations in 1990 establishing a mandatory testing program for egg-producing breeder flocks and commercial flocks implicated in causing human illnesses. This testing should lead to a reduction in cases of gastroenteritis caused by eating eggs.

Salmonella spp. are also mesophilic organisms which grow best at moderate temperatures and pH, and under conditions of low salt and of high water activity. They are killed rapidly by moderate heat treatment, yet mild heat treatment might enable them to develop heat resistance, up to 185°F. Similarly, they can adapt to an acidic environment, as in the case of the orange juice outbreak mentioned earlier.

Ordinary household cooking, personal hygiene to prevent recontamination of cooked food, and control of time and temperature are generally adequate to prevent salmonellosis.

Shigella 

There are four species of Shigella. Because there is little difference in their behavior, they will be discussed as a group.

The onset time for shigellosis can range from 12-96 hours. Typical symptoms include fever, cramps, tenesmus, inflammation and ulceration of the intestine, and diarrhea. Sometimes the diarrhea can lead to dysentery, which is not usually a life-threatening illness.  In malnourished children, immunocompromised individuals and older adults, the disease might be lethal. Ordinary shigellosis might also be severe, but it is self-limiting. If left untreated, it might last 1 to 2 weeks. The infectious dose might be as few as 10 cells, depending on the age and condition of the host, and the disease is easily transmitted from person to person, with a high secondary attack rate. Approximately 14,000 cases of shigellosis are reported every year, with an estimated total of 300,000 cases. Of the 14,000, an average of four will result in death.

The only significant reservoir for Shigella is humans. Foods associated with shigellosis include salads (potato, tuna, shrimp, macaroni, and chicken), raw vegetables, milk and dairy products, poultry, fruits, bakery products, hamburger, and finfish. 

In 1985, a large outbreak of foodborne shigellosis occurred in Texas, involving as many as 5,000 persons. The implicated food was chopped, bagged lettuce, prepared in a central location for a Mexican restaurant chain. A number of  outbreaks of shigellosis occurred on college campuses throughout the U.S.. Fresh vegetables from the salad bar were usually implicated, and an ill foodservice worker proved to be the cause.

The growth conditions for Shigella, which are mesophilic organisms, are similar to those of Salmonella. Shigella can survive under various environmental conditions, including low-acid conditions.

Shigella can spread rapidly under the crowded and unsanitary conditions often found in such places as summer camps, refugee camps, and camps for migrant workers, and at mass gatherings, such as music festivals. The primary reasons for the spread of Shigella in foods are poor personal hygiene on the part of food handlers, and the use of improper holding temperatures for contaminated foods.  The best preventive measures would be good personal hygiene and health education. Chlorination of water and sanitary disposal of sewage would prevent waterborne outbreaks of shigellosis.

Shigella are closely genetically related to E. coli and could be considered the same species. But, because all strains of Shigella cause severe diarrhea, while most strains of E. coli do not, microbiologists differentiate between the two.

Escherichia coli

E. coli have long been used as indicator organisms because E. coli are present in the normal gut flora of warm-blooded animals.  There are four classes of pathogenic

E. coli -- enteropathogenic (EPEC), enterotoxigenic (ETEC), enteroinvasive (EIEC), and enterohemorrhagic (EHEC).  All four types have been associated with foodborne disease. Although the growth requirements are similar for each class, the diseases differ, so we will cover each separately.

EPEC.  The onset time of EPEC is 17-72 hours, and the disease can last anywhere from six hours to three days. Outbreaks most often affect infants, especially those who are bottle-fed, suggesting that contaminated water is often used to rehydrate infant formulae in underdeveloped countries. Occasionally diarrhea in infants is prolonged, leading to dehydration, electrolyte imbalance and death. Diarrhea can be either watery or bloody.  A 50% mortality rate has been reported in third-world countries.

EPEC is highly infectious for infants, and the infectious dose is presumably very low. In the few documented cases of adult disease, the infective dose was greater than 1,000,000 total. Symptomatic and asymptomatic human carriers are believed to be a principle reservoir of EPEC. Many outbreaks of infantile diarrhea due to EPEC were reported in the 1950s, but these have largely disappeared, due to improvements in sanitation and hygiene. EPEC has been implicated in day-care and nursery outbreaks, and some cases of travelers' diarrhea. Examples of outbreaks affecting adults include an outbreak in Sweden where EPEC was isolated from both the water supply and the feces of ill individuals, an outbreak in Britain linked to cold pork, and another outbreak in Britain related to meat pie.  Immunity might explain the rare occurrence of EPEC illness in adults.

