FOOD POISONING

FOOD POISONING:

Food poisoning syndrome results from ingestion of water and wide variety of food contaminated with pathogenic microorganisms (bacteria, viruses, protozoa, fungi), their toxins and chemicals. Food poisoning must be suspected when an acute illness with gastrointestinal or neurological manifestation affect two or more persons, who have shared a meal during the previous 72 hours.

Food poisoning and food-borne infections.  

In microbial food poisoning, the microbes multiply readily in the food prior to consumption, whereas in food-borne infection, food is merely the vector for microbes that do not grow on their transient substrate. While food poisoning could be as intoxication of food by chemicals or toxins from bacteria or fungi.

 Consumption of poisonous mushroom leads to mycetism, while consumption of food contaminated with toxin producing fungi leads to mycotoxicosis.

The Most Common Causes of Food Poisoning

1. Inadequate Cooling and Cold Holding

More than half of all food poisoning is due to keeping food out at room temperature for more than 2-4 hours.

2. Preparing Food Too Far Ahead of Service

Food prepared 12 or more hours before service increases the risk of

food temperature abuse.

3. Poor Personal Hygiene and Infected Personnel

Poor handwashing habits and foodhandlers working while ill are

implicated in 1 out of every 4 food poisonings.

4. Inadequate Reheating

When leftovers are not reheated to above 165 F for 15 seconds.

5. Inadequate Hot Holding

Cooked foods not held at above 140 F until served can become highly

contaminated.

Food and water microorganisms  

Food borne infections by bacteria can also be classified as toxicosis and food-infections. In toxicosis, the toxins are released by bacteria such as Clostridia, Bacillus and Staphylococcus. In food-infections, the bacteria are ingested, which later initiate the infection.

Staphylococcus aureus:

S. aureus is   gram positive cocci grape like clusters. It is present in the environment. Only those strains that produce enterotoxin can cause

food poisoning. Food is usually contaminated from infected food handler.

Incriminated food: Custard and cream filled bakery food, ham, chicken, meat, milk, fish, salads, puddings, pie etc.

Pathogenesis: If the food is stored for some time at room temperature the organism may multiply in the food and produce toxin (enterotoxin) . S.aureus is known to produce six serologically different types of enterotoxins (A, B, C, C2, D and E) that differ in toxicity. Most food poisoning is caused by enterotoxin A. These enterotoxins tend to be heat stable, with type B being most heat resistant. Ingestion of as little as 23 µg of enterotoxin can induce vomiting and diarrhoea.

Incubation period: is usually 1-6 hours.

Clinical features: vomiting and diarrhea watery but no fever. The illness lasts less than 12 hours. There are no complications and treatment is usually not necessary.

Laboratory diagnosis: The presence of a large number of S.aureus organisms in a food may indicate poor handling or sanitation; however.

Staphylococcal food poisoning can be diagnosed if they are isolated in large numbers from the food and their toxins demonstrated in the food or the isolated S. aureus must be shown to produce enterotoxins. Dilutions of food may be plated on Baird-Parker agar or Mannitol Salt agar.

Enterotoxin may be detected and identified by gel diffusion.

Lactobacillus

Lactobacilli are non-sporeforming Gram-positive long rods. There are more than thirty species in the genus. Most are microaerophillic, although some are obligate anaerobes. Cells are catalase-negative and obtain their energy by the fermentation of sugars, producing a variety of acids, alcohol and carbon dioxide. Lactobacilli very rarely cause infections in humans.

Enterococci

Enterococci are Gram-positive, non-sporeforming, catalase-negative ovoid cells. Cells occur singly, in pairs or short chains. Optimal growth for most species is 35–37 °C. Some will grow at 42–45 °C and withstand 60 °C for 30m. Cells are resistant to 40% bile, 0.4% azide, 6.5% sodium chloride, have β-glucosidase and hydrolyze esculin. The enterococci are facultative anaerobic but prefer anaerobic conditions ,.

Enterococci are naturally present in many kinds of foods, especially those of animal origin such as milk and milk products, meat and fermented sausages. Enterococci are often play a positive role in ripening and aroma development of some types of cheeses.  

Environmental waters are not a natural habitat for enterococci and their presence in this milieu is considered the result of fecal pollution

The intestinal enterococci group has been used as an index of fecal pollution.

Bacillus cereus:

B. cereus is a gram positive aerobic spore bearing bacilli. It is found abundantly in environment and vegetation.

Incriminated food: Rice and vegetables.

Pathogenesis: During the slow cooling, spores germinate and vegetative bacteria multiply, then they sporulate again. Sporulation is also associated with toxin production. The toxin is heat-stable, and can easily withstand the brief high temperatures used to cook fried rice.

Food poisoning also associated with meat or vegetable-containing foods after cooking, the illness is mediated by a heat-labile enterotoxin, which activates intestinal adenylate cyclase and causes intestinal fluid secretion.

Incubation period: 1-6 hours in and 8-16 hours in long incubation form.

Clinical features: nausea, vomiting and abdominal cramps and resembles S. aureus and diarrhea (watery).

Laboratory diagnosis: The short-incubation or emetic form of the disease is diagnosed by the isolation of B. cereus from the incriminated food. The long-incubation or diarrheal form is diagnosed by isolation of the organism from stool and food. Isolation from stools alone is not sufficient because 14% of healthy adults have been reported to have transient gastrointestinal colonization with B. cereus.

Clostridium perfringens:

It is a gram positive anaerobic spore bearing bacilli that is present abundantly in the environment, vegetation, sewage and animal feces.

Incriminated food: Meat products that are eaten 1- 2 days after preparation. Meats that have been cooked, allowed to cool slowly, and then held for some time before eating are commonly incriminated. Fish pastes and cold chicken too have been incriminated.

Like E. coli, C. perfringens does not multiply in most water environments and is a highly specific indicator of fecal pollution.

Pathogenesis: Spores in food may survive cooking and then germinate when they are improperly stored. The bacterium is known to produce at least 12 different toxins. Food poisoning is mainly caused by Type A strains. The toxins result in excessive fluid accumulation in the intestinal lumen.

Incubation period: 8-24 hours.

Clinical features: Illness is characterized by acute abdominal pain, diarrhea, and vomiting. Illness is self- limiting and patient recovers in 18-24 hours.

Laboratory diagnosis: Since the bacterium is present normally in the intestine, their isolation from feces may not be sufficient to implicate it. Similarly, isolation from food except in large numbers (>10⁵/gram) may not be significant. The homogenized food is diluted and plated on selective medium as well as cooked meat medium and incubated anaerobically. The isolated bacteria must be shown to produce enterotoxin.

Clostridium botulinum:

It is a gram positive anaerobic spore bearing bacilli that is widely distributed in soil, sediments of lakes and ponds, and decaying vegetation.

Incriminated food: home canned or bottled meat, vegetables and fish. In general, the low and medium acid canned foods are often incriminated. The anaerobic environment produced by the canning process may further encourage the outgrowth of spores.

Pathogenesis:  Not  all  strains  of  C. botulinum  produce  the  botulinum  toxin. Seven  toxigenic  types  of  the organism exist. The toxins are designated A, B, C1, D, E, F, and G. Food-borne botulism is not an infection but an intoxication since it results from the ingestion of foods that contain the preformed clostridial toxin. If contaminated food has been insufficiently sterilized or canned improperly, the spores may germinate and produce botulinum toxin. The toxin is released only after the death and lysis of cells. The toxin resists digestion and is absorbed by the upper part of the GI tract and then into the blood. It then reaches the peripheral neuromuscular synapses where the toxin binds to the presynaptic stimulatory terminals and blocks the release of the neurotransmitter acetylcholine.

