The food-poisoning group of organisms include Staphylococcus aureus, Salmonella species, Clostridium perfringens, Bacillus cereus, Clostridium botulinum, and Vibrio parahaemolyticus.
The term “food poisoning” is generally used in a restricted way to exclude clinical entities like typhoid and cholera, but it includes those conditions in which intestinal disturbances commonly occur, resulting in diarrhea and vomiting (with or without other symptoms). It is caused by ingesting either live bacteria or products of bacterial growth - the latter arising after the microbes have multiplied in the food for some hours before it is eaten.
This vital initial period of bacterial multiplication in food is an essential feature in the natural history of all bacterial food poisoning, as it ensures that a vastly increased number of bacteria are present in the food. The “challenge dose” of bacteria (the number that have to be ingested in order to cause disease) is much higher in the food-poisoning group than it is with other diseases, like typhoid and shigellosis, where food merely acts as a vehicle for transmission of the infection. C. botulinum is included in the food-poisoning group, although the symptoms produced here are neurological rather than gastrointestinal, because this pathogen from the soil must grow and multiply for several hours in food before it is able to cause disease.
Crucial factors in the evolution of an outbreak of food poisoning include the type of food involved, the way in which it has been prepared, and the duration and temperature of storage prior to eating. As with most other food-borne infections, the food looks and tastes normal.
Bacterial food poisoning may belong to three different types, depending on the mechanism of pathogenesis observed:
1. In the infective type, as caused by Salmonella, large numbers of live organisms must be consumed. They then invade the cells lining the small intestine, where further multiplication occurs, causing inflammation, diarrhea, and vomiting. The time interval between ingesting the food and developing symptoms (the incubation period) is between 14 and 36 hours.
2. In the toxic type, caused by S. aureus, B. cereus, and C. botulinum, the phase of multiplication in food results in the release of toxins. When ingested, these toxins (preformed in the food) are responsible for producing the symptoms. It is not necessary to ingest the live organisms. Rapid heating of the food may destroy the bacteria but allow heat-stable toxins to remain active. In contrast to the previous type, the incubation period here is fairly short - a few hours - because the preformed toxin is able to act immediately on the gut.
3. The intermediate type, caused by C. perfringens, has an incubation period of 8 to 20 hours. The toxin is released not in the food but in the gut of the host, following ingestion. The release of toxins here coincides with the formation of spores (a resistant form of the bacterium). Prolonged low-temperature cooking or inadequate reheating of infected, leftover meat stews and casseroles increases the occurrence of this disease, because it stimulates sporulation of the C. perfringens in the gut of the host.
It is apparent that three important criteria have to be satisfied for the successful development of food poisoning. These are as follows:
1. Introduction of organisms into the food at some stage during its preparation.
2. Inadequate cooking. Most microbial agents in food are destroyed by cooking at high temperatures for the prescribed length of time. When frozen food, like meat, is cooked before it is completely thawed out, the temperature reached in the center of the meat may not be sufficient to kill the bacteria even when the outside looks well done.
3. Leaving food standing at an ambient temperature for several hours, enabling the bacteria to multiply and attain the infective dose. Refrigeration of food stops multiplication of most of the harmful pathogens, with the exception of species like Yersinia enterocolitica and L. monocytogenes, which are particularly adapted for growth at temperatures of 0 to 4 degrees Celsius (°C). Freezing suspends replication of all pathogens but fails to kill them. After frozen food is allowed to thaw, bacteria may start to multiply again if kept standing at room temperature. Hence, careful attention is important at all stages of food preparation to ensure adequate cooking and prevent contamination. Furthermore, if food is not to be eaten without delay, it should be rapidly chilled before freezing for storage.
Clearly, even contaminated food is not harmful if properly cooked and eaten right away. But when eaten raw, inadequately cooked, or after remaining for a long time in a warm environment, contamined food may transmit disease. Bacteria in food replicate rapidly by binary fission, doubling their numbers progressively at intervals of less than 20 minutes by a simple process of division in which each cell gives rise to two daughter cells. Food provides the ideal nutrient for bacteria, which continue to multiply, so that in just over 3 hours a starting population of 1,000 organisms (which is too low a dose to cause food poisoning) may reach the level of millions and constitute an effective challenge dose. In most outbreaks, food has been left for longer than 3 hours and the starting population may be higher than 1,000. Organisms like Campylobacter, Shigella, and the typhoid bacillus are infective at a much smaller dose level than the food-poisoning group, and in this case it is not necessary for the infected food to remain at room temperature for several hours in order to be able to transmit infection. Indeed, organisms like Campylobacter and viruses do not multiply in food. As already mentioned, in this case, food merely acts as a vehicle for transmission of these agents.
Most pathogenic bacteria are unable to grow in acidic food (pH less than 4.5) or in food with a low moisture content. Similarly, high salt or sugar concentration in preserved food inhibits many bacteria. However, they may survive for long periods of time in dried food products.
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