ETEC.  ETEC is perhaps the most widely known.  It is also commonly called traveler’s diarrhea. ETEC frequently causes diarrhea in infants in less developed countries, and in visitors from industrialized countries. The most frequent symptoms include watery diarrhea, abdominal cramps, low-grade fever, nausea, vomiting, and malaise.

Volunteer feeding studies show that a relatively large dose -- 100 million to 10 billion bacteria -- is probably necessary for infection. The onset of symptoms ranges between 8 and 44 hours. With a high dose, diarrhea can be induced within 24 hours. Infants might require fewer organisms for infection. The disease can last anywhere from 3-19 days, but is usually self-limiting, in infants or debilitated elderly persons, electrolyte replacement therapy might be necessary.

Contamination of food or water does occasionally lead to outbreaks of ETEC. In 1975, more than 2000 staff members and visitors at a park in Oregon developed gastroenteritis caused by ETEC. The source was the park's water supply, which had been contaminated by raw sewage. In 1983, a multi-state outbreak of ETEC gastroenteritis with 169 reported cases was associated with imported French Brie cheese. As is also the case with EPEC, human carriers are believed to be a principle reservoir.

EIEC.  EIEC resembles Shigella in many ways.  Like Shigella, EIEC produces in humans an invasive, dysenteric form of diarrhea known as bacillary dysentery. The infectious dose of EIEC is believed to be as few as 10 cells.

Dysentery caused by EIEC usually occurs within 12-72 hours following eating contaminated food. This illness is characterized by abdominal cramps, diarrhea, vomiting, fever, chills, and a generalized malaise. Dysentery caused by this organism is generally self-limiting, with no known complications.

EIEC is associated with water, cheese, potato salad, and canned salmon. Again, human carriers are believed to be the reservoir. A major foodborne outbreak in the United States, involving 387 individuals, was traced to imported French Brie and Camembert cheese; it appeared that a water filtering system  was malfunctioning at the time the cheese was produced. A second major outbreak in the U.S. occurred on a cruise ship -- potato salad served at a cold buffet was implicated. The contamination might have occurred through preparation by an infected food handler or through the use of contaminated raw ingredients.

EHEC.  Judging solely by medical records, one might assume that hemorrhagic colitis, caused by EHEC, is not particularly common, but this is probably inaccurate. In the Pacific Northwest, one strain --  E. coli O157:H7 -- is thought to be second only to Salmonella as a cause of bacterial diarrhea. Because of the unmistakable symptoms of profuse, visible blood in severe cases, victims are more likely to seek medical attention, but less severe cases are probably more numerous and less likely to be reported.

Onset of the disease is anywhere from 3-9 days. The infective dose is unknown, but is suspected to be similar to that of Shigella (10 organisms). The illness is characterized by severe cramping (abdominal pain) and diarrhea, which is initially watery but becomes grossly bloody. Occasionally vomiting occurs. Fever is either low-grade or absent.  The illness is usually self-limited and lasts for an average of eight days. Some individuals exhibit watery diarrhea only.

All people are believed to be susceptible to hemorrhagic colitis. Some victims, particularly the very young, have developed hemolytic uremic syndrome (HU.S.), characterized by renal failure and hemolytic anemia. The disease can lead to permanent loss of kidney function. In older adults, this illness can have a mortality rate as high as 50%.

The intestinal tract of cattle and other food animals are  reservoirs for EHEC. EHEC has been associated with ground beef, raw milk, fermented sausage, apple cider, unpasteurized apple juice, mayonnaise, water, raw vegetables, and club sandwiches. In 1986, an outbreak of hemorrhagic colitis occurred in Washington. Thirty-seven (37) people, ages 11 months to 78 years, developed diarrhea traced to E. coli O157:H7.  Of the 17 patients hospitalized, two died. Ground beef was implicated.

In 1993, a similar outbreak involving undercooked hamburger occurred in Washington, resulting in 447 illnesses and three deaths. During the same year, several other outbreaks along the Pacific Coast were reported; these involved salad bars, raw milk, a church dinner, and a Mexican fast food chain. The most recent outbreak again occurred in Washington in October 1996. The product implicated was unpasteurized apple juice; 70 people became ill, and there was one death.

The growth and survival conditions and controls are similar for all four classes of E. coli so these conditions will be discussed collectively.  E. coli are mesophilic organisms.  They grow best at moderate temperatures, at moderate pH, and in conditions of high water activity. It has, however, been shown that some E. coli strains are very tolerant of acidic environments and freezing.

