Salmonellosis
Salmonellosis is the most frequently occurring bacterial food-borne illness.
Salmonellae are gram-negative non-spore-forming rods that ferment glucose, usually with gas, but usually do not ferment lactose or sucrose. They grow over a wider range of temperature, pH, and water activity. They grow well at room temperatures, optimum is about 37oC. The pH range for growth is 4.1 to 9.0. The lowest aw for growth varies with the food but is about 0.93 to 0.95.
The likelihood of infection by consumption of a food containing Salmonellae depends on the resistance of the consumer, the infectiveness of the particular strain of Salmonella. and the number of organisms ingested. Salmonellae can attain considerable numbers in foods without causing detectable alterations in appearance, odor, or even taste. Human beings and animals are directly or indirectly the source of the contamination of foods with salmonellae. The organisms may come from cases of the disease or from carriers.
Most frequently isolated serovars, such as S. typhimurium cause human gastroenteritis.
A large variety of foods are involved in causing outbreaks of Salmonella infections. Most common are various kinds of meats, poultry and products from them, especially if they are held unrefrigerated for long periods. Fresh meats may carry Salmonella bacteria that caused disease in the slaughtered animals or may be contaminated by handlers. Meat products, such as meat pies, hash, sausages, cured meats (ham, bacon, and tongue), sandwiches, and chili, often are allowed to stand at room temperatures, permitting the growth of salmonellae. Milk and milk products, including fresh milk, fermented milks, ice cream, and cheese, have caused infections. Since eggs may carry the Salmonellae, foods made with eggs and not sufficiently cooked or pasteurized may carry live organisms, e.g., pastries filled with cream or custard, cream cakes, etc.
The organisms also may come from cats, dogs, swine, and cattle, but more important sources for foods are poultry and their eggs and rodents. About one-third of all the food products involved in Salmonella outbreaks are meat and poultry products, eggs etc. Infected rodents, rats and mice, may contaminate unprotected foods with their feces and thus spread Salmonella bacteria. Flies may play an important role in the spread of Salmonella, especially from contaminated fecal matter to foods.
Changes in processing, packaging, and compounding of foods and feeds in recent years have resulted in an apparent increase in salmonellosis from these products. Salmonellae have been introduced by the incorporation of cracked and dried eggs in baked goods, candy, ice cream, and convenience foods such as cake and cookie mixes.
Large-scale handling of foods, increase the spread of trouble, precooked foods and food vending machines add to the risk. Feeds, especially those from meat or fish by-products, may carry Salmonellae to poultry or meat animals. The susceptibility of humans varies with the species and strain of the organism and the total numbers of bacteria ingested but in general morbidity is high in any outbreak.
Salmonellosis has a longer incubation period - usually 12 to 36 hr. The principal symptoms of a Salmonella gastrointestinal infection are nausea, vomiting, abdominal pain, and diarrhea that usually appear suddenly. This may be preceded by a headache and chills. Other evidences of the disease are watery, greenish foul-smelling stools, prostration, muscular weakness, faintness, usually a moderate fever, restlessness, twitching, and drowsiness. The mortality is low, being less than 1 percent. The severity and duration vary not only with the amount of food eaten and hence the numbers of Salmonella bacteria ingested and with the individual.
Intensity may vary from slight discomfort and diarrhea to death in 2 to 6 days. Usually, the symptoms persist for 2 to 3 days, followed by uncomplicated recovery, but they may linger for weeks or months. About 0.2 to 5 percent of the patients may become carriers of the Salmonella organism.
The laboratory diagnosis of the disease is difficult unless Salmonella can be isolated from the suspected food and from the stools of individuals.
For the prevention of outbreaks of food-borne Salmonella infections:
(I) avoid contamination of the food with salmonellae from sources such as diseased human beings and animals and carriers and ingredients carrying the organisms, e.g., contaminated eggs
(2) destroy the organisms in foods by heat (or other means) when possible, as by cooking or pasteurization, paying special attention to held-over foods
(3) prevent the growth of Salmonella in foods by adequate refrigeration or by other means.
In the prevention of contamination, care and cleanliness in food handling and preparation are important. The food handlers should be healthy (and not be carriers) and clean. Rats and other vermin and insects should be kept away from the food. Ingredients used in foods should be free of salmonellae, if possible.
Foods should not be allowed to stand at room temperature for any length of time, but if this happens, thorough cooking will destroy the Salmonella organisms unlike Staphylococcus enterotoxin.
Warmed-over leftovers, held without refrigeration, often support the growth of Salmonella, as may canned foods that have been contaminated and held after the cans were opened. Inspection of animals and meats at packing houses may remove some Salmonella-infected meats but is not in itself a successful method for the prevention of human salmonellosis.
Staphylococcus Food Intoxication
One of the most commonly occurring food poisonings is caused by the ingestion of the enterotoxin formed in food during growth of certain strains of Staphylococcus aureus. The toxin is termed an enterotoxin because it causes gastro- enteritis or inflammation of the lining of the intestinal tract.
Staphylococcus, typically appear as clusters of grapes or in pairs and short chains. Growth on solid media usually is golden or yellow but may be unpigmented in some strains. Most enterotoxin-producing S. aureus cultures are coagulase-positive (coagulating blood plasma), produce a thermal stable nuclease, and are facultative in their oxygen requirements in a complex glucose medium but grow better aerobically than anaerobically. Some of the toxigenic cocci are very salt-tolerant (10 to 20 percent NaCl), and also tolerate nitrites fairly well and therefore can grow in curing solutions and on curing and cured meats if other environmental conditions are favourable. They also are fairly tolerant of dissolved sugars (50 to 60 percent sucrose). They are fermentative and proteolytic but usually do not produce obnoxious odors in most foods or make them appear unattractive.
S. aureus produces six enterotoxins (A, B, Cl , C2 , D, and E) that differ in toxicity; most food poisoning is from type A. The range of conditions permitting growth of the staphylococcus, and hence toxin production, varies with the food involved. The better medium the food is for the coccus, the wider the range of temperature, pH, or aw over which growth can take place. The temperature range for growth and toxin production is about 4 to 46oC, depending on the food. S. aureus grows most rapidly between 20 and 45oC. A and D are more often associated with food-poisoning outbreaks. Toxin production and growth of the Staphylococcus is best at 40oC.
The sources from which the food-poisoning staphylococci enter foods are human or animal. The nasal passages of many persons are laden with these organisms, which are a common cause of sinus infections. Also, boils and infected wounds may be sources. Staphylococci are becoming increasingly important in causing mastitis in cows, and some of these cocci can form enterotoxin in milk or milk products.
Production of enterotoxin by the Staphylococci is more likely when competing microorganisms are absent, few, or inhibited for some reason. Therefore, a food that had been contaminated with the Staphylococci after a heat process would be favourable for toxin production. The type of food has an influence on the amount of enterotoxin produced; much is produced in meat products and custard-filled bakery goods. The presence of starch and protein in considerable amounts enhance toxin production by the staphylococci. Type B enterotoxin is the most heat-resistant. The normal cooking of foods will not destroy the toxin formed therein before the heat process. Such foods might cause poisoning, although no live staphylococci could be demonstrated.
About 75 percent of all staphylococcal food-poisoning outbreaks occur because of inadequate cooling of foods. Other foods incriminated include other meats and meat products, fish and fish products, milk and milk products, cream sauces, salads, puddings, custards, pies, and salad dressings. The fillings in bakery goods usually are good culture media in which the staphylococci can grow during the time that these foods are held at room temperatures. Toxin production has even been reported in imitation cream filling. The contaminated leftover turkey, or other fowl, along with the gravy and dressing, is kept out of the refrigerator, it may cause poisoning.
Foods that ordinarily are too acid for good growth of the staphylococci may have this acidity reduced by added ingredients, such as eggs or cream, and then become dangerous. Growth and toxin production by staphylococci may take place in the steam tables in cafeterias and restaurants and in food-vending machines that keep foods heated for extended periods if temperatures and times are not properly controlled.
Individuals differ in their susceptibility to staphylococcus poisoning, so that of a group of people eating food containing toxin some may become very ill and few may be affected little or not at all. The incubation period for this kind of poisoning usually is brief, 2 or 4 hr unlike the other common food poisonings and infections, which usually have longer incubation periods.
The most common human symptoms are nausea, vomiting, retching, abdominal cramping of varying severity, and diarrhea. Blood and mucus may be found in stools and vomitus in severe cases. Headache, muscular cramping, sweating, chills, prostration, weak pulse, shock, and shallow respiration may occur. Usually a subnormal body temperature is found rather than fever. The duration is brief, usually only a day or two, and recovery ordinarily is uneventful and complete. The mortality is extremely low.
For the most part no treatment is given, except in extreme cases, when saline solutions may be given parenterally to restore the salt balance and counteract dehydration. Diagnosis of the poisoning would depend, on isolation of staphylococci and demonstration that this produce enterotoxin or isolation and detection of the enterotoxin.
The means of prevention of outbreaks of staphylococcus food poisoning include
(1) prevention of contamination of the food with the staphylococci (2) prevention of the growth of the staphylococci and (3) killing staphylococci in foods.
Contamination of foods can be reduced by general methods of sanitation, by using ingredients free from the cocci, e.g., pasteurized rather than raw milk, and by keeping employees away from foods when these workers have staphylococcal infections in the form of colds, boils, carbuncles, etc. Growth of the cocci can be prevented by adequate refrigeration of foods and, in some instances, by adjustment to a more acid pH. Also the addition of a bacteriostatic substance, such as serine or an antibiotic, has been suggested. Some foods may be pasteurized to kill the staphylococci before exposure of the foods to ordinary temperatures, e.g., pasteurization of custard filled puffs and eclairs for 30 min at 190.6 to 218.3 C oven temperature.
Enteropathogenic Escherichia Coli
E. coli is generally regarded as part of the normal flora of the human intestinal tract and that of many animals. Serotypes of E. coli have been implicated in human diarrheal diseases or foodpoisoning outbreaks are designated as enteropathogenic E. coli (EEC).
The human disease syndromes resulting from the ingestion of EEC have been divided into two main groups. The first group consists of strains which produce an enterotoxin and result in a choleralike or enterotoxigenic illness in humans. These enterotoxigenic strains usually produce two enterotoxins, a heat-stable (ST) and a heat-labile (LT) toxin, and are thought to be responsible for infantile diarrheal diseases and traveller’s diarrhea. EEC serotypes capable of elaborating the enterotoxins if ingested, are colonized in the upper small intestine and produce the enterotoxins. The enterotoxins cause fluid accumulation in the intestinal lumen.
The second major group consists of invasive strains which produce a cytotoxin and result in the invasive illness, colitis, or dysentery like syndrome. These serotypes are non-enterotoxigenic, grow in the colon, and invade or penetrate the epithelial cells of colonic mucosa, resulting in fever, chills, headache, abdominal cramps etc
A large infective dose of EEC is required for either the enterotoxigenic or invasive illness to occur. Therefore, foods must be highly contaminated or inadequately preserved or refrigerated to allow for prolific growth. The optimal temperature for growth is 37oC, with a temperature range for growth of 10 to 40oC. The optimal pH for growth is 7.0 to 7.5, with the minimum at pH 4.0 and the maximum at pH 8.5. The organism is relatively heat sensitive and can readily be destroyed at pasteurization temperatures and by the proper cooking of foods.
In addition, the hemorrhagic E. coli (EHEC) strains can result in illness in humans as manifested by bloody diarrhea and severe abdominal pain.
Thorough and sanitary methods of cooking, chilling foods after use, maintaining personal hygiene, treating water and ensuring sanitary disposal of sewage can control E. coli infections
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