Foodborne Infections

Characteristics of Foodborne Infections
Live cells of the enteric pathogens(bacteria and viruses)
have to be consumed through food.
The surviving cells(from gastric environment) penetrate
through the membrane and establish in the epithelial
cells of the intestines, multiply and produce
Dose levels that cause infection vary greatly. e.g.
10 cells from virulent E. coli 0157:H7 might be
required for disease.
Symptoms generally occur after 24 h, which, depending
on pathogen, can be both enteric and nonenteric in
Enteric symptoms are local and due to enteric
infection and the effect of toxins. Symptoms include
abdominal pain, diarrhea, nausea, vomiting, and
fever. Examples of pathogens include Salmonella,
Shigella, enteroinvasive E. coli(EIEC), Vibrio
parahaemolyticus, Campylobacter jejuni, and
Yersinia enterocolitica.
Nonenteric symptoms result when the pathogens
or their toxins pass through the intestine and
invade other internal organs. Symptoms depend on
the affected tissue, but are accompanied by fever.
Examples of pathogens include Listeria
monocytogenes, enterohemorrhagic E. coli(EHEC),
hepatitis A virus.
Pathogenic Escherichia coli
E. Coli is Gram negative, motile, nonsporulating,
rod-shaped, facultative anaerobic bacterium.
E.Coli is a normal inhabitant of the intestinal tract
of humans, warm-blooded animals and birds.
E.Coli is used as an index organism for possible
fecal contamination and presence of enteric
pathogens in food and water.
Certain E.coli strains are pathogenic and cause
Pathogenic Groups Of E. coli
Enteropathogenic E. coli(EPEC)
These strains are important in infant diarrhea
worldwide, especially in places with poor
They cause waterborne and foodborne disease
outbreaks in different countries.
They have the ability to form a close contact with
the intestinal epithelial cells and cause lesions. The
predominant symptom is gastroenteritis.
The infectious dose exceeds several million cells.
Enterotoxigenic E. coli(ETEC)
These strains are the major cause of diarrhea in
travelers and infants where there is poor
The pathogens produce an invasive factor and heat
labile(HL) and heat stable(HS) or both
The symptom is gastroenteritis, like a mild form of
The pathogen is spread by human carriers.
Ingestion of several millions of cells is necessary to
cause disease.
Enteroinvasive E. coli(EIEC)
These strains cause dysentery similar to that
caused in shigellosis.
They produce an invasive factor which is the
cause of the disease.
Human carriers spread the disease.
Ingestion of one million cells is necessary for
an individual to develop symptoms.
Enterohemorrhagic E. coli(EHEC)
The strains in this group(a principal serogroup is (0157:H7)
produce powerful toxins causing bloody diarrhea(hemorrhagic
colitis) due to the damage of the lining of large intestine and
hemorrhagic uremic syndrome (HUS) in humans. HUS results
from hemolyzed RBCs and clotting in small blood vessels of
the kidney leading to kidney damage and occasionally kidney
failure, it can be fatal , particularly in children.
Animals, particularly dairy cattle, are thought to be the
Ingestion of as few as 10-100 cells can produce the disease.
Enterotoxins(verotoxins or shiga toxin ;VTEC or STEC)
produced by the serotype cause the disease symptoms. Toxins
are absorbed into the blood stream and damage the small
blood vessels in the intestine, kidneys and brain.
Pasteurization of milk
Refrigeration or freezing of perishable foods
Never thaw a food at room temperature or keep a
refrigerated food at room temperature over 2h
Wash hands, utensils, and work areas with hot
soapy water after contact with raw meat
Prevent fecal-oral contamination through proper
personal hygiene.
Proper sanitation at all stages of food processing
and handling
Individuals suspected of being carriers should not
handle food, especially ready-to-eat.
Shigellosis(Bacillary Dysentery)
The genus Shigella contains four species:
Shigella dysenteriae
Shigella flexneri
Shigella boydii
Shigella sonnei
The organisms are either transmitted directly
through fecal-oral routes or indirectly through
fecal-contaminated food and water.