This results in flaccid paralysis. Even 1-2 µg of toxin can be lethal to humans.

Incubation period: 12-36 hours

Clinical features: vomiting, thirst, dryness of mouth, constipation, ocular paresis (blurred-vision), difficulty in speaking, breathing and swallowing. Coma or delirium may occur in some cases. Death may occur due to respiratory paralysis within 7 days.

Laboratory diagnosis: Spoilage of food or swelling of cans or presence of bubbles inside the can indicate clostridial growth. Food is homogenized in broth and inoculated in Robertson cooked meat medium and blood agar or egg-yolk agar, which is incubated anareobically for 3-5 days at 37oC. The toxin can be demonstrated by injecting intraperitoneally the extract of food or culture into mice or guinea pig.

Enterotoxigenic E.coli (ETEC)

E. coli are gram negative enteric bacilli that are carried normally in the intestine of humans and animals. The enterotoxin production is limited to following O serotypes: O6, O8, O15, O25, O63, O78, O148 and O159.

Incriminated food: Infection is acquired by ingestion of food or water contaminated with ETEC. Contamination of water with human sewage may lead to contamination of foods. Infected food handlers may also contaminate foods. The infective dose is 10⁶-10¹⁰ bacilli.

Pathogenesis: Enterotoxins produced by ETEC include the LT (heat-labile) toxin and or the ST (heat-stable) toxin. LTs are similar to cholera toxin in structure and mode of action, causing hyper secretion of water and electrolytes into the lumen and inhibition of sodium reasborption.

Incubation period: 16-72 hours

Clinical features: Sudden onset of watery diarrhea associated with nausea, vomiting, abdominal cramping and bloating is commonly observed. This bacterium is responsible for majority of traveler’s diarrhea. The disease is self-limiting and resolves in few days.

Laboratory diagnosis: The sample of feces is cultured on McConkey's agar. The ETEC stains are indistinguishable from the resident E. coli by biochemical tests.  These  strains  are  differentiated  from nontoxigenic E.coli present in the bowel by a variety of in vitro immunochemical, tissue culture, or DNA hybridization  tests  designed  to  detect  either  the  toxins  or  genes  that  encode  for  these  toxins. ELISA, immunodiffusion, coaglutination etc.

Enterohemorrhagic E.coli (EHEC( .

EHEC strains have been associated with many serogroups including O4, O26, O45, O91, O111, O145 and O157. The most serotype is O157:H7.

Incriminated food: Cattle appear to be the main source of infection; most cases being associated with the consumption of undercooked beef burgers and similar foods. meat, raw milk, cream and cheeses made from raw milk. In addition outbreaks are associated with the consumption of fruits and vegetables (sprouts, lettuce, coleslaw, salad) contaminated with feces from domestic or wild animals.

Pathogenesis: EHEC strains may produce one or more types of cytotoxins, known as Shiga-like toxins (SLT) since they are antigenically and functionally similar to Shiga toxin produced by Shigella dysenteriae. SLTs were previously known as verotoxin. The toxins provoke cell secretion and kill colonic epithelial cells.

Incubation period: 72-120 hours (3-4 day)

Clinical features: Initial symptoms may be diarrhea with abdominal cramps, which may turn into grossly bloody diarrhoea in a few days. There is however, no fever.

Laboratory diagnosis: Laboratory diagnoses involve culturing the faeces on McConkey's agar or on sorbitol McConkey's agar, where they don't ferment sorbitol. Strains can then be identified by serotyping using specific antisera. SLTs can be detected by ELISA and genes coding for them can be detected by DNA hybridization techniques.

Enteroinvasive E. coli (EIEC) Strains

Enteroinvasive E. coli (EIEC) behave in many respects like shigellae. They are capable of invading and multiplying in the intestinal epithelial cells of the distal large bowel in humans. The illness is characterized by abdominal cramps, diarrhea, vomiting, fever, chills, a generalized malaise, and the appearance of blood and mucus in the stools of infected individuals. The dysentery was more common among the older age groups. Any food contaminated with human feces from an ill individual, either directly or via contaminated water, could cause disease in others. Outbreaks have been associated with hamburger meat and unpasteurized milk.

Vibrio

Several Vibrio species can infect humans .  V. cholerae is the most important of these species. V. alginolyticus, V. fluvialis, Grimontia hollisae (V. hollisae), and V. mimicus can cause diarrhea or infections of the gastrointestinal tract. V. furnissii has been isolated from a few individuals with diarrhea, but there is evidence that it can actually cause this pathology. V. parahaemolyticus is a well-documented cause agent of acute food-borne gastroenteritis. V. vulnificus is an important cause of (often fatal) septicemia and wound infections.

Cases arassociated with the consumption of raw or undercooked shellfish

Vibrios are primarily aquatic bacteria. Species distribution depends on sodium concentration and water temperature.

Vibrio cholerae cells can grow at 40 °C with pH 9–10. The growth is stimulated by the presence of sodium chloride. Vibrio cholerae is a very diverse bacterial species (Table below). It is divided in  200 serovarieties, characterized by the structure of the lipopolysaccharide (LPS) (O antigens). Only serovarieties O1 and O139 are involved in true  cholera. Some other serovarieties can cause gastroenteritis, but not cholera. The distinction between Classical and El Tor biotypes is based on biochemical and virological characteristics .

Pathogesis : The Infective dose 10⁸ -10¹², and the incubation period for cholera is 1–3 days. The disease is characterized by an acute and very intense diarrhea that can exceed one liter per hour. Cholera patients feel thirsty, have muscular pains and general weakness, and show signs of oliguria, hypovolemia, hemoconcentration, followed by anuria. Potassium in blood drops to very low levels. Patients feel lethargic. Finally, circulatory collapse and dehydration with cyanosis occurs.

In the absence of treatment, the mortality of cholera-patients is  50%. It is mandatory to replace not only lost water but also lost salts, mainly potassium. In light dehydrations, water and salts can be orally-administered, but in severe conditions, rapid and intravenous-administration is obligatory. The most efficient antibiotic is currently doxicyclin. If no antibiotic is available for treatment, the administration of water with salts and sugar can, in many cases, save the patient and help in the recovery.

Vibrio parahemolyticus:

They are straight or curved gram negative bacilli. In morphology and staining it resembles V.cholerae and is actively motile in liquid cultures. It is commonly found in coastal seas.

Incriminated food: Infections are associated with consumption of uncooked or undercooked crabs, prawns, shrimps and other seafoods.

Pathogenesis: No enterotoxin has been demonstrated in the bacterium. The infection is thought to result from invasion of intestinal epithelium.

Incubation period: 7-48 hours

Clinical features: The clinical infection is characterized by a sudden onset of acute gastroenteritis. Infection may also result in diarrhoea, abdominal pain, vomiting and fever.

Laboratory diagnosis: Homogenized food may be inoculated into TCBS agar or into double strength alkaline peptone water and incubated overnight at 37oC. This bacterium is positive for Kanagawa phenomenon where isolates from human feces show hemolysis on blood agar.

Shigellosis or Bacillary Dysentery

Shigella are Gram-negative, non-sporeforming, non-motile, straight rod-like members of the family Enterobacteriaceae. Cells ferment sugars without gas production. Cells do not utilize citrate, malonate and acetate and do not produce HS. Lysine is not decarboxylated. Cells are oxidase-negative and catalase-positive.

The incubation period is 1–4 days and the infected dose 10² - 10⁶.

The disease usually begins with fever, anorexia, fatigue and malaise. Patients display frequent bloody stools of small volume (sometimes grossly purulent) and abdominal cramps. Twelve to 36 hours later, diarrhea progresses to dysentery, blood, mucus and pus appearing in feces. White blood (WBC) cells increased (leukomoid reaction).   

Ecology of Shigellae and the Cycle of Shigellosis

Shigella is typically an inhabitant of the intestinal tract of humans. Typically spread by fecal-contaminated drinking water or food, or by direct contact with an infected person. In water, shigellae can survive for at least six months at room temperature, and this high survival favors transmission through water. Flies have been implicated on the transmission of Shigella cells from human feces to foods. The hand is an important vehicle for transmission of shigellosis, since S. dysenteriae serotype 1 cells survives for up to one hour on a human‘s skin and a very small inoculum is required to cause infection . Resistance of Shigella to gastric juice certainly accounts, although not exclusively, for this high infectivity. Asymptomatic and inappropriately-treated patients with shigellosis can harbor the bacteria in the gut and these appear to be the main reservoirs of the bacteria in the environment.

Salmonella enteritidis:

These are gram negative rod enterobacteriaceae. This species does not occur normally in humans but several animals act as reservoirs.

Incriminated food: Most important sources are chicken and poultry. Chicken, duck, turkey and goose may be infected with Salmonella, which then find its way into its feces, eggs or flesh of dressed fowl. Milk and milk products including ice creams have been incriminated.

Pathogenesis: Organism penetrates epithelial cells lining small  intestine produces  inflammatory  mediators,  recruits neutrophils  and  triggers  inflammation.  Release  of  LPS  causes  fever.  Inflammation  causes  release  of prostaglandins from epithelial cells causing electrolytes to flow into lumen of the intestine. Water flows into lumen in response to osmotic imbalance resulting in diarrhea.

Incubation period: 12-36 hours the infective dose 10⁶ - 10⁹

Clinical features: Sudden onset of abdominal pain, nausea, vomiting and diarrhea, which may be watery, greenish and foul smelling. This may be preceded by headache and chills. Other findings include prostration, muscular weakness and moderate fever. In most cases the symptoms resolve in 2-3 days without any complications.

Laboratory  diagnosis:  Homogenized  food  is  cultured  in  selenite  F  broth  and  then  sub-cultured  on deoxycholate citrate agar. Plates are incubated at 37oC overnight and growth identified by biochemical tests and slide agglutination test.

Yersinia enterocolitica:

It is a gram negative rod shaped , motile only at temperature below 30^oC. Yersinia enterocolitica is widely distributed in environment and have been isolated frequently from soil, water and animals. The major animal reservoir is pigs, but may also found in many other animals including rodents, rabbits, sheep, cattle, horses, dogs, and cats. Serogroups that predominate in human illness are O:3, O:8, O:9, and O:5.

Incriminated food: Raw or undercooked pork products. Drinking contaminated unpasteurized milk or untreated water can also transmit the infection.

Pathogenesis:  This organism may survive and grow during refrigerated storage. Ingested bacteria adhere and invade cells or epithelial cells. They exhibit resistance to complement and phagocytosis. They produce ST only at temperatures below 30ºC. The role of ST in the disease process remains uncertain.

Incubation period: 4-7 days

Clinical features: fever, abdominal pain, and diarrhea, which is often bloody. Illness generally lasts from 1 to 2 weeks but chronic cases may persist for up to a year. Apart from gastroenteritis it may also cause pseudoappendicitis, mesenteric lymphadenitis, and terminal ileitis.

Laboratory diagnosis: Suspected food is homogenized in phosphate-buffered saline and inoculated into selenite F broth and held at 4^oC for six weeks. The broth is sub-cultured at weekly intervals on DCA or Yersinia selective agar plates. This is termed as cold enrichment technique.

Campylobacter jejuni:

These are small, curved-spiral comma, S-shaped or “gull-wings/sea-gull” Gram negative form. Campylobacter are harbored in reproductive and alimentary tracts of some animals.

Incriminated food: Transmission to humans occurs via a fecal-oral route, originating from farm animals, birds, dogs, and processed poultry, with chicken preparation. The organism is transmitted to man in milk, meat products and contaminated water. Undercooked poultry and unpasteurized dairy are most often implicated as a source of C.jejuni.

Pathogenesis: As few as 500 organisms can cause enteritis. The organism is invasive but generally less so than Shigella. Campylobacter produces adenylate cyclase-activating toxins same as of E. coli LT and cholera.

Incubation period: Ranges from 2 to11 days.

Clinical features: abdominal pain and cramps, diarrhea, malaise, headache, and usually fever and bacteremea. Watery diarrhoea, but in severe cases bloody diarrhea may occur. Diarrhea may last 2-7 days. The disease is usually self-limiting.

Laboratory diagnosis: The feces may be inoculated in enrichment medium or on selective media such as Campy BAP or Skirrow's medium. The plates are incubated in microaerophilic conditions at 42oC for 2-5 days.

Helicobacter pylori 

Helicobacter pylori is curved-spiral comma, S-shaped or “gull-wings/sea-gull” microaerophelic Gram negative

Pathogenesis: It is the major etiologic agent for gastritis peptic and duodenal ulcer disease and gastric carcinoma. However, most individuals that are infected by this pathogen remain asymptomatic. How the organism is transmitted is still not fully understood. However, the fact that it has been recovered from saliva, dental plaques, the stomach, and fecal samples strongly indicates oral-oral or fecal-oral transmission. Water and food appear to be of lesser direct importance.

Lab diagnosis: Using culture-based methods, H. pylori has not been isolated from environmental sources, including water.

Polymerase chain reaction has also been used to detect the presence of H. pylori DNA in drinking water, especially associated with biofilms. In drinking-water biofilms, H. pylori cells rapidly lose culturability, entering a viable but non-culturable state. In these biofilms, cells can persist for more than one month, with densities exceeding 10⁶cells per cm².

Mycobacterium avium Complex (Mac)

The Mycobacterium avium complex (Mac) consists of 28 serovars of two distinct species: Mycobacterium avium and Mycobacterium intracellulare. The importance of Mac organisms was recognized with the discovery of disseminated infection in immunocompromised people, particularly people with HIV and AIDS. Members of MAC are considered opportunistic human pathogens. Mac organisms have been identified in a broad range of environmental sources, including marine waters, rivers, lakes, streams, ponds, springs, soil, piped water supplies, plants, and house dust. Mac organisms have ability to survive and grow under varied conditions. Mac organisms can proliferate in water at temperatures up to 51 °C and can grow in natural waters over a wide pH range. These mycobacteria are highly resistant to chlorine and the other chemical disinfectants used for the treatment of drinking-water. Standard drinking-water treatments will not eliminate Mac organisms but, if operating satisfactorily, will significantly reduce the numbers that may be present in the source water to a level that represents a negligible risk to the general population.

Aeromonas hydrophyla 

A. hydrophila is opportunistic pathogen. It cause gastroenteritis, septicemia, meningitis, and wound infections. It can play a significant role in intestinal disorders in children under five years old, the elderly, and immunosuppressed people.

Aeromonas hydrophila are Gram-negative, non-sporeforming, rod-shaped, facultative anaerobic bacilli belonging to the family Aeromonadaceae.

Aeromonas species, including A. hydrophila, are frequently isolated from food, drinking water, and aquatic environments. In clean rivers and lakes, concentrations of Aeromonas spp. are usually around 10 colony-forming units (CFU)/mL. Groundwater generally contain less than 1 CFU/mL. Drinking water immediately leaving the treatment plant has been found to contain between 0 and 10²  CFU/mL. Aeromonas spp. have been found to grow between 5 °C and 45 °C. A. hydrophila is resistant to standard chlorine treatments, probably surviving inside biofilms.

The common routes of infection suggested for Aeromonas are the ingestion of contaminated water or food or contact of the organism with a break in the skin.