Etiket Arşivleri: Gram-positive

Bacillus Cereus ( Mehrdad Tajkarimi )

Bacillus cereus
Mehrdad Tajkarimi
Materials from Maha Hajmeer


Bacillus cereus is a Gram-positive, spore-forming microorganism capable of causing foodborne disease At present three enterotoxins, able to cause the diarrheal syndrome, have been described: hemolysin BL (HBL), nonhemolytic enterotoxin (NHE) and cytotoxin K. HBL and NHE are three-component proteins, whereas cytotoxin K is a single protein toxin. Symptoms caused by
the latter toxin are more severe and may even involve necrosis. In general, the onset of symptoms is within 6 to 24 h after consumption of the incriminated food. B. cereus food poisoning is underestimated probably because of the short duration of the illness (~24 h).


In 1887, Bacillus cereus isolated from air in a cowshed by Frankland and Frankland. Since 1950, many outbreaks from a variety of foods including meat and vegetable soups, cooked meat and poultry, fish, milk and ice cream were described in Europe. In 1969, the first well-characterized B. cereus outbreak in the USA was documented. Since 1971, a number of B. cereus poisonings of a different type, called the vomiting type, were reported. This type of poisoning was characterized by an acute attack of nausea and vomiting 1–5 h after consumption of the incriminated meal. Sometimes, the incubation time was as short as 15–30 min or as long as 6–12 h. Almost all the vomiting type outbreaks were associated with consumption of cooked rice. This type of poisoning resembled staphylococcal food poisoning.


Introduction to Lab Ex. Differential Stains – Gram Staining

Introduction to Lab Ex. Differential Stains – Gram Staining

Basic classification of bacteria is based on the cell wall structure. There are 2 main groups: Gram positive and Gram negative. Gram staining is a differential staining technique that provides an easy differentiation of bacteria into one of two groups.

The staining technique, developed in the late 1700’s by Christian Gram classifies the rigid cell walled bacteria into one of two groups based on whether they are able to resist the decolorizing  action of an alcoholic solution.

Those that resist decolorization by 95% ethanol are arbitrarily termed Gram positive and those that do not are Gram negative (the terms positive and negative have nothing to do with charges of the cell but based on differences in the cell wall structure of  these two groups of bacteria).

The characteristic compound found in all true bacterial cell walls is peptidoglycan. The amount of PPG is among one of the differences between the GP and GN cell walls.

Gram-positive cell walls

  • Thick peptidoglycan

  • 90% peptidoglycan

  • Teichoic acids

  • 1 layer

  • Not many polysaccharides

  • In acid-fast cells, contains mycolic acid

Gram-negative cell

  • Thin peptidoglycan

  • 5-10% peptidoglycan

  • No teichoic acids

  • 3 layers

  • Outer membrane has lipids, polysaccharides

  • No acid- fast cells (mycolic acid)