Shigellosis occurs more frequently in places with
poor sanitation.
Gram-negative, nonmotile, facultative anaerobic rods,
catalase positive, oxidase and lactose negative. They
ferment sugars without gas.
The strains grow between 7-46C with an optimum at
They survive for days under refrigeration, freezing, 5%
NaCl, pH 4.5.
They are killed by pasteurization.
The strains can multiply in many foods when stored at
growth temperature range.
Following recovery, a person can remain a carrier for
Shigella can invade epithlial mucosa of the small
and large intestines.
Once engulfed by the epithelial cells, they can
produce an exotoxin that has an enterotoxigenic
The toxin is known as Shiga toxin(ST). It is
expressed at 37C.
The engulfed shigellae cells kill the epithelial cells
and then attack fresh cells, causing ulcers and
The infective dose is very low(10-1000 cells)
Symptoms occur in 1-3 days
The symptoms are the consequence of both
invasiveness of epithelial mucosa and the
enterotoxin and include abdominal pain, diarrhea
often mixed with blood, mucus and pus, fever, chills,
and headache. Children are more susceptible than
Foods implicated in shigellosis are those that are
handled too much and are ready to eat. As the
infective dose is very low, growth in food is probably
not an important factor for the disease.
Prevention like pathogenic E. coli.
Foodborne salmonellosis is the leading cause of
foodborne bacterial and viral diseases in the U.S.A and
other developed countries.
It may be related to the large number of serotypes
present in high frequency in carrier state in food
animals, birds, pets, insects, humans, and their ability to
grow in foods, the way the food animals and birds are
raised, processed, and marketed.
The present increase in salmonellosis could be related to
four factors; increase in number of antimicrobialresistant Salmonella isolates, increase in
immunodeficient individuals, increase in laying hens
with infected ovaries, food production in centralized
facilities that can lead to, if contamination occurs,
extremely large and widespread outbreaks.
There are more than 2000 serovars(based on somatic,
flagellar, and capsular antigen types) of Salmonella,
potentially capable of causing salmonellosis in humans.
Along with fecal-oral direct transmission,
contaminated food and water can cause salmonellosis.
Salmonella cells are Gram-negative, nonsporulating,
facultative anaerobic, motile rods, they form gas while
growing in media containing glucose, they ferment
dulcitol but not lactose, utilize citrate as a carbon
source, produce H2S, decarboxylate lysine and
ornithine, do not produce indole and are negative for
Salmonella are mesophilic with optimum growth
temperature between 35-37C, with growth range of 546C, they are killed by pasteurization temperature,
sensitive to low pH.
The cells survive in frozen and dried states for a long
They can multiply in many foods without affecting the
acceptance qualities.
Salmonellae are natural inhabitants of the GIT of
domisticated and wild animals, birds and pets and
insects. In humans and animals they cause salmonellosis
and then persist in a carrier state. They have also been
isolated from soil, water and sewage contaminated with
fecal matters.
Toxins and disease
Following ingestion of Salmonella cells, the
pathogens invade mucosa of the small intestine,
proliferate in the epithelial cells, resulting in an
inflammatory reaction and fluid accumulation in the
The ability of the pathogens to invade and damage
the cells is due to the production of a thermostable
cytotoxic factor.
Once inside the epithelial cells, the pathogen
multiply and produce a thermolabile enterotoxin
that causes the secretion of fluid and electrolytes.
For foodborne salmonellosis, the infectious
dose is around one million cells, for more
virulent strains, ingestion of fewer cells can
cause the disease.
Symptoms appear in 24-36 h, symptoms last
for 2-3 days, an individual remains in a carrier
state for several months following recovery.
Symptoms are abdominal cramps, diarrhea,
nausea, vomiting, chills, fever. It can be fatal,
especially to the sick, infants, and the elderly.
Food Association
Foods of animal origin have been associated with
large numbers of outbreaks. These include beef,
chicken, turkey, pork, eggs, milk, and products made
from them.
Salmonella have also been isolated from many foods
of plant origin( use of sewage as fertilizer or washing
products with polluted water), and fish harvested
from polluted water.
