(http://medic.med.uth.tmc.edu/path/00001437.htm) Oct. 24
BIOL 447
Assignment 2:
A Scientific Study of Bacillus Cereus
Paul M. Jaworsky (95147514)
Nomenclature and Taxonomy
Bacillus cereus (pronounced "bah-sill-us" "serious") is a bacterium that is common in the natural environment and in a variety of foods. The organism (genus = bacillus; species = cereus) is so widespread that it is almost impossible to keep it from contaminating certain foods. B. cereus is able to form spores that can survive long periods of dryness and mild heat treatments such as cooking; temperatures under 212°F will allow for the survival of some Bacillus spores.. Not all strains of B. cereus are able to cause foodborne illness; only those strains that are able to produce one or more of two toxins are able to cause illness. (http://www.cehs.siu.edu/fix/medmicro/bacil.htm) Oct. 22
Bacillus cereus is an aerobic (needs oxygen to grow) spore-forming bacteria that is widely distributed in nature and in foods. It is commonly found in soil, milk, cereals, starches, herbs, spices, and other dried food stuffs. Foods most often implicated in outbreaks include meat pies, fried rice and puddings. The cells of this pathogenic microorganism are large and rod-shaped, as seen in the following image (seen under a common microscope):
(http://medic.med.uth.tmc.edu/path/00001437.htm) Oct. 24
The bacteria in this image appear purple in colour, and points to another characteristic of Bacillus cereus: they are gram positive. This means that they take up the original purple test-staining dye (and reject red), and as such, have a cell wall but no cell membrane. B. cereus also has a wide pH survival range of 4.5-9.3 and a short generation time of 20-30 min.
(http://www.ag.iastate.edu/departments/foodsci/classes/fshn420/bