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Bacillus cereus

In microbiology, the term bacillus means any rod-shaped microbe (and coccus means a spherical microbe). However, Bacillus (written with a capital letter and italicized) refers to a specific genus of bacteria. The family Bacillaceae are all Gram-positive, rod-shaped bacteria which form endospores, with two main divisions:

Characteristically, Bacillus cultures are Gram-positive when young, but may become Gram-negative as they age. Bacillus species are aerobic, sporulating, rod-shaped bacteria which are ubiquitous in nature. Gram-stained cells, 1 µm wide, 5-10 µm long, arranged singly or in short chains.

Bacillus endospores are resistant to hostile physical and chemical conditions, but in addition various Bacillus species have a wide range of physiologic adaptations which enable them to survive or thrive in harsh environments, ranging from desert sands and hot springs to Arctic soils and from fresh waters to marine sediments. Because the spores of many Bacillus species are resistant to heat, radiation, disinfectants, and desiccation, they are difficult to eliminate from medical and pharmaceutical materials and are a frequent cause of contamination. Bacillus species are well known in the food industry as spoilage organisms. At the start of this video, spores can be seen as the bright, refractile objects seen under phase contrast microscopy. The second part of the video show green spores differentiated from pink vegetative cells by a spore staining procedure:

Spore stainOnly a few genera of bacteria such as Bacillus and Clostridium are capable of forming endospores. These are dormant form of the bacterium that allows it to survive sub-optimal environmental conditions. Spores have a tough outer covering made of keratin and are highly resistant to heat and chemicals. The keratin also resists staining, so specialized procedures are necessary to stain endospores:

- Malachite green stain is forced into the spore by heating the cells.
- Vegetative cells are then decolorized with water and stained pink with safranin counterstain.

Endospores may be located in the middle of the cells (central), at the end (terminal), or between the end and the middle of the cells (subterminal). The endospores themselves may be round or oval. B. cereus produces terminal endospores as can be seen in the video.

So what's all the fuss about?

Bacillus cereus causes two distinct food poisoning syndromes:

Bacillus food poisoning usually occurs because heat-resistant endospores survive cooking or pasteurization and then germinate and multiply when the food is inadequately refrigerated. The symptoms of B. cereus food poisoning are caused by toxins produced in the food during bacterial growth, principally a necrotizing enterotoxin and potent haemolysins (especially cereolysin). Phospholipases produced by B. cereus may act as exacerbating factors by degrading host cell membranes following exposure of their phospholipid substrates in wounds or other infections. Emetic food poisoning probably results from the release of emetic factors from specific foods by bacterial enzymes.

Food poisoning can largely be prevented by proper food handling:

 


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