Salmonella species Introduction
Salmonella spp. are a group of bacteria which reside in the intestinal tract of human beings and warm
blooded animals and are capable of causing disease. They are the second most common cause of
bacterial foodborne illness in Ireland (Campylobacter spp. is the most frequent cause). They are
facultative anaerobic Gram-negative rods. Salmonella spp. are members of the Enterobacteriaceae
group. The genus Salmonella contains 2 species:
• Salmonella enterica • Salmonella bongori
Salmonella enterica is an important agent of foodborne illness. This species is sub-classified into 6
subspecies of which S. enterica subspecies enterica is the most important for human health. The genus
Salmonella can be subdivided into more than 2,400 serotypes. Salmonella enterica subsp. enterica
serotype Typhimurium (S. Typhimurium) and Salmonella enterica subsp. enterica serotype Enteritidis (S.
Enteritidis) are the most frequently isolated serotypes in humans in Ireland. Serotypes are further subdivided by their resistance to bacteriophages (phage types or lystotypes), antibiotics or heavy metals;
their biochemical characteristics (biovars or biotypes) or their sensitivity to or production of bacteriocins.
Growth and survival characteristics
Table1 the Factors affecting the growth of Salmonella spp.
Conditions
Minimum
Optimum
Maximum
Temperature (ºC)
5.2*
35 – 43
46.2
pH
3.8
7-7.5
9.5
Water activity (aw)
0.94
0.99
>0.99
* Most serotypes fail to grow at < 7ºC
Salmonella spp. are not particularly heat resistant and most serotypes are killed by normal cooking
conditions, i.e. cooking to a core temperature of 75ºC instantaneously or an equivalent time
temperature combination, e.g.70ºC for 2 minutes. However, a few highly heat resistant serotypes have
been reported, e.g. S. Senftenberg 775W and S. Irumu. Heat resistance is influenced by water activity
(aw), nature of the solutes and pH of the suspending medium. Greater heat resistance is observed for
cells in sucrose compared with NaCl at the same aw values. Heat resistance also increases with
increasing concentration of milk solids.
Occurrence in food
Many foods have been identified as vehicles for the transmission of this pathogen to humans,
particularly foods of animal origin but also foods of non-animal origin which may be subject to faecal
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contamination. Those of particular importance include eggs, pig meat, poultry meat, milk, chocolate,
fruit and vegetables. It is important to note that although Salmonella spp. do not form spores, they can
survive for long periods in foods.
The Illness (salmonellosis)
Symptoms usually appear 12-36 hours after the ingestion of contaminated food. Symptoms include
acute entercolitis with sudden onset of headache, abdominal pain, diarrhoea, nausea, vomiting and
fever. Dehydration may occur particularly among vulnerable populations, e.g. infants, the immunocompromised and the elderly. The condition usually lasts 2-5 days and is generally self limiting.
Complications can lead to systemic infections and can result in various chronic conditions such as
reactive arthritis. The infective dose can vary depending on the strain, the immuno-competence of the
individual and the nature of the food. Data from outbreaks of foodborne diseases suggest that infections
may be caused by the ingestion of as few as 10-45 cells. S. Typhi and S. Paratyhi can cause enteric
(typhoid) fever, a severe systemic life threatening condition, but it is very rare
Control of Salmonella spp. in the Food Chain
Since low numbers of Salmonella spp. can cause illness, it is important that control measures are taken
at all stages in the food chain. These are essential to protect the health of consumers and the integrity
of the business. Examples of control measures include:
• Implementation of Good Hygiene Practices (GHP) and Good Manufacturing Practices (GMP) at all
stages in the food chain, i.e. at farm level, slaughterhouse, manufacturing, processing, catering, retail etc.
Particular attention should be paid to the prevention of cross contamination between raw and ready-toeat foods
• Implementation of a food safety management system based on the principles of HACCP. This includes
good process control, e.g. temperature control during cooking, storage
• Testing against microbiological criteria, as appropriate, when validating and verifying the correct
functioning of their HACCP based procedures and other hygiene control measures
Shigella species
Shigella spp. are bacteria that cause shigellosis, also known as bacillary dysentery. They are a
highly infectious organism, with foodborne outbreaks often involving infected food handlers.
Unlike other common foodborne pathogens, humans are the only natural hosts of Shigella spp.
Shigella spp. are Gram-negative, non-spore forming rod-shaped bacteria and are members of
the family Enterobacteriaceae. The genus Shigella is divided into four species based on their O
antigen type and biochemical characteristics: S. dysenteriae (comprising
15 serotypes), S. flexneri (comprising 14 serotypes), S. boydii (comprising 20 serotypes) and S.
sonnei (1 serotype) (Lampel and Maurelli 2003; Levine et al. 2007). The most severe form of
shigellosis is caused by S. dysenteriae serotype 1. S. sonnei causes the mildest form of disease,
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while S. flexneri and S. boydii can cause either severe or mild illness (FDA 2012). In Australia, S.
sonnei was the most frequently reported species in 2010, representing 55.6% of all notified
Shigella infections (OzFoodNet 2012).
S. dysenteriae serotype 1 is very rare in Australia, with all reported cases acquired overseas
(Lightfoot 2003).
Growth and survival characteristics
The growth and survival of Shigella spp. in foods is influenced by a number of factors such as
temperature, pH, salt content and the presence of preservatives
Table 2: Limits for growth of Shigella spp. when other conditions are near optimum (ICMSF
1996; Lightfoot 2003)
Conditions
Minimum
Optimum
Maximum
Temperature (°C)
6–8
-
45–47
pH
5
6–8
9
NaCl (%)
-
-
4–5
Occurrence in food
There is very little published surveillance data on the presence of Shigella in food. Some
international surveys have been performed in which Shigella spp. have been found in a range in
foods. For example, Ghosh et al. (2007) isolated Shigella spp. from 15% of coconut slices
(n=150), 9% of ready-to-eat salads (n=150) and 7% of samples of coriander sauces (n=150)
from Indian street vendors. Shigella spp. have also been detected in 11% of raw meat samples
(n=250) from retail outlets in Pakistan (Hassan Ali et al. 2010). In Mexico, Shigella spp. have
been isolated from 6% of freshly squeezed orange juice samples (n=100) and from the surface
of 17% of oranges sampled (n=75). All four Shigella spp. were isolated from the surface of the
oranges, whereas only S. sonnei and S. dysenteriae were isolated from the orange juice
samples (Castillo et al. 2006). Although Shigella can be isolated from a range of food, outbreaks
often occur due to an infected food handler contaminating food that is served cold or raw. A
study of foodborne shigellosis outbreaks in the US demonstrated that 20% of outbreaks were
due to exclusively raw food (e.g. lettuce based salads) and 30% of outbreaks were from partially
raw food (e.g. potato salad) (Nygren et al. 2012).
The Illness
Once ingested, Shigella spp. must survive the acidic environment of the stomach and invade
the epithelial cells of the colon to enable infection. Shigella spp. multiply inside the colonic
epithelial cells and spread to adjacent cells, leading to the death of the infected cells. The colon
becomes inflamed and ulcerated and the dead mucoid cells are shed, resulting in the bloody
mucoid diarrhoea often characteristic of Shigella infection (Lightfoot 2003; Montville and
Matthews 2005; Warren et al. 2006).
Shigella spp. have a virulence plasmid that encodes genes involved in the invasion process and
intra- and inter-cellular spread. Other genes involved in the invasion process are located on the
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chromosome (Warren et al. 2006). S. flexneri 2a produce the chromosome encoded shigella
enterotoxin 1, while most Shigella serotypes produce the virulence plasmid encoded shigella
enterotoxin 2. S. dysenteriae serotype 1 strains produce the potent Shiga toxin. Shiga toxin is
chromosomally encoded and has cytotoxic, enterotoxic and neurotoxic effects (Niyogi 2005;
Warren et al. 2006).
E.coli Introduction
Escherichia coli (E. coli) is a bacterium that is commonly found in the gut of humans and warm-blooded
animals. Most strains of E. coli are harmless. Some strains however, such as Shiga toxin-producing E. coli
(STEC), can cause severe foodborne disease. It is transmitted to humans primarily through consumption
of contaminated foods, such as raw or undercooked ground meat products, raw milk, and contaminated
raw vegetables and sprouts.
Growth and survival characteristics
Table3 the Factors affecting the growth of E.coli.
