Bacterial Food Infections

 Food infection occurs when a pathogen enters the gastrointestinal tract and
multiplies. Microorganisms can penetrate into the intestinal mucosa and grow
there, or they can pass through other systemic organs. Infections are
characterized by a delay in the appearance of gastrointestinal disturbance while
the pathogen increases in numbers or affects invaded tissue.

Bacterial Food Infections

There is also
usually a fever, one of the body’s general responses to an infective organism.
Foodborne infections remain a major public health problem. The Council for
Agricultural Science and Technology estimated in its 1994 report, Foodborne
Pathogens: Risks and Consequences, that as many as 9,000 deaths and 6.5 to
33 million illnesses in the United States each year are food-related.

Zoonotic Diseases

The World Health Organization defines Zoonoses (Zoonosis, sing.) as “Those
diseases and infections which are naturally transmitted between vertebrate
animals and man”. 
Mode of transmission: Feces, urine, saliva, blood, milk, via aerosol, oral,
contact with bedding or animals, etc.
Approximately 150 zoonotic diseases are known to exist. Wildlife serves as a
reservoir for many diseases common to domestic animals and humans. Persons
working with wildlife should be alert to the potential for disease transmission
from animals. 
Generally, disease is more easily prevented than treated. Many
zoonotic diseases are so common in nature, so rare in humans, or so mild in
their symptoms, that wild animals pose a minimal health risk to people.
Zoonotic diseases include: 
• Those which can be transmitted directly from animals to humans (e.g.
• Diseases that can be acquired indirectly by humans through ingestion,
inhalation or contact with infected animal products, soil, water, or other
environmental surfaces which have been contaminated with animal waste
or a dead animal (e.g. salmonellosis, leptospirosis, anthrax). Campylobacter
infection is mainly found in chicken meat. Listeriosis and E.coli
gastroenteritis are two other common infections caused by zoonotic agents.
All these will be discussed one by one. 
• A disease which has an animal reservoir, but requires a mosquito or other
arthropod to transmit the disease to humans (e.g. St. Louis encephalitis,
Rocky Mountain spotted fever).


Salmonellosis is the most reported zoonotic disease in European countries.
Salmonellosis (Salmonella gastroenteritis) results from the ingestion of foods
that contain significant numbers of viable cells of the members of the genus
Salmonella. It is the most frequently occurring food borne infection. 
Salmonella are small gram negative, motile, non-spore forming rods that
ferment glucose, usually with gas, but usually do not ferment lactose or
sucrose. They are widely distributed in nature, with humans and animals being
their primary reservoir. Generally large number of salmonellae typically 106
bacterium must be ingested to cause illness.
Occurrence: The initial source of the bacteria is the intestinal tract of animals
such as birds, reptiles, farm animals, humans and occasionally insects. As
intestinal form, the organisms are excreted in feces from which they maybe
transmitted by insects and other living creatures to a large number of places,
polluted water and contaminated food. 
The organism may get transferred from
actual infected cases of the disease or from carriers. A carrier is defined as a
person or an animal that repeatedly sheds bacteria, usually through feces,
without showing any signs or symptoms of the disease. 
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. Humans
acquire the bacteria from contaminated food such as beef products, poultry,
eggs, egg products or water.
Symptoms: The susceptibility of humans varies with the species and strains of
the organism and the total number of bacteria ingested. A longer incubation
period usually distinguishes salmonellosis from staphylococcus poisoning:
usually 12-36 hours for the former and about 2-4 hours for the latter. 
The principle symptoms of a salmonella gastroenteritis infection are nausea,
vomiting, abdominal pain and diarrhoea that usually appear suddenly. This
may be preceded by a headache and chills. Other evidences of the disease are
watery, greenish-fowl-smelling stools, prostration, muscular weakness,
faintness, usually a moderate fever, restlessness, twitching and drowsiness. 
mortality is less than 1%. Intesibility may vary from slight discomfort and
diarrhoea to death in 2 to 6 days. About 0.2 to 5.0% of the patients may
become carriers of the Salmonella organism. During the acute phase of the
disease, as many as one billion salmonellae can be found per gram of feces.
Associated foods: Raw meats, poultry, eggs, milk and dairy products, fish,
shrimp, coconut, sauces and salad dressings, cake mixes, cocoa, peanut butter
and chocolate

Conditions Necessary for Outbreak

The food must contain or become contaminated with the Salmonella bacteria.
These bacteria must be there in considerable numbers i.e., food should be a
good culture media, temperature favourable and enough time allowed for
appreciable growth.
The viable organism must be ingested
Prevention of Outbreak: The control of food borne salmonella infection
requires the following: 
1. Preventing food contamination by human carriers, especially food handlers. 
2. Avoiding the use of animal products from domestic livestock that are
grossly infected with salmonellae. 
3. Avoiding the use of food ingredients that contain salmonellae. 
4. Processing all foods susceptible to Salmonella contamination at timetemperature schedules sufficient to destroy the organism. Heating foods so
that all portions reach 66ºC for 12-15 minutes will assure destruction of
even most resistant Salmonella types. 
5. Refrigerating all foods susceptible to Salmonella contamination and
avoiding prolonged holding of these foods at room temperature.


