Advanced Food Microbiology

Module 9: Course assessment

Bacillus cereus

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Bacillus cereus was originally isolated in 1887 from an agar plate that was left exposed in a cow shed.((Frankland, Grace C.; Frankland, Percy Faraday (1 January 1887). “Studies on some new micro-organisms obtained from air”Philosophical Transactions of the Royal Society B: Biological Sciences178: 257–287. Bibcode:1887RSPTB.178..257Fdoi:10.1098/rstb.1887.0011JSTOR 91702.))

Characteristics of Bacillus cereus are:

  • gram-positive
  • distinctive rod shape
  • facultatively anaerobic (it can grow in both the presence or absence of oxygen)
  • motile
  • spore forming

B. cereus belongs to a group of bacilli that includes 7 closely related species, which also includes Bacillus anthracis (responsible for Anthrax – a deadly disease of livestock and potentially humans).

B. cereus is known to cause two different types of foodborne illness: the diarrhoeal type and the emetic type.

The diarrhoeal type of foodborne illness is caused by an enterotoxin(s) produced during vegetative growth of B. cereus in the small intestine, whereas the emetic illness is caused by a toxin, preformed by B. cereus whilst growing in the food.

For both types of foodborne illness, the food involved has generally been heat treated and it is the surviving spores that are the source of the food poisoning, following germination. B. cereus tends not to be a very competitive microorganism but grows well in food following cooking and cooling (<48°C), when other organisms have been destroyed by the cooking process.

Once germinated, B. cereus cells can duplicate rapidly under optimum conditions and with no competition – doubling in numbers every 12 minutes.

As an inhabitant of the soil, B. cereus can easily spread to many food commodities, including: rice, pasta, meat, eggs and dairy.

It is a food safety hazard in products utilising any of the above ingredients and undergoing a heat treatment (followed by inadequate cooling times), such as: pies, soups, curries, stews, pasta and rice dishes.