Salmonella of Food Poisoning Infections
1. Salmonella
Organisms of the salmonella group are divisible into those that cause enteric fever; S typhi and S. paratyphi A, B, and C, and those that cause food poisoning. The salmonellae that cause food poisoning are classified into more than 2000 serotypes. They reach food directly or indirectly from animal excreta at the time of slaughter, from human excreta, or water contaminated with sewage. Also, in the kitchen, they may be transferred from raw to cooked food by hands, surfaces, utensils, and other equipment. Illness is more likely to occur if the organism is ingested in large numbers. A chance contamination of food by a small number of bacilli may not be harmful unless they are allowed to multiply e.g. if the food is allowed to stand for hours in a warm room.
The onset of illness occurs usually within 6-36 hours of the consumption of contaminated food. The symptoms are characterised by fever, headache, and general aching of the limbs, as well as diarrhoea and occasionally vomiting. The illness lasts 1-7 days or longer. The strain of salmonella involved can be typed by serological methods or by phage typing. Typing is useful in the investigation of outbreaks.
2. Staphylococcus aureus
Staphylococcal food poisoning follows the consumption of food heavily contaminated with certain types of S. aureus which produce a toxin. The food is often contaminated by food handlers, whose hands and noses frequently harbour S. aureus. Meat and poultry intended to be eaten cold, and prepared foods such as custards, trifles, and cream products, are especially prone to such contamination. Since the toxin is formed in the food before it is eaten, the incubation period may be as short as 1-2 hours. There is a rapid onset of symptoms characterised predominantly by severe vomiting, with diarrhoea, abdominal pain and cramps. Recovery usually occurs within 6 to 24 hours. S. aureus may be typed by phage and serological methods. There are several types of enterotoxin which can be extracted from culture filtrates and foods and they can be identified serologically.
3. Clostridium perfringens
C. perfringens is a common organism frequently found in excreta from humans and animals and in raw meats, poultry and other foods, including dehydrated products. It can survive heat and dehydration by means of spores that remain dormant in food, soil, and dust. Illness occurs after eating food contaminated with large numbers of C. Perfringens grown from spores: the spores had been activated by heat shock. Multiplication of bacteria take place during long slow cooking and warm storage of food in the kitchen or canteen. Cooked meat, poultry, fish, stews, pies, and gravies are excellent media for growth at temperatures up to 50oC. There is little growth below 15oC. The spores of some strains of C. Perfringens can withstand long hours of boiling. The organism may be isolated in large numbers from boiled, stewed, steamed, braised, or even roasted foods, particularly those cooked in bulk and stored unrefrigerated for a few hours.
Symptoms occur 8 to 22 hours after consumption of the contaminated food. They include abdominal pain, profuse diarrhoea, and nausea, but rarely vomiting. Symptoms may continue for 12 to 48 hours. The symptoms occur as a result of an enterotoxin produced by a large number of ingested organisms. An effective amount of toxin is not usually formed in the food before it is eaten. There are 5 types of C. Perfringens (A to E) classified according to the various toxins they produce. Only two of these produce the enterotoxin which cause gastroenteritis: type A is the more common agent of food poisoning whereas type C is responsible for a more serious but rare condition known as enteritis necroticans.
4. Bacillus cereus
B. cereus is a common aerobic sporing organism. Its spores are often found in cereals and other foods, some of which will survive cooking and germinate into bacilli, which under warm storage conditions in cooked foods, will multiply and produce toxin. A wide variety of foods have been associated with outbreaks, particularly cornflour sauce in Norway and fried rice in Britain and other countries. As with C. perfringens, long moist storage of warmed cooked food.
There is no serological typing scheme for B. Cereus: certain serotypes occur more commonly in outbreaks than others and there are two different patterns of symptoms. The outbreaks arising from rice typically have a short incubation period of 1-3 hours with sudden onset of acute vomiting and some diarrhoea: the symptoms resembling those of staphylococcal enterotoxin food poisoning. The other pattern resembles C. perfringens food poisoning with an incubation period of 9-18 hours and diarrhoea as the chief symptom. The toxin responsible for the vomiting syndrome, which is probably preformed when the organism grows in food, is extremely heat resistant; it is not destroyed after 1 ½ hours at 121oC. Other aerobic spore-bearing organisms such as B. Subtilis have also been described as agents of food poisoning with similar characteristics to those caused by B. Cereus.
5. Escerichia coli
The majority of E. coli in the GI tract are harmless unless they are displaced to other parts of the body such as the urinary tract or meninges. The pathogenic, or enterovirulent E. coli are divided into 4 groups according to their action in the body:
1. Enteropathogenic (EPEC) - responsible for severe infantile diarrhoea.
2. Enterotoxigenic (ETEC) - produces a heat labile (LT) or heat stable (ST) toxin. Both may be produced by the same organism. They are responsible for cases of paediatric diarrhoea, severe cholera-like illness, and traveller’s diarrhoea
3. Enteroinvasive (EIEC) - pathogenicity is due to invasion of the gut mucosa. They give rise to a dysentery-like illness.
4. Enterohaemorrhagic (EHEC) - responsible for bloody diarrhoea and colitis. It differs from bacillary dysentery in that fever is not prominent and the bloody discharges are copious rather than scanty. The O157:H7 had been implicated.
The pathogenic E. Coli can be differentiated from the non-pathogenic types by immunological and other methods. The incubation period is 12 hours to 3 days. The symptoms consist mainly of diarrhoea with blood and mucus in the stool. Infants acquire the infection by direct oral-faecal spread in maternal units and also by contaminated feeds. Enterotoxigenic E. Coli are prevalent in infants in tropical countries. Food and water in addition to faeces are sources of these organisms. The symptoms in adults are probably initiated by large doses of enterotoxigenic E. Coli producing heat stable toxin. E. Coli may enter kitchens in many raw foods and readily pass to cooked foods by hands, surfaces, containers and other equipment. They may also be water-borne also. During epidemics, human excreta may play a part in direct spread but unlike other gram-negative organisms, E. Coli will not survive long on the skin.
Since E. Coli are always present in faeces, they are used as a marker for the faecal pollution of water, milk, and food. Intestinal pathogens such as Shigella and Salmonella may be found intermittently in water and sewage. However, they are only present in small numbers and thus difficult to find.
6. Campylobacter jejuni
Campylobacter is commonly implicated in outbreaks involving predominantly children. The vehicle for transmission is usually raw or imperfectly pasteurized milk, and sometimes water. Isolation of the organism from poultry has been described frequently and infection occurs probably through the cross contamination from raw to cooked material. There may also be direct infection from the raw material to the food handler. The unusual conditions required to grow the organism have hindered its isolation and delayed epidemiological investigations. Person-to-person spread has been demonstrated and thus the dose level to initiate symptoms may be small.
The onset of symptoms may be sudden with abdominal cramps followed by passage of foul-smelling stools. Blood and mucus may be present and the diarrhoea may persist for 1-3 days. A prodrome consisting of fever, headache, and dizziness may occur and this may last for a few hours to several days. The abdominal pain may persist once the diarrhoea has eased and symptoms may be more severe in adults than children. The incubation period is probably 3-5 days with a fairly wide range. Pet dogs and cats may be affected at the same time.
7. Vibrio parahaemolyticus
V. parahaemolyticus are reported to be responsible for 50% of food-poisoning incidents in Japan. In the warmer weather, it can be isolated from fish, shellfish, and other seafoods. Both raw and cooked seafoods such as shrimps, crabs, and lobsters are vehicles of infection. Cooked foods may be contaminated by the raw products in kitchens. Food poisoning following dinner parties with prawns on the menu are common. The illness resembles a mild form of cholera with an average incubation period of 15 hours. There is a rapid onset of symptoms with profuse diarrhoea often leading to dehydration, some vomiting , and fever. There is acute abdominal pain. The illness usually lasts 2-5 days. There are many serotypes.
8. Aeromonas hydrophilia
Reports of outbreaks of A. Hydrophilia infections have come from India and Ethiopia. The organism was isolated from patients with diarrhoea. Long term surveys of diarrhoea in children and adults in the USA indicate both A. Hydrophilia and A. Sobria play significant roles. Water, milk, and seafoods were considered to be basic sources of the organism.