Food might be contaminated by infected food handlers who practice poor personal hygiene or by contact with water contaminated by human sewage. Control measures to prevent food poisoning, therefore, include educating food workers on safe food handling techniques and proper personal hygiene, properly heating foods, and holding foods under appropriate temperature controls. Additionally, untreated human sewage should never be used to fertilize vegetables and crops used for human consumption, nor should unchlorinated water be used for cleaning food-contact surfaces.

Prevention of fecal contamination during the slaughter and processing of foods of animal origin is paramount to control foodborne infection of EHEC. Foods of animal origin should be heated sufficiently to kill the organism. Consumers should avoid eating raw or partially cooked meats and poultry and drinking unpasteurized milk or fruit juices.

Vibrios

There are many species of Vibrios, but only four will be covered -- Vibrio parahaemolyticus; Vibrio cholerae 01; Vibrio cholerae non-01; and Vibrio vulnificus. 

Vibrio parahaemolyticus.  V. parahaemolyticus is naturally occurring in estuaries and other coastal waters. Implicated foods are fish and shellfish that are raw, improperly cooked or recontaminated after cooking. The symptoms associated with Vibrio parahaemolyticus are diarrhea, abdominal cramps, nausea, vomiting, and fever. Illness is usually mild or moderate, with onset in 4-96 hours, and it lasts for 2.5 days. It can also result in septicemia. The infective dose is one million organisms.

Vibrio cholerae 01.  Epidemic cholera is caused by Vibrio cholerae 01. Onset ranges from 6 hours to 5 days, and symptoms vary from mild, watery diarrhea to acute diarrhea with characteristic rice water stools.  Illness can include abdominal cramps, nausea, vomiting, dehydration and shock, while severe fluid and electrolyte loss might result in death.  The infective dose -- determined by healthy human volunteer feeding studies – is one million organisms. Poor sanitation and contaminated water supplies spread the disease. Proper sewage treatment is responsible for the near­eradication of epidemic cholera in the U.S.  At present, about 20 cases are reported per year, and these are usually the result of travelers returning from developing countries. The organism is found in sewage-contaminated water, and has been associated with various feces-contaminated foods and beverages, including seafood.

Vibrio cholerae non-01.  V. cholerae non-01 is generally a less severe gastroenteritis than that caused by V. cholerae 01. The symptoms are diarrhea, abdominal cramps, fever, nausea, and vomiting. Onset of symptoms occurs between 6 hours and 3 days, and these symptoms can last from 6-7 days. This organism can also cause septicemia.  As with V. parahaemolyticus, the reservoir for this organism is estuarine water. Illness is associated with raw oysters, but the bacterium has also been found in crabs.

Vibrio vulnificus.  V. vulnificus is an estuarine species that can cause wound infections, gastroenteritis, or primary septicemia. It is one of the most severe foodborne infectious diseases, with a fatality rate of 50% for those who contract septicemia. Healthy individuals are most often susceptible to gastroenteritis, while high-risk individuals (those with cirrhosis or other liver disease, diabetes, leukemia or immunosuppression) are particularly susceptible to primary septicemia and so should not eat raw shellfish. The onset of fever, chills, and nausea can occur within 24-48 hours; where septicemia develops, death has been reported to occur within 36 hours. There have been no major outbreaks -- only sporadic individual cases, usually during the warm weather months. Again, as with V. parahaemolyticus, this organism occurs naturally in estuarine waters. So far, only oysters from the Gulf of Mexico have been implicated in illness, but the organism itself has been isolated from both the Atlantic and Pacific Oceans.

Like the other gram negatives, Vibrios are mesophilic and require relatively warm temperatures, high water activity and neutral pH for growth; unlike the others, they also require some salt for growth, and are quite salt-tolerant. They are, however, easily eliminated by a mild heat treatment.

All the Vibrios can be controlled by thorough cooking and the prevention of cross-contamination afterward. Proper refrigeration prevents proliferation, which is particularly important because of the short generation times for these species. The high infective dose of V. parahaemolyticus makes such control even more vital. To guard against cholerae, food handlers should know the source of their product and be cautious about importing from countries experiencing an epidemic.

