Staphylococcus aureus

The name Staphylococcus comes from the Greek staphyle, meaning a bunch of grapes, and kokkos, meaning berry, and that is what staph bacteria look like under the microscope, like a bunch of grapes or little round berries.

There are over 30 different types of staphylococci that can infect humans, but most infections are caused by Staphylococcus aureus. These bacteria are commonly found in the nose, as well as on the skin on around 25%-30% of healthy adults and animals. The bacteria can also produce 9 toxins (enterotoxins) and is one of the most common causes of food poisoning. In America, there are almost 250 000 reported cases a year of Staphylococcal food poisoning. These bacteria have a high salt tolerance and the toxins produced are highly stable, heat-resistant and resistant to environmental conditions such as freezing and drying. These bacteria can grow in a wide range of temperatures from 6 degrees Celsius to 46 degrees Celsius and in a pH of 4.2 to 9.3.

Staphylococcus aureus

Sources of contamination

Staphylococcus aureus
Staphylococcus aureus
Staphylococcus aureus
Staphylococcus aureus

The most common source of contamination to food products is from workers/food handlers who carry the bacteria, and the highest risk comes in when workers have a skin infection and handle foods that are uncooked or left out at room temperature. High-risk food products are ones that require a high amount of handling and are often stored at room temperature.

Some examples of typical foods are:

  •             Sandwich fillings
  •             Cold salads
  •             Pastries/cakes (dairy-filled)
  •             Dairy products (milk, cheese)
  •             Meats
  •             Meat products / processed meats
  •             Poultry
  •             Egg products

Staphylococcus aureus

How we test for S.aureus in our SANAS accredited laboratory

At FCS the method of testing for Staphylococcus aureus is performed by taking a sample  (swab or food) and growing a measured portion of it on a selective media called Baird Parker Agar (BPA). This sample is spread evenly over the surface of the media and then incubated at ±37 degrees Celsius for ±48 hours. Typical colonies are black, shiny, convex, smooth and ±1.5mm in size, with a narrow white edge surrounded by a clear zone 2-5mm wide. Atypical colonies look black, shiny with or without a narrow white edge; the clear zone is absent or barely visible.

If typical or atypical colonies are found on the media, then a catalase test is done on the colonies by adding a drop of hydrogen peroxide to it. Effervescence is an indication of the presence of catalase. Pathogenic staphylococci are catalase positive. A Haemagglutination test is used after on the catalase positive colonies for confirmation.

Symptoms of food poisoning

When Staphylococcus aureus grows on food it produces toxins, thus when Staphylococcal food poisoning does occur it is not from ingesting the bacteria itself but rather from ingesting the toxins present on the contaminated food.

Symptoms usually develop within 1 to 6 hours after ingesting the contaminated food and generally lasts from 1 to 3 days.

Some of the most common symptoms of Staphylococcal food poisoning are:

  • Upset stomach
  • Stomach cramps
  • Nausea
  • Vomiting
  • Diarrhoea
  • Fever

Staphylococcus aureus

Preventing Staphylococcal food poisoning

Key aspects of preventing Staphylococcal food poisoning are:

  • Maintaining the cold chain of foods to prevent the growth of the bacteria on food products.
  •  Control of raw ingredients.
  • Personal hygiene practices by workers and food handlers.
  • Proper handling and processing in order to prevent cross-contamination.
  • Adequate cleaning and disinfecting of equipment.
  • Following guidelines such as:
  • Good Manufacturing Practice (GMPs)
  • Good Hygienic Practices (GHPs)
  • Hazard Analysis and Critical Control Points (HACCP)
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LEIGHTON BROWN

Chief Consultant

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