B io Factsheet
www.curriculum-press.co.uk
Number 216
The Biology of Salmonella
There are many reasons why cases of Salmonella food poisoning
are on the increase:
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•
•
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Over 2000 antigenic strains of Salmonella bacteria have been
associated with the disease in humans and animals. Each strain has
slightly different proteins on the outer surface of the bacteria. This
huge number of strains explains why vaccines are ineffective –
every strain would need a different vaccine.
Animals being transported longer distances to abattoirs
collecting bacteria in transit
Spread of abattoir waste on grazing fields
Lack of training of meat handlers
Insufficient inspection of premises
Lysozyme in saliva kills bacteria such as Salmonella and in healthy
individuals large numbers of bacteria are required to cause disease
– in other words, Salmonella is not highly infective.
There are basically three types of exam question on Salmonella:
1. How Salmonella enters and affects the human body
2. How food poisoning by Salmonella can be avoided
3. Comparisons of food poisoning and typhoid fever – both of
which are caused by species of Salmonella
Typical Exam Question
Salmonella can be spread in infected milk. Pasteurisation of milk
is intended to prevent the bacteria surviving. The diagram shows
how milk is collected and processed in the UK.
How Salmonella enters and affects the body
There are 8 key points:
• Salmonella bacteria (e.g. S. entiriditis) enter the body in
contaminated food or drink
• e.g. raw eggs, raw or undercooked meat or faecally –
contaminated food
• The bacteria enter the epithelium cells lining the ileum and
caecum
• After each bacterium dies, it releases endotoxins
• Thus there is a time delay before symptoms appear
• The endotoxin lowers the water potential in the intestines
causing water to move out of epithelial cells by osmosis, leading
to diarrhoea
• Other symptoms include vomiting / abdominal pain / fever /
headache
• The resulting dehydration and inability to absorb food may
cause serious harm to infants and the elderly. Infants’ immune
system may not be fully developed and that of elderly people
may be impaired, thus Salmonella infection may allow the entry
of more pathogenic bacteria
milk line
vacuum pump
receiving
vessel
milk
pump
filter
cooler
bulk tank
Extract from Chief Examiner’s Markscheme
Some candidates were uncertain of the region of the gut affected
– stomach was not awarded a mark. Many did not seem to
release that the bacteria enter the cells and precisely when the
toxins were released.
purity
tests
pasteurisation
bottling
Typical Exam Question
(a) Suggest one way in which the milk in the bulk tank could
become infected by salmonella (1)
(a) The endotoxin released by dead Salmonella causes epithelial
cells lining the intestine to secrete sodium ions and chloride
ions into the lumen. This causes diarrhoea. Explain how (2).
(b) Explain how pasteurisation attempts to prevent infection (2)
(b) Explain why there is usually an interval of several hours
between someone ingesting Salmonella bacteria and their
suffering from diarrhoea and sickness (2)
(a) contaminated udders/ faecal contamination during milking;
(b) held in a heated vat at 65°C for 30 minutes, followed by quick
cooling to about 4°C;
combination of high heat, length of time and rapid cooling
kills bacteria;
(b) Salmonella only release endotoxins when they die;
takes time for enough bacteria to die/for bacteria to multiply;
Markscheme
Markscheme
(a) Water potential of intestine is lowered;
So water moves out of the cells/cannot enter the cells;
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Bio Factsheet
216. The Biology of Salmonella
www.curriculum-press.co.uk
Antibiotic-resistant Salmonella
Typhoid fever
Salmonella typhimurium DT 104, which first appeared in cattle in
the late 1980s and has now spread to pigs, sheep and poultry,
has become resistant to at least four antibiotics. This strain already
accounts for around 15% of all salmonella cases.
Scientists are blaming the strain’s increasing resistance on the
overuse of antibiotics in farming.
•
Highly infective means that only a small number of the bacteria
are required to cause disease (symptoms) / infection
•
Highly invasive means that, once it has entered the body, S.
typhi spreads widely in blood / tissue fluid
Typhoid fever is caused by the bacterium, Salmonella typhi. This is
a highly infective and highly invasive pathogen.
Typical Exam Question
Whereas the strains of Salmonella that cause food poisoning only
infect the gut, S.Typhi spreads widely. This helps explain why
symptoms take a lot longer in typhoid fever – it takes time for the
bacteria to spread throughout the body.
How can the overuse of antibiotics in farming lead to a strain
of Salmonella becoming antibiotic resistant?
Initial use of antibiotic kills large proportion of bacteria;
Small e.g. 1% have mutation that allows them to survive;
Repeated use of antibiotic kills non-mutated strains, reducing
competition and allowing rapid growth of mutated, resistant
form;
Extract from Chief Examiner’s Markscheme
Although many candidates knew that typhoid was highly
infective, they also beliebed that this meant that this required
large numbers of bacteria. Few realised that salmonella food
poisoning is rapid because it is confined to the gut whereas the
symptoms of typhoid takes time to develop, as the bacteria have
to move out of the gut and infect other body tissues.
Markscheme
How can food poisoning by Salmonella be avoided?
Most cases of Salmonella occur in the summer. This is because the
bacteria can multiply more rapidly at warmer temperatures. Good
refrigeration and cooking techniques, combined with careful personal
hygiene minimize the risk of infection.
People who have recovered from typhoid fever may still have large
numbers of S. typhi bacteria in their large intestines. These people
do not show any symptoms of typhoid fever but can infect others
with S. typhi because the bacteria may be present in their faeces
and they may contaminate food, drinking water or people e.g. when
shaking hands.
Exam Hint:- When describing how food poisoning can be
avoided, make sure you explain why each technique works
• Wash hands after going to the toilet because Salmonella
may be present in faeces
• Store food in fridge because low temperatures reduces rate
of growth of bacteria
• Thaw and cook food thoroughly to kill the bacteria
• Store uncooked meat below cooked food in the fridge to
prevent transfer of bacteria to cooked foods
• Wash hands/utensils in contact with raw meat before
touching cooked foods to prevent transfer of bacteria to
cooked foods
• Use different chopping boards for raw and cooked meats to
prevent transfer of bacteria to cooked foods
• Boil/throw away dish cloths after use because bacteria can
multiply / spread in cloth
Typically, exam questions ask you to compare typhoid with
Salmonella food poisoning (Table 1)
Table 1.
Typical Exam Question
(a) Eggs are a common source of Salmonella food poisoning.
Outline how the way in which eggs are used may increase the
risk of Salmonella food poisoning (2)
(b) Explain how each of the following rules minimises the risk of
Salmonella food poisoning.
(i) Keeping raw meat at the bottom of the refrigerator and
cooked meat at the top (1)
(ii) Using disposable dish cloths and throwing them away
after use (1)
Salmonella food
poisoning
(e.g. S.enteriditis)
Typhoid fever
(S. typhi)
Transmission
Ingestion of
contaminated food
Ingestion of
faecallycontaminated
water
Infectivity
Low infectivity –
high number of
bacteria needed to
establish the
disease
High infectivity –
low number of
bacteria needed to
establish disease
Time between
infection and
symptoms appearing
Short – within 24
hours
Long – 10 to 14
days as the
bacteria have to
leave the gut
Cause of symptoms
Endotoxin
Endotoxin
(a) Contamination from shell;
Cooking at low temperature does not kill bacteria / raw eggs
may contain bacteria;
(b) (i) bacteria in raw meat;
bacteria/exudates from raw meat cannot fall on cooked
food;
(ii) bacteria may (get into cloth) and be transferred elsewhere/
bacteria grow/multiply (in moisture/food solution) on
cloth;
Acknowledgements:
This Factsheet was researched and written by Kevin Byrne.
Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU.
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permission of the publisher. ISSN 1351-5136
Markscheme
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