SALMONELLA FOOD POISONING

Salmonella food poisoning, consistent with all food poisoning, results from the growth of the bacterium in food. This is in contrast to food intoxication, were illness results from the presence of toxin in the food. While food intoxication does not require the growth of the contaminating bacteria to reasonably high numbers, food poisoning does.

Salmonella is a Gram negative, rod-shaped bacterium. The gastrointestinal tracts of man and animals are common sources of the bacterium. Often the bacterium is spread to food by handling the food with improperly washed hands. Thus, proper hygiene is one of the keys to preventing Salmonella food poisoning.

The food poisoning caused by Salmonella is one of about ten bacterial causes of food poisoning. Other involved bacteria are Staphylococcus aureus, Clostridium perfringens, Vibrio parahaemolyticus, and certain types of Escherichia coli. Between 24 and 81 million cases of food borne diarrhea due to Salmonella and other bacteria occur in the United States each year. The economic cost of the illnesses is between 5 and 17 billion dollars.

Poultry, eggs, red meat, diary products, processed meats, cream-based desserts, and salad-type sandwich filling (such as tuna salad or chicken salad) are prime targets for colonization by species of Salmonella. The high protein content of the foodstuffs seems to be one of the reasons for their susceptibility. Contamination is especially facilitated if improperly cooked or raw food is held at an improper storage temperature, for example at room temperature. Proper cooking and storage temperatures will prevent contamination, as Salmonella is destroyed at cooking temperatures above 150° F (65.5 °C) and will not grow at refrigeration temperatures (less than 40°F, or 4.4°C). Also, contamination can result if the food is brought into contact with contaminated surfaces or utensils.

The vulnerable foods offer Salmonella a ready source of nutrients and moisture. If the temperature conditions are right for growth, the increase in numbers of Salmonella can be explosive. For example, from a starting population of a single live bacterium with a division time of 30 minutes, a population of over 500 million bacteria can be generated in just 15 hours.

The ingestion of contaminated foods leads, within hours, to the development of one or all of the following ailments: stomach cramps, vomiting, fever, headache, chills, sweating, fatigue, loss of appetite, and watery or bloody diarrhea. Prolonged diarrhea is dangerous, as the body can be depleted of fluids and salts that are vital for the proper functioning of organs and tissues. The resulting shock to the body can be intolerably lethal to infants and the elderly. As well, there is a possibility that the bacteria can spread from the intestinal tract to the bloodstream, leading to infections in other parts of the body.

There are hundreds are different forms, or strains, of Salmonella, varying in the antigenic composition of their outer surface and in the maladies caused. Concerning food poisoning, Salmonella enteriditis is of particular concern. This strain causes gastroenteritis and other maladies because of several so-called virulence factors the organism is armed with.

One virulence factor is called adhesin. An adhesin is a molecule that functions in the recognition and adhesion of the bacterium to a receptor on the surface of a host cell. In the case of Salmonella, the tube-like structures called fimbriae can perform this function. Other molecules on the surface of the bacterium can be involved also.

Another virulence factor is a compound called lipopolysaccharide (LPS for short). Depending on the structure, LPS can help shield the Salmonella surface from host antibacterial compounds. As well, part of the LPS, can lipid A, can be toxic to the host. The lipid A toxic component is also referred to as endotoxin. Salmonella also produces another toxin called enterotoxin. Other bacteria produce enterotoxin as well. The Salmonella enterotoxin is readily degraded by heat, so proper cooking of food will destroy the activity of the toxin. The enterotoxin remains inside the bacteria, so the toxin concentration increases with the increase in bacterial numbers.

Salmonella is not particularly difficult to identify, as it produces distinctive visual reactions on standard laboratory growth media. For example, on bismuth sulfide media the bacteria produce hydrogen sulfide, which produces jet-black colonies. Unfortunately for the individual who experiences a food poisoning event, the diagnosis is always "after the fact." Knowledge of the cause often comes after the miseries of the poisoning have come and gone. But, in those instances where the spread of the bacteria beyond the gastrointestinal tract has occurred, diagnosis is helpful to treat the infection.

The prospects of eliminating of Salmonella food poisoning using vaccination are being explored. The most promising route is to block the adhesion of the bacteria to host epithelial cells of the intestinal tract. Such a strategy would require the development of a vaccine with long lasting immunity. However, vaccine development efforts will likely be devoted to other illnesses. For the foreseeable future, the best strategy in preventing Salmonella food poisoning will remain the proper cooking of foods and the observance of good hygiene practices when handling food.