GRAM POSITIVE RODS AND COCCI

Bacillus cereus 

Bacillus cereus is a gram-positive, aerobic spore former that causes an intoxication. Two types of toxins can be produced -- one results in diarrheal syndrome and the other in the emetic syndrome. Onset for the diarrheal syndrome is 6-15 hours after ingestion, with a duration of 24 hours.  The primary symptom is diarrhea; vomiting is rare. Onset for the emetic syndrome is earlier -- 30 minutes to 6 hours after eating. As with the diarrheal syndrome, the duration is 24 hours. Food counts of B. cereus greater than 10⁶/gram (1,000,000) indicate active growth and a potential hazard to health.

B. cereus is widely distributed throughout the environment. It has been isolated from a variety of foods, including meats, dairy products, vegetables, fish, and rice. The bacteria can also be found in starchy foods such as potato, pasta, and cheese products, and in food mixtures, such as sauces, puddings, soups, casseroles, pastries, and salads.

Fried rice is a leading cause of B. cereus emetic-type food poisoning in the U.S. The organism is frequently present in uncooked rice, and its heat-resistant spores survive cooking. If the rice is then held at room temperature, the spores might germinate and multiply. The toxin produced can survive heating (for instance, stir frying), and many people are unaware that cooked rice is a potentially hazardous food.

This organism will grow at temperatures as low as 39°F, at a pH as low as 4.3, and at salt concentrations as high as 18%. Unlike other pathogens, it is an aerobe, and will grow only in the presence of oxygen. Both the spores and the emetic toxin are heat-resistant.

Unlike B. cereus, Listeria monocytogenes is not easily controlled by refrigeration. Listeriosis, the disease caused by this organism, can produce mild flu-like symptoms in healthy individuals. In susceptible individuals, including pregnant women, newborns, and the immunocompromised, the organism might enter the blood stream, resulting in septicemia. Ultimately, listeriosis can result in meningitis, encephalitis, spontaneous abortion, and stillbirth. The onset of disease might range from a few days to three weeks. The infectious dose is unknown.

Listeria monocytogenes 

L. monocytogenes can be isolated from soil, silage, and other environmental sources. It can also be found in man-made environments, such as food processing establishments. Generally, the drier the environment, the less likely the environmental will harbor this organism.

L. monocytogenes has been associated with raw or inadequately pasteurized milk, cheeses (especially soft-ripened types), ice cream, raw vegetables, fermented sausages, raw and cooked poultry, raw meats, and raw and smoked fish. In 1985, Mexican-style cheese led to at least 46 stillbirths in California. The consumption of large quantities of smoked mussels in New Zealand is reported to have caused two women to experience spontaneous abortions. The CDC has linked  listeriosis with eating raw hot dogs or undercooked chicken.

L. monocytogenes is a psychotropic facultative anaerobe. It can survive some degree of thermal processing, but can be destroyed by  cooking to an internal temperature of 158°F for two minutes. It can also grow at refrigerated temperatures below 31°F. Reportedly, it has a doubling time of 1.5 days at 40°F.

There is nothing unusual about Listeria’s pH and water activity range for growth.

L. monocytogenes is salt-tolerant: it can grow in up to 10% salt and has been known to survive in 30% salt. It is also nitrite-tolerant.

Prevention of recontamination after cooking is a necessary control; even if the product has received thermal processing adequate to inactivate L. monocytogenes, the widespread nature of the organism provides the opportunity for recontamination. Furthermore, if the heat treatment has destroyed the competing microflora, L. monocytogenes might find itself in a suitable environment without competition.

Clostridium perfringens 

Clostridium perfringens is an anaerobic spore former and is a common cause of foodborne gastroenteritis. Perfringens poisoning is characterized by intense abdominal cramps and diarrhea, which begin 8-22 hours after eating contaminated food. The food must contain large numbers (100,000,000 or more) of the bacteria in order to produce toxin in the intestine. The illness is usually over within 24 hours, but less severe symptoms might persist in some individuals for 1-2 weeks. A few deaths have been reported as a result of dehydration and other complications.

CDC estimates that there are 10,000 cases per year. Of these, approximately 1,200 are reported. The large number of cases and the small number of outbreaks are jointly attributable to institutional feeding, such as school cafeterias and nursing homes. Perfringens poisoning most frequently occurs in the young and older adults.

C. perfringens is widely distributed in the environment and is frequently found in the intestines of humans and many domestic and feral animals. Spores of the organism persist in soil and sediments. C. perfringens has been found in beef, pork, lamb, chicken, turkey, stews, casseroles and gravy. In one 1984 outbreak involving 77 prison inmates, the implicated food was roast beef.  Soon afterward, there was a second outbreak which involved many of the same people, and on that occasion, the food implicated was ham. The cause in these instances was determined to have been inadequate refrigeration and insufficient reheating of the implicated foods.

In 1985, a large outbreak of C. perfringens gastroenteritis occurred among factory workers in Connecticut, and some 600 employees were affected. In that case, gravy that was prepared 12-24 hours before serving and inadequately cooled was implicated.

Clostridium perfringens is a mesophilic organism. Because  it is also a spore-former, it is quite resistant to heat, and temperatures for growth range from 50°F to 125°F. The pH, water activity and salt ranges for growth are fairly typical.

Cooking the spores does not kill them.  Cooking encourages them to germinate when the food reaches suitable temperature. Rapid, uniform cooling after cooking is critical. In virtually all outbreaks, the principal cause of perfringens poisoning is failure to properly refrigerate previously cooked foods, especially when it is prepared in large portions. Proper hot-holding (above 135°F)  and adequate reheating of cooked, chilled foods (to a minimum internal temperature of 75°F) are also necessary controls. Educating food handlers remains a critical aspect of control.

C. botulinum 

Like perfringens, C. botulinum is an anaerobic spore-former. There are seven types of C. botulinum -- A, B, C, D, E, F, and G -- but the types that will discussed be are type A, which represents a group called proteolytic botulinum, and type E, which represents the nonproteolytic group. The reason for the distinction is the proteolytic organisms' ability to breakdown protein.

This organism is one of the most lethal foodborne pathogens. The infectious dose is exceedingly low; a few nanograms of toxin can cause illness, and everyone is susceptible. Typically, the onset might be from 18-36 hours after eating contaminated food, but this can vary from 4 hours to 8 days. Symptoms include weakness and vertigo, followed by double vision and progressive difficulty in speaking, breathing, and swallowing. There might also be abdominal distention and constipation. The toxin eventually causes paralysis, which progresses symmetrically downward, starting with the eyes and face, and proceeding to the throat, chest, and extremities. When the diaphragm and chest muscles become involved, respiration is inhibited, and death from asphyxia results. Treatment includes early administration of antitoxin and mechanical breathing assistance. Mortality is high; without antitoxin, death is almost certain.

There is a variation of botulism known as infant botulism. In this case, the toxin is formed in the intestinal tract rather than preformed in the food. Honey is the only food that has been definitely linked to this disease, and it has occurred only in infants. The symptoms begin with constipation, followed by loss of appetite, lethargy, general weakness, pooled oral secretions, altered cry, and loss of head control, which is striking. Given the potential danger, one should never feed an infant honey.

C. botulinum is widely distributed in nature and can be found in soils, sediments from streams, lakes and coastal waters, the intestinal tracts of fish and mammals, and the gills and viscera of crabs and other shellfish. Type E is most prevalent in fresh water and marine environments, while type A is generally found terrestrially.

C. botulinum has been a problem in a wide variety of foods -- canned foods, acidified foods, smoked and uneviscerated fish, stuffed eggplant, garlic-in-oil, baked potatoes, sauteed onions, black bean dip, meat products, and marscapone cheese.

Two outbreaks in the 1960’s involved vacuum-packaged fish (smoked ciscos and smoked chubs). The causative agent in each case was C. botulinum type E. The food was packed without nitrites, with low levels of salt, and were temperature-abused during distribution, all of which contributed to the formation of the toxin. There were no obvious signs of spoilage because aerobic spoilage microorganisms were inhibited by the vacuum packaging, and because type E does not produce any offensive odors.

In 1987, there were eight cases in which kapchunka -- an uneviscerated, salted, air-dried whitefish ­- was implicated. It was believed that the fish contained low levels of salt during air drying at room temperature, which allowed for the toxin formation. The outbreak resulted in one death. Three cases of botulism in New York were traced to chopped garlic bottled in oil, which had been held at room temperature for several months before it was opened. Presumably, the oil created an anaerobic environment.

Type A and type E vary in their growth requirements. Minimum growth temperature for type A is 50°F, while type E will tolerate conditions down to 38°F. Type A’s minimum water activity is 0.94, and type E’s is -.97 -- a small difference but important when controlling the organism.  The acid-tolerance of type A is reached at a pH of 4.6, while type E can grow at a pH of 5. And type A is more salt-tolerant; it can handle up to 10%, while 5% is sufficient to stop the growth of type E.

Although the vegetative cells are susceptible to heat, the spores are heat-resistant and able to survive many adverse environmental conditions. Type A and type E differ in the heat-resistance of their spores.  Compared to type E, type A's resistance is relatively high. By contrast, the neurotoxin produced by C. botulinum is not resistant to heat, and can be inactivated by heating for 10 minutes at 176°F.

There are two primary strategies to control C. botulinum. The first is destruction of the spores by heat (thermal processing). The second is to alter the food to inhibit toxin production ­- something that can be achieved by acidification, controlling water activity, adding salt and/or preservatives, and refrigeration. Water activity, salt, and pH can each be individually considered a full barrier to growth, but very often these single barriers -- a pH of 4.6 or 10% salt -- are not used because they result in a product which is unacceptable to consumers. For this reason, multiple barriers are used.

One example of a product using multiple barriers is pasteurized crabmeat stored under refrigeration.  Type E is destroyed by the pasteurization process, while type A is controlled by refrigerated storage. (NOTE:  Type E is more sensitive to heat, while type A’s minimum growth temperature is 50°F.)

Another example of multiple barriers is hot-smoked, vacuum-packed fish. Vacuum-packaging provides the anaerobic environment necessary for the growth of C. botulinum, even as it inhibits the normal aerobic spoilage flora that would otherwise offer competition and exhibit signs of spoilage. So heat is used to weaken the spores of type E, which are then further controlled by the use of salt, sometimes in combination with nitrites. Finally, the spores of type A are controlled by refrigeration.

Vacuum-packaging of foods that are minimally processed, like sous vide foods, allows the survival of C. botulinum spores while wiping out competing microflora. If no control barriers are present, the C. botulinum might grow and produce toxin, particularly if there is temperature abuse. Given the frequency of temperature abuse documented at the retail and consumer levels, this process is safe only if temperatures are carefully controlled to below 38^oF throughout distribution.

Vacuum-packing is also used to extend the shelf-life of the product. Because  this provides additional time for toxin development, such food must be considered a high risk. Controls can be used to prevent the recurrence of such incidents as the 1987 outbreak caused by uneviserated fish. Any seafood product which will be preserved using salt, drying, pickling, or fermentation must be eviscerated prior to processing; the only exception is small fish (less than five inches in length), which will instead be processed to inhibit the formation of C. botulinum toxin -- something that can be done by  maintaining a water phase salt of 10%, a water activity of below 0.85, or a pH of 4.6 or less.

Salmonella 

Staphylococcus. aureus is a gram-positive cocci that grows in irregular clusters and produces a highly heat-stable toxin. Staphylococcal food poisoning is one of the most economically important foodborne diseases in the U.S., costing approximately $1.5 billion each year in medical expenses and loss of productivity.

Onset is rapid, usually within four hours of ingestion, and the most common symptoms are nausea, vomiting, abdominal cramps, diarrhea, and prostration. Recovery usually takes two days, but can take longer in severe cases. S. aureus food poisoning is usually considered a mild, self-limiting illness with a low mortality rate; however, death has been known to occur among infants, older adults, and severely debilitated individuals. The infective dose is less than 1.0 microgram of toxin, and this toxin level is reached when the S. aureus population reaches 100,000 cells per gram in the food.

S. aureus can be found in air, dust, sewage, and water, although humans and animals are the primary reservoirs. S. aureus is present in and on the nasal passages, throats, hair and skin of at least one out of two healthy individuals. Food handlers are the main source of contamination, but food equipment and the environment itself can also be sources of the organism.

Foods associated with S. aureus include poultry, meat, salads, bakery products, sandwiches, and dairy products. Due to poor hygiene and temperature abuse, a number of outbreaks have been associated with cream-filled pastries and salads such as egg, chicken, tuna, potato, and macaroni.

S. aureus grows and produces toxin at the lowest water activity (0.85) of any food pathogen. Like type A botulinum and Listeria, S. aureus is salt-tolerant and will produce toxin at 10%.

Foods that require considerable handling during preparation and that are kept at slightly elevated temperatures after preparation are frequently involved in staphylococcal food poisoning. And, while S. aureus does not compete well with the bacteria normally found in raw foods, it will grow both in cooked food and in salted food where the salt inhibits spoilage bacteria. Because S. aureus is a facultative anaerobe, reduced oxygen packaging can also give it a competitive advantage.

The best way to control S. aureus is to ensure proper employee hygiene and to minimize exposure to uncontrolled temperatures. While the organism can be killed by heat, the toxin cannot be destroyed even by thermal processing.

Prepared by: 

Angela M. Fraser, Ph.D., Associate Professor/Food Safety Education Specialist, NC State University.  All content was adapted from the FDA course “Food Microbiological Control” prepared in 1998.