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FOOD MICRO-ORGANISMS THEIR SIGNIFICANCE
Bacteria are not visible to
the naked eye. They exist in and on the human body, in soil
and in air, but only a few types can grow in food and cause
food poisoning.
Under favourable conditions
bacteria can multiply very rapidly simply by dividing in two
every 20 to 30 minutes, so that one cell could produce up
to 16 million cells within 8 hours.
With some bacteria, toxins
are produced which are not destroyed by cooking – a food
does not have to look, smell or taste ‘off’ to be
potentially hazardous to health.
Total Viable Count (TVC)
TVC reflects the conditions in which the food was produced,
stored or abused. With experience, this count can be used
to predict the shelf life or keeping quality of the product.
The spoilage of many foods may be imminent when the total
viable count reaches 10- 100 million per g.
Coliform
Counts
Coliform counts are generally
used as an indicator of possible faecal contamination, and
reflect the hygiene standards adopted in the food’s preparation.
Improper handling and storage can allow the level to increase.
Coliforms are also found on many types of plant material since
the organisms are usually found at high levels in soil.
Escherichia
coli (E. coli)
This is a faecal coliform which
exists in the intestine of animals and man, and is an indicator
of poor hygienic practice. Some strains eg E coli O157 can
cause illness when present at levels as low as 10 per gram
of food. These strains would not necessarily be included in
traditional E coli tests: the very low infective dose means
that cross contamination between foods is a particular hazard.
Staphylococcus
aureus
This organism is commonly found
on the skin, hair and mucous membranes, and very high concentrations
are often found in wounds, sores or septic spots. They can
readily be transferred to food by poor handling. Incubation
time is short, and violent vomiting and diarrhoea can occur
within 2 to 6 hours.
Salmonella
This organism accounts for
over 50 per cent of all reported cases of food poisoning.
Symptoms usually develop between 12 and 36 hours after ingestion,
causing pain, diarrhoea, fever and vomiting which can last
for 6 days or more.
Listeria
Although Listeria is widespread
in the environment, it is only in recent years that the public
have become aware of this organism. It has been involved in
cases of food poisoning from the ingestion of affected milk,
vegetables, meats, soft cheeses, ice cream and seafood. Listeria
monocytogenes is the most significant species and can grow
at refrigerator temperatures.
Yeasts
and Moulds
Yeasts do not cause food poisoning,
but some types are capable of causing food spoilage. Moulds
can produce mycotoxins, some of which can affect man adversely.
Mould spores can be carried by the wind, and hence can have
easy entry to a factory.
"Faecal
streptococci"
These organisms occurs in man
and other animals. They do not multiply in water, but are
more resistant to sanitation and heat than E. coli and therefore
persist longer. The presence of faecal streptococci always
acts as an indicator of faecal contamination.
Bacillus
cereus
Bacillus cereus occurs in stools
and is common in soil. Hence, foods such as rice, cereals
and spices often contain this organism as can milk from cattle
with mastitis. The organism is a spore former and can survive
heat processing. Rapid growth of vegetative cells occurs at
ambient temperatures. These cells produce toxins, both in
the food and after ingestion.
Campylobacter
This organism is found in the
digestive tract of animals and birds, and is the commonest
cause of diarrhoea. Small numbers of these organisms (eg.
500) are sufficient to cause infection in man. The commonest
sources of poisoning are poultry, eggs and dairy products.
Contamination is usually caused by storing near raw meats
or inadequate heat treatment. Symptoms usually occur between
2 and 7 days.
Clostridium
perfringens
Clostridium perfringens is
anaerobic, ie. it grows in the absence of oxygen. It is a
very common organism, and is found in soil and the excreta
of most mammals. The spores can survive in most conditions,
and are found throughout the environment. They can survive
cooking and the organism
is therefore significant in cold meats. Its rate of growth
is slow below 15 °C. It produces a toxin, causing symptoms
of abdominal pain, diarrhoea and nausea between 8 and 24 hours after infection.
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