Different serotypes of salmonella are geographically
distributed and vary in their pathogenecity.
In the U.S., Salmonella Typhimurium has been
considered the major causative agent of foodborne
Foodborne salmonellosis from Salmonella
Enteritidis has increased, mainly from
contaminated shell eggs. The exact cause of
predominance of Salmonella Enteritidis is not
clearly understood. The methods used in raising
food of animals and birds and in processing of
foods of animal origin are suspected to have an
important role.
Prevention and Control
Raw foods of animal origin that are heat treated before
consumption can have Salmonella.
Many food processing industries have in-house
Control measures include proper cooking of
foods(minimum to pasteurization temperature and
time; 71.7C for 15 seconds and prompt cooling(3-4C or
freezing if not used in 2 h).
Preventing cross-contamination of ready-to-eat food
through cutting boards, equipment, utensils and
Using proper sanitation and personal hygiene.
Properly reheating a food refrigerated for a long time.
Detection Methods
The methods involve preenrichment of a sample in a
nutrient broth, followed by selective enrichment,
streaking on a selective-differential agar medium, and
biochemical and serological confirmation.
Case Study
A group of people(24) attended a party, 6-57h later, 21
persons had gastroenteritis, with severe diarrhea,
vomiting, fever and cramps. Twenty people needed
medical help, of which 18 were hospitalized.
Salmonella Enteritidis was isolated from all 21
people(rectal swab). Epidemiological investigation
revealed that the implicated food was homemade
pasta dish consisting of one raw egg,
Ricotta cheese, cooked tomato and meat sauce.
They were mixed together in a pan and
refrigerated overnight. Before serving, the
preparation was baked for 30 min at 177C. Several
attendees observed that the center of the dish
was cold when served. Salmonella Enteritidis was
isolated from the leftover and from the unused
eggs from the carton. Salmonella Enteritidis was
also isolated from laying hens at the poultry farm
that supplied the eggs.
Several Campylobacter species can cause human
The most common causative agents are :
Campylobacter jejuni
Campylobacter coli
They are Gram-negative, motile, nonsporulating, rod-shaped
bacteria. The cells are small, fragile, and spirally curved. The
strains are microaerophilic and catalase and oxidase positive.
Growth temperature ranges between 32-45C, with optimum
42C. They survive well under refrigeration and for months in
the frozen state.
Campylobacter jejuni is an enteric organism, it has
been isolated from feces of animals and birds.
Human carriers were found to shed the organisms in
Fecal materials from poultry were found to contain
more than one million cells/g.
Water, sewage, vegetables, and foods of animal origin
are easily contaminated with campylobacter jejuni
through feces.
Campylobacter jejuni has a thermolabile enterotoxin
which cross –reacts with cholera toxin. In addition, the
strains produce an invasive factor that enables the cells
to invade and establish in epithelial cells in both the
small and large intestines in humans.
Disease and Symptoms
The infective dose of campylobacteriosis is
low(500 cells). Following ingestion, symptoms of
the disease occur in 2-5 days and last for 2-3 days.
Persons with no visible symptoms can shed the
cells in feces for a long time.
The main symptoms are enteric and include
abdominal cramps, diarrhea, nausea and vomiting.
Other symptoms include fever, headache, and
chills. In some cases bloody diarrhea has been
Food Association
Because the organism is present in high frequency
in animals, birds, and the environment, many
foods, both from plant and animal sources, can be
contaminated with Campylobacter jejuni.
The organism has been isolated frequently from
raw meats(beef, lamb, pork, chicken, and turkey),
milk, eggs, vegetables and mushrooms.
The use of animal feces as fertilizers was found to
contaminate vegetables.
Proper sanitation reduces food load of the organism
during production, processing and future handling.
Preventing consumption of raw foods of animal
Heat-treatment of food.
Prevention of post-heat contamination.
Contamination of vegetables can be controlled by not
using animal fertilizer and contaminated water to
wash vegetables.
Adopting good personal hygiene to control
By humans.