Conditions
Minimum
Optimum
Maximum
Temperature (ºC)
7
37
50
pH
4.4
-
-
Water activity (aw)
0.95
-
-
E.coil can grow in temperatures ranging from 7 °C to 50 °C, with an optimum temperature of 37 °C.
Some STEC can grow in acidic foods, down to a pH of 4.4, and in foods with a minimum water activity
(aW) of 0.95. E.coli is destroyed by thorough cooking of foods until all parts reach a temperature of
70 °C or higher.
Occurrence in food
The reservoir of this pathogen appears to be mainly cattle. In addition, other ruminants such as sheep,
goats, deer are considered significant reservoirs, while other mammals (such as pigs, horses, rabbits,
dogs, and cats) and birds (such as chickens and turkeys) have been found infected.E coli is transmitted to
humans primarily through consumption of contaminated foods, such as raw or undercooked ground
meat products and raw milk. Faecal contamination of water and other foods, as well as crosscontamination during food preparation (with beef and other meat products, contaminated surfaces and
kitchen utensils), will also lead to infection. Examples of foods implicated in outbreaks of E. coli include
undercooked hamburgers, dried cured salami, unpasteurized fresh-pressed apple cider, yogurt, and
cheese made from raw milk
Prevention
The prevention of infection requires control measures at all stages of the food chain, from agricultural
production on the farm to processing, manufacturing and preparation of foods in both commercial
establishments and household kitchens.
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Industry
The number of cases of disease might be reduced by various mitigation strategies for ground beef (for
example, screening the animals pre-slaughter to reduce the introduction of large numbers of pathogens
in the slaughtering environment). Good hygienic slaughtering practices reduce contamination of
carcasses by faeces but do not guarantee the absence of STEC from products. Education in hygienic
handling of foods for workers at farms, abattoirs and those involved in the food production is essential
to keep microbiological contamination to a minimum. The only effective method of eliminating STEC
from foods is to introduce a bactericidal treatment, such as heating (for example, cooking or
pasteurization) or irradiation.
Household
Preventive measures for E. coli O157:H7 infection are similar to those recommended for other
foodborne diseases. Basic good food hygiene practices, as described in the WHO “Five keys to safer
food”, can prevent the transmission of pathogens responsible for many foodborne diseases, and also
protect against foodborne diseases caused by STEC.
The five keys to safer food are:





Keep clean.
Separate raw and cooked.
Cook thoroughly.
Keep food at safe temperatures.
Use safe water and raw materials.
Such recommendations should in all cases be implemented, especially "cook thoroughly" so that the
centre of the food reaches at least 70 °C. Make sure to wash fruits and vegetables carefully, especially if
they are eaten raw. If possible, vegetables and fruits should be peeled. Vulnerable populations (such as
small children and the elderly) should avoid the consumption of raw or undercooked meat products,
raw milk, and products made from raw milk. Regular handwashing, particularly before food preparation
or consumption and after toilet contact, is highly recommended, especially for people who take care of
small children, the elderly or immunocompromised individuals, as the bacterium can be passed from
person to person, as well as through food, water and direct contact with animals. A number of STEC
infections have been caused by contact with recreational water. Therefore, it is also important to
protect such water areas, as well as drinking-water sources, from animal waste (4).
Producers of fruits and vegetables
WHO’s "Five keys to growing safer fruits and vegetables" provides rural workers who grow fresh fruits
and vegetables for themselves, their families and for sale in local markets, with key practices to prevent
microbial contamination of fresh produces during planting, growing, harvesting and storing.
The five keys to growing safer fruits and vegetables are:
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




Practice good personal hygiene.
Protect fields from animal faecal contamination.
Use treated faecal waste.
Evaluate and manage risks from irrigation water.
Keep harvest and storage equipment clean and dry.
Reference: WHO WEBSITE,FDA (2012) Bad bug book: Foodborne pathogenic microorganisms and
natural toxins handbook, 2nd ed, US Food and Drug Administration, Silver Spring, p. 25–28.
Veterinary World, Vol.1(3): 83-87. European Commission (2003) Opinion of the Scientific
Committee on Veterinary Measures relating to Public Health on Salmonellae in Foodstuffs
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