Bacillus Cereus Food Poisoning Biology Essay


Recent outbreaks caused by this pathogen that have made the headlines include the fatal family outbreak associated with food Poisoning in August 2003. Five children fell sick to the illness after consuming pasta salad. Approximately after 6 hours of the meal the youngest started vomiting. She was taken to the nearby hospital emergency room after she complained of having respiratory problems. As the family arrived at the hospital, the rest of the children started vomiting too. All children had different severity of symptoms and needed immediate medical attention in a bigger and more convenient hospital. They were taken to the University Hospital in Leuven. As they were transferring the children to the hospital the youngest fell into a coma because of her severe pulmonary hemorrhage and so she needed continuous resuscitation. After their arrival for about twenty minutes she was pronounced dead as a result of diffusive bleeding, and severe muscle cramps. An autopsy was carried out and B. cereus was found in the contents of her gut. Her spleen was also infected, most likely by the translocation of bacteria after she had died. Her liver had microvascular and extensive coagulation necrosis and also showed metabolic acidosis and liver failure. The older boy was also affected severely by the bacteria, although his symptoms were lighter than his younger sister. He was moved to the pediatric intensive care unit, where he was put on a mechanical ventilation and invasive hemodynamic observation. His blood lactate levels decreased slowly after he underwent fluid resuscitation. The rest of the children had mild symptoms and recovered fully quickly.7

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Another major outbreak in another part of the world was between 3rd and 5th of May 2008 in Oman. Fifty eight people were reported sick with gastroenteritis in a referral hospital. Patients and their attendants were among the reported cases. They all had consumed meals at the hospital the prior day. An investigation team interviewed the patients and their attendants about their symptoms and what they had eaten the previous day while they were in the hospital. Samples were taken from the food in the kitchen and kitchen staff fecal matter was also sampled and those affected and then these were cultured. A kitchen environmental assessment was conducted. The majority of those affected were adult females, mainly patient attendants. The symptoms among those infected were mainly diarrhea and little vomiting. Most of them had mild symptoms and so needed mild medication and little attention except for two patient attendants who needed intravenous rehydration. The staff had violated many of the basic food hygiene protocols in the kitchen. B. cereus was detected in 25 % of patients and 76% of kitchen staff, and 57% of the kitchen food samples.1

B. cereus colonies are isolated by the presence of lecithinase, an enzyme found in B. cereus on MEP agar. A halo appears because of the insoluble lipids released by the action of lecithinase. They can also be isolated by their incapability of fermenting mannitol with the appearance of a pink color as a result of an increase in pH, the phenol red turns pink. When using a blood agar, clear zones appear as a result of Beta-hemolysis of the red blood cells. The ability to lyse red blood cells resembles toxin production in B. cereus strains. Consequent immunological detection of B. cereus toxin is done by removing the nitrocellulose membrane in blood agar plates, as the nitrocellulose membrane starts to appear, indicates the presence of the B. cereus toxins. Detecting B. cereus can be done by PCR and the Tecra VIA. B. cereus contains three genes nheA, nheB, and nheC, encoding the nonhemolytic enterotoxin. So depending on these genes, detection of the different strains can be accomplished. The nheA gene was not detected by PCR, although it can be by the Tecra VIA. This is most likely due to sequence differences among the nheA genes of certain strains. Five different sets of primers are used for PCR-based detection of the bceT gene inferred from the B. cereus B-4ac sequence. B. cereus strains yield PCR products with sizes that correspond to the PCR products of B. cereus B-4ac. The Southern analysis detects all B. cereus strains that are PCR positive with primer sets 113 and/or 114 and not PCR negative strains for all primer sets.2

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B. cereus is transmitted to foods mostly by cross contamination or if that food has not been cooked to a high enough temperature. B. cereus is a common soil inhabitant that is found mostly on harvested crops such as grains, vegetables and fruits. Illnesses are most likely to be associated with ingesting foods that have not been cooked to a temperature that destroys the spores that are heat-resistant or refrigerated not as quick as needed and left out at room temperature.Common sources include Soil, unpasteurized milk, cereals and starch, Herbs and spices as I had mentioned before, a spice example (papirika) was mentioned in the "factors that affect B. cereus" section. Associated foods include meat pies, cooked rice and fried rice, starchy foods (potato, pasta), food mixtures (soups, casseroles, sauces) and puddings.1

According to the type of illness symptoms are different, diarrheal illness and vomiting illness express different symptoms. The diarrheal illness includes watery diarrhea, abdominal pain and cramps, nausea and rarely vomiting. These symptoms usually last 24 hours. The vomiting illness usually lasts less than 24 hours and is characterized by nausea and vomiting. Occasionally abdominal pain and diarrhea may occur. This illness is sometimes mistaken for an illness caused by Staphylococcus aureus. The vomiting illness is also known as the Emetic syndrome. The onset of its symptoms starts within about 1 to 5 hours after consumption of the contaminated food, and the duration of the symptoms is 24 hours at most. It is a bout of nausea and vomiting. The onset of symptoms of the Diarrheic syndrome starts within 8 to 12 hours after consumption of the contaminated food, and the symptoms last also 24 hours at most. The infective dose is a large numbers (more than 105 CFU/gm) of viable B. cereus cells need to be consumed for symptoms of the illness to develop.6

The pathogenic mechanisms of the emetic form of disease are not well understood. After the food contaminated with the bacteria is ingested, the bacteria forms toxins that it requires for its survival. This form is mainly caused when consumption of inappropriately refrigerated rice that had not been cooked at a sufficient temperature and time appropriate to kill the B. cereus spores. Soon spores start to produce a toxin to survive on called cereulide. This toxin is made while the food is refrigerating but does not get destroyed if later heating was used. A person then consumes the rice and soon experiences nausea and vomiting within 1-5 hours after eating. These symptoms are mistaken sometimes for Staphylococcus aureus symptoms. There is little known about this mechanism pathway and further research is required. Sometimes the toxins enter the blood stream and interact with the Vagus nerve leading to emesis ( just as the first outbreak I mentioned). Regarding the mechanism pathway for the diarrheic form of the illness, four enterotoxins are produced by B. cereus, hemolysin BL (HBL), nonhemolytic enterotoxin (NHE), enterotoxin-T and cytotoxin-K, as previously mentioned.

The HBL, NHE, and cytotoxin-K are related to food poisoning outbreaks. HBL and NHE are very similar, they are vegetative growth metabolites. Research has proven that HBL and NHE do not play a role in the pathogenicity of B. cereus when they are produced in the food. Furthermore, the majority of the vegetative cells of B. cereus also do not cause pathogenicity of B. cereus. In most cases the pH in the stomach is too low for vegetative cells to survive. However, consumption of different foods alters the pH in the stomach, even though the pH is mainly low it may get higher enabling the bacteria to survive and even grow. And so, spores are able to survive and enter the intestines with the chyme. As the spores are in the small intestine, they germinate, grow and simulate the production of enterotoxins. This is believed to be the route for the diarrheal syndrome caused by B. cereus. The spores are able to adhere to the intestines epithelial cells. 29

Factors that affect the growth and survival of B. cereus include higher temperatures and high water activity (aw) values. Its temperature range is from 10-49°C and its optimum temperature is 30°C. A temperature increase from 90 to 95 C has a large impact on the inactivation of the bacteria's spores. B. cereus can grow in a pH range of 4.9-9.3, but any pH lower than 4.9 and higher than 9.3 is detrimental to its growth. The lowest reported water activity level for B. cereus growth is 0.93; anything lower will not provide the appropriate free water available for its growth. So for example B. cereus spores in paprika powder could be reduced by 4.5 log10 CFU/g within 6 min at an aw value of 0.88 and heated to product temperatures of 95-100 C. Lowering pH does not result in a significant reduction in the concentration of B. cereus spores. Using homogenizing sterilizer, which mixes heated layers of powdery foods intensively, is a good method of destroying B. cereus bacteria and spores. Also Using high dry heat temperatures have proven to be a good method of destroying B. cereus while it's in foods 22, 26

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Prevention is a major key that benefits a community when outbreaks are nearby. Consumers should pay attention to food recalls and as soon as there is an active recall get rid of any recalled food. Refrigerating cooked foods as soon as there is no need for them will minimize the possibility of emetic illness (cool cooked products to 41°F within 4 hours). Food that is cooked and then frozen should be thawed inside the refrigerator to prevent any contamination that would occur if food was thawed at room temperature. Foods should be stored at sufficient temperatures. Infants should not be fed formula that has been sitting out at room temperature. Produce should be washed thoroughly and handled well. Hands should be washed always before and after handling produce and any other foods I mentioned that were associated with B. cereus. Ground meat products should be thoroughly cooked. 22

Further research: Improved knowledge of the pathogenic mechanism for the emetic form of B. cereus is necessary for two reasons. Firstly, better understanding of the pathogenic mechanism may lead to a better description of the dose-response relationship in humans. Secondly, a better insight into the pathogenic mechanism in combination with quantitative data concerning the occurrence of pathogenic B. cereus may lead to adjusting the tolerance level set for B. cereus in food commodities.2