Escherichia coli is generally regarded as part of the normal flora of the human
intestinal tract and that of many animals. Serotypes of E. coli which have been
implicated in human diarrhoeal diseases or food poisoning outbreaks and have
been designated enteropathogenic E. coli (EEC). 
They grow over a wide range
of temperatures, 20-40ºC with a minimum growth temperature at 10ºC and an
optimum at 37ºC. Heating at 65ºC for 15-20 minutes is lethal. The pH range for
growth is 4.2-8.50, with an optimum in the range of pH 7.2-7.5. E. coli will
grow in the presence of 5.0% salt at 37ºC but 10% is inhibitory.
Symptoms: The E. coli gastroenteritis syndrome is caused by the ingestion of
-1010 viable cells/g that must colonize the small intestine and produce
enterotoxin. The syndrome is characterized primarily by non-bloody, watery
diarrhoea without inflammatory exudates in stools. Incubation time of disease is around 2 days after eating the contaminated food and may last for 8 days.
Common symptoms included are cramps, chills, vomiting, aches and headache.
Associated Foods: E. coli is the etiologic agent of food poisoning involves
variety of foods such as cream pie, mashed potatoes, cream puffs and creamed
fish. Other E. coli food poisoning outbreaks have been attributed to the
consumption of milk, cheese, ice cream, meats, fish and macaroni. E. coli is
relatively sensitive to destruction by drying or freezing but some survivors may
exist for extended periods.
“Enteropathogenic” strains colonize in the small intestine and cause acute
gastroenteritis in newborns and in infants up to two years of age.
“Enteroinvasive” strains invade the epithelial cells of large intestine and cause
diarrhoea in older children and adults. 
“Enterotoxigenic” (enterotoxin
producing) strains produce one or both of two different toxins:a heat stable
toxin (ST) and a heat labile toxin (LT). Both toxins cause diarrhoea in adults
and infants. Enterotoxigenic strains of E. coli are often associated with
Travellers’ diarrhoea, a common disease contracted by tourists when visiting
developing countries. 
Diagnosis of travellers’ disease is based on the past
travel history and symptoms. Laboratory diagnosis is by isolation of the
bacteria from feces. Treatment is with fluid and electrolytes. Other strains of E.
coli which are usually harmless in their normal habitat (the intestine) can cause
disease when they gain access to other sites or tissues. These diseases include
urinary tract infections, septic infections, bacteremia, meningitis, pulmonary
infections, abscesses, skin and wound infections.
Prevention and Control: Involves avoiding contaminated food and water that
have high coliform counts, avoiding unpasteurized juices, washing fresh fruits
and vegetables thoroughly before eating raw, using adequate cooking
procedures for destruction and prompt refrigeration. Most people recover from
E. coli infection within 5-10 days without treatment. Antibiotics and
antidiarrhoeal drugs are usually not helpful.


Bacillus cereus is not a common cause of food poisoning. It is a Gram positive,
aerobic, spore forming rod shaped bacteria normally present in soil, dust and
water. The bacterium has a minimum growth temperature around 4-5ºC, with
maximum around 48-50ºC. Optimum pH range for growth is 4.9 to 9.3.
Symptoms: Extremely large numbers (108
per gram) of viable cells of
B.cereus must be ingested to develop signs and symptoms of the syndrome.
The bacterial cells produce intoxication characterized by acute abdominal pain,
flatulence and watery diarrhoea. 
Headache and dizziness are common,
dehydration and prostration may occur but nausea, vomiting, fever and chills
are rare. The illness appears within 6-15 hours after consumption of food and
the symptoms usually last less than 24 hours.
Associated Foods: Vehicle foods consist of cereal dishes that contain corn and
corn starch, mashed potatoes, vegetables, minced meat, liver sausage, milk,
cooked meat. Food mixtures such as sauces, puddings, soups, pastries and
salads have frequently been incriminated in outbreaks.


Cholera is caused by the gram negative, V. cholerae, which is acquired by
ingesting food or water contaminated by fecal material from patients or carriers
(shellfish and plankton may be the natural reservoir).
Symptoms: Once the bacteria enter the body, the incubation period is from
several hours to three or more days. An infective dose of around one million
organisms should be ingested to cause illness. The bacteria adhere to the small
intestine wall, where they secrete the cholera enterotoxin, choleragen. 
As a
result, there is hyper secretion of water and chloride ions, while inhibiting
absorption of sodium ions. The patient experiences an outpouring of fluid and
electrolytes with associated abdominal muscle cramps, vomiting, fever and
watery diarrhoea. 
The diarrhoea can be so profuse that a person can lose 10-15
liter of fluid during the infection. Death may result from the elevated
concentration of blood proteins, caused by reduced fluid levels, which leads to
circulatory shock and collapse. Onset of the illness is generally sudden, with
incubation periods varying from 6 hours to 5 days.
Associated Foods: Cholera is generally a disease spread by poor sanitation,
resulting in contaminated water supplies. Sporadic cases occur when shellfish
harvested from fecally polluted coastal waters are consumed raw.
Diagnosis: Cholera can be confirmed only by the isolation of the causative
organism from the diarrheic stools of infected individuals.
Prevention: Following recommendations are there to prevent cholera
• Drink only water that you have boiled or treated with chlorine or iodine.
Other safe beverages include tea and coffee made with boiled water and
carbonated, bottled beverages with no ice. 
• Eat only those foods that have been thoroughly cooked and are still hot, or
fruit that you have peeled yourself. 
• Avoid undercooked or raw fish or shellfish. 
• Make sure all vegetables are cooked, avoid salads. 
• Avoid foods and beverages from street vendors.
A simple thumb rule is “Boil it, cook it, peel it, or forget it”
Control: Individuals infected with cholera require oral rehydration therapy
with NaCl plus sucrose, sodium bicarbonate and potassium chloride to
stimulate water uptake by the intestine. The antibiotics of choice are a
tetracycline or aproflaxin. 
The most reliable control methods are based on
proper sanitation, especially of water supplies. The mortality rate without
treatment is often over 50%. Medical treatment to prevent dehydration prevents
all complications.

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