Bacillus cereus Food Poisoning (page 1)
Bacillus cereus spore stain
Bacillus cereus has been recognized as an agent of food poisoning since 1955. There are only a few outbreaks a year reported by CDC. Between 1972 and 1986, 52 outbreaks of food-borne disease associated with B. cereus were reported to the CDC (in 2003, there were two), but this is thought to represent only 2% of the total cases which have occurred during these periods. It is not a reportable disease, and usually goes undiagnosed.
B. cereus causes two types of food-borne illnesses. One type is characterized by nausea and vomiting and abdominal cramps and has an incubation period of 1 to 6 hours. It resembles Staphylococcus aureus (staph) food poisoning in its symptoms and incubation period. This is the "short-incubation" or emetic formof the disease.
The second type is manifested primarily by abdominal cramps and diarrhea following an incubation period of 8 to 16 hours. Diarrhea may be a small volume or profuse and watery. This type is referred to as the "long-incubation" ordiarrheal form of the disease, and it resembles food poisoning caused byClostridium perfringens. In either type, the illness usually lasts less than 24 hours after onset. In a few patients symptoms may last longer.
The short-incubation form is caused by a preformed, heat-stable emetic toxin, ETE. The mechanism and site of action of this toxin are unknown, although the small molecule forms ion channels and holes in membranes. The long-incubation form of illness is mediated by the heat-labile diarrheagenic enterotoxin Nheand/or hemolytic enterotoxin HBL, which cause intestinal fluid secretion, probably by several mechanisms, including pore formation and activation of adenylate cyclase enzymes.
TOXINS
Bacillus cereus produces one emetic toxin (ETE) and three different enterotoxins: HBL, Nhe,and EntK.
Two of the three enterotoxins are involved in food poisoning. They both consist of three different protein subunits that act together. One of these enterotoxins (HBL) is also a hemolysin; the second enterotoxin (Nhe) is not a hemolysin. The third enterotoxin (EntK) is a single component protein that has not been shown to be involved in food poisoning. All three enterotoxins are cytotoxic and cell membrane active toxins that will make holes or channels in membranes.
The emetic toxin (ETE) is a ring-shaped structure of three repeats of four amino acids with a molecular weight of 1.2 kDa. It is a K+ ionophoric channel, highly resistant to pH between 2 and 11, to heat, and to proteolytic cleavage.
The nonhemolytic enterotoxin (Nhe) is one of the three-component enterotoxins responsible for diarrhea in Bacillus cereus food poisoning. Nhe is composed of NheA, NheB and NheC. The three genes encoding the Nhe components constitute an operon. The nhe genes have been cloned separately, and expressed in eitherBacillus subtilis or Escherichia coli. Separate expression showed that all three components are required for biological activity.
The hemolytic enterotoxin, HBL, is encoded by the hblCDA operon. The three protein components, L1, L2 and B, constitute a hemolysin. B is for binding; L1 and L2 are lytic components. This toxin also has dermonecrotic and vascular permeability activities, and it causes fluid accumulation in rabbit ileal loops.
EPIDEMIOLOGY
B. cereus food poisoning occurs year-round and is without any particular geographic distribution. The short-incubation form is most often associated with rice dishes that have been cooked and then held at warm temperatures for several hours. It is often associated with Mexican and Chinese restaurants, but in one reported outbreak, macaroni and cheese made from powdered milk turned out to be the source of the bacterium. "Mac and cheese"- you can't get much more American than that.
Long-incubation B. cereus food poisoning is frequently associated with meat or vegetable-containing foods after cooking. The bacterium has been isolated from 50% of dried beans and cereals and from 25% of dried foods such as spices, seasoning mixes and potatoes. One outbreak of the long-incubation form was traced to a "meals-on-wheels" program in which food was held above room temperature for a prolonged period before delivery to consumers.
The short-incubation or emetic form of the disease is diagnosed by the isolation of B. cereus from the incriminated food. The long-incubation or diarrheal form is diagnosed by isolation of the organism from stool and food. Isolation from stools alone is not sufficient because 14% of healthy adults have been reported to have transient gastrointestinal colonization with B. cereus. Because B. cereusgastroenteritis is generally a benign, self-limited illness, antimicrobial agents are of no value in management. Since the bacteria grow best at temperatures ranging from 40 to 140°F, infection may be prevented if cold food is refrigerated and if hot food is held at greater than 140°F before serving.
CASE STUDY:
Summary and Analysis of a Report of Bacillus cereus Food Poisoning Associated with Fried Rice at Two Child Day Care Centers -- Virginia, 1993 (from CDC).
Summary:
On July 21, 1993, a regional public health facility received reports of acute gastrointestinal illness that occurred among children and staff at two jointly owned child day care centers following a catered lunch.
The catered lunch was served on July 21 to 82 children aged less than or equal to 6 years, and to nine staff; dietary histories were obtained for 80 persons. 67 ate the catered lunch. A case was defined as vomiting by a person who was present at either day care center on July 21. Fourteen (21%) persons who ate the lunch became ill, compared with none of 13 who did not. Symptoms included nausea (71%), abdominal cramps or pain (36%), and diarrhea (14%). Twelve of the 14 cases occurred among children aged 2.5-5 years, and two occurred among staff. The median incubation period was 2 hours (range: 1.5-3.5 hours). Symptoms resolved a median of 4 hours after onset (range: 1.5-22 hours).
Chicken fried rice prepared at a local restaurant was the only food significantly associated with illness; illness occurred in 14 (29%) of 48 persons who ate chicken fried rice, compared with none of 16 who did not.
The rice had been cooked the night of July 20 and cooled at room temperature before refrigeration. On the morning of the lunch, the rice was pan-fried in oil with pieces of cooked chicken, delivered to the day care centers at approximately 10:30 a.m., held without refrigeration, and served at noon without reheating.
Following the outbreak, health officials recommended to day care staff and restaurant food handlers that the practice of cooling rice or any food at room temperature be discontinued, food be maintained at proper temperatures (i.e., below 41 F {5 C} or above 140 F {60 C}), and a thermometer be used to verify food temperatures.
Analysis:
The emetic ("short incubation") form of the disease, which occurred in this outbreak, is mediated by a highly stable toxin that survives high temperatures and exposure to trypsin, pepsin, and pH extremes; the diarrheal syndrome is mediated by a heat- and acid-labile enterotoxin that is sensitive to proteolytic enzymes.
The diagnosis of B. cereus food poisoning can be confirmed by the isolation of greater than or equal to 105 B. cereus organisms per gram from epidemiologically- implicated food. Underreporting of such outbreaks is likely because illness associated with B. cereus is usually self-limiting and not severe. In addition, findings of a recent survey about culture practices for outbreaks of apparent foodborne illness indicate that 20% of state public health laboratories do not make B. cereus testing routinely available.
Fried rice is a leading cause of B. cereus emetic-type foodpoisoning in the United States. B. cereus is frequently present in uncooked rice, and heat-resistant spores may survive cooking. If cooked rice is subsequently held at room temperature, vegetative forms multiply, and heat-stable toxin is produced that can survive brief heating, such as stir frying. In the outbreak described in this report, vegetative forms of the organism probably multiplied at the restaurant and the day care centers while the rice was held at room temperature.
The day care staff and restaurant food handlers in this report were unaware that cooked rice was a potentially hazardous food. This report underscores the ongoing need to educate food handlers about basic practices for safe food handling.
Bacillus cereus colonies on blood agar.
Bacillus cereus spore stain
Bacillus cereus has been recognized as an agent of food poisoning since 1955. There are only a few outbreaks a year reported by CDC. Between 1972 and 1986, 52 outbreaks of food-borne disease associated with B. cereus were reported to the CDC (in 2003, there were two), but this is thought to represent only 2% of the total cases which have occurred during these periods. It is not a reportable disease, and usually goes undiagnosed.
B. cereus causes two types of food-borne illnesses. One type is characterized by nausea and vomiting and abdominal cramps and has an incubation period of 1 to 6 hours. It resembles Staphylococcus aureus (staph) food poisoning in its symptoms and incubation period. This is the "short-incubation" or emetic formof the disease.
The second type is manifested primarily by abdominal cramps and diarrhea following an incubation period of 8 to 16 hours. Diarrhea may be a small volume or profuse and watery. This type is referred to as the "long-incubation" ordiarrheal form of the disease, and it resembles food poisoning caused byClostridium perfringens. In either type, the illness usually lasts less than 24 hours after onset. In a few patients symptoms may last longer.
The short-incubation form is caused by a preformed, heat-stable emetic toxin, ETE. The mechanism and site of action of this toxin are unknown, although the small molecule forms ion channels and holes in membranes. The long-incubation form of illness is mediated by the heat-labile diarrheagenic enterotoxin Nheand/or hemolytic enterotoxin HBL, which cause intestinal fluid secretion, probably by several mechanisms, including pore formation and activation of adenylate cyclase enzymes.
TOXINS
Bacillus cereus produces one emetic toxin (ETE) and three different enterotoxins: HBL, Nhe,and EntK.
Two of the three enterotoxins are involved in food poisoning. They both consist of three different protein subunits that act together. One of these enterotoxins (HBL) is also a hemolysin; the second enterotoxin (Nhe) is not a hemolysin. The third enterotoxin (EntK) is a single component protein that has not been shown to be involved in food poisoning. All three enterotoxins are cytotoxic and cell membrane active toxins that will make holes or channels in membranes.
The emetic toxin (ETE) is a ring-shaped structure of three repeats of four amino acids with a molecular weight of 1.2 kDa. It is a K+ ionophoric channel, highly resistant to pH between 2 and 11, to heat, and to proteolytic cleavage.
The nonhemolytic enterotoxin (Nhe) is one of the three-component enterotoxins responsible for diarrhea in Bacillus cereus food poisoning. Nhe is composed of NheA, NheB and NheC. The three genes encoding the Nhe components constitute an operon. The nhe genes have been cloned separately, and expressed in eitherBacillus subtilis or Escherichia coli. Separate expression showed that all three components are required for biological activity.
The hemolytic enterotoxin, HBL, is encoded by the hblCDA operon. The three protein components, L1, L2 and B, constitute a hemolysin. B is for binding; L1 and L2 are lytic components. This toxin also has dermonecrotic and vascular permeability activities, and it causes fluid accumulation in rabbit ileal loops.
EPIDEMIOLOGY
B. cereus food poisoning occurs year-round and is without any particular geographic distribution. The short-incubation form is most often associated with rice dishes that have been cooked and then held at warm temperatures for several hours. It is often associated with Mexican and Chinese restaurants, but in one reported outbreak, macaroni and cheese made from powdered milk turned out to be the source of the bacterium. "Mac and cheese"- you can't get much more American than that.
Long-incubation B. cereus food poisoning is frequently associated with meat or vegetable-containing foods after cooking. The bacterium has been isolated from 50% of dried beans and cereals and from 25% of dried foods such as spices, seasoning mixes and potatoes. One outbreak of the long-incubation form was traced to a "meals-on-wheels" program in which food was held above room temperature for a prolonged period before delivery to consumers.
The short-incubation or emetic form of the disease is diagnosed by the isolation of B. cereus from the incriminated food. The long-incubation or diarrheal form is diagnosed by isolation of the organism from stool and food. Isolation from stools alone is not sufficient because 14% of healthy adults have been reported to have transient gastrointestinal colonization with B. cereus. Because B. cereusgastroenteritis is generally a benign, self-limited illness, antimicrobial agents are of no value in management. Since the bacteria grow best at temperatures ranging from 40 to 140°F, infection may be prevented if cold food is refrigerated and if hot food is held at greater than 140°F before serving.
CASE STUDY:
Summary and Analysis of a Report of Bacillus cereus Food Poisoning Associated with Fried Rice at Two Child Day Care Centers -- Virginia, 1993 (from CDC).
Summary:
On July 21, 1993, a regional public health facility received reports of acute gastrointestinal illness that occurred among children and staff at two jointly owned child day care centers following a catered lunch.
The catered lunch was served on July 21 to 82 children aged less than or equal to 6 years, and to nine staff; dietary histories were obtained for 80 persons. 67 ate the catered lunch. A case was defined as vomiting by a person who was present at either day care center on July 21. Fourteen (21%) persons who ate the lunch became ill, compared with none of 13 who did not. Symptoms included nausea (71%), abdominal cramps or pain (36%), and diarrhea (14%). Twelve of the 14 cases occurred among children aged 2.5-5 years, and two occurred among staff. The median incubation period was 2 hours (range: 1.5-3.5 hours). Symptoms resolved a median of 4 hours after onset (range: 1.5-22 hours).
Chicken fried rice prepared at a local restaurant was the only food significantly associated with illness; illness occurred in 14 (29%) of 48 persons who ate chicken fried rice, compared with none of 16 who did not.
The rice had been cooked the night of July 20 and cooled at room temperature before refrigeration. On the morning of the lunch, the rice was pan-fried in oil with pieces of cooked chicken, delivered to the day care centers at approximately 10:30 a.m., held without refrigeration, and served at noon without reheating.
Following the outbreak, health officials recommended to day care staff and restaurant food handlers that the practice of cooling rice or any food at room temperature be discontinued, food be maintained at proper temperatures (i.e., below 41 F {5 C} or above 140 F {60 C}), and a thermometer be used to verify food temperatures.
Analysis:
The emetic ("short incubation") form of the disease, which occurred in this outbreak, is mediated by a highly stable toxin that survives high temperatures and exposure to trypsin, pepsin, and pH extremes; the diarrheal syndrome is mediated by a heat- and acid-labile enterotoxin that is sensitive to proteolytic enzymes.
The diagnosis of B. cereus food poisoning can be confirmed by the isolation of greater than or equal to 105 B. cereus organisms per gram from epidemiologically- implicated food. Underreporting of such outbreaks is likely because illness associated with B. cereus is usually self-limiting and not severe. In addition, findings of a recent survey about culture practices for outbreaks of apparent foodborne illness indicate that 20% of state public health laboratories do not make B. cereus testing routinely available.
Fried rice is a leading cause of B. cereus emetic-type foodpoisoning in the United States. B. cereus is frequently present in uncooked rice, and heat-resistant spores may survive cooking. If cooked rice is subsequently held at room temperature, vegetative forms multiply, and heat-stable toxin is produced that can survive brief heating, such as stir frying. In the outbreak described in this report, vegetative forms of the organism probably multiplied at the restaurant and the day care centers while the rice was held at room temperature.
The day care staff and restaurant food handlers in this report were unaware that cooked rice was a potentially hazardous food. This report underscores the ongoing need to educate food handlers about basic practices for safe food handling.
Bacillus cereus colonies on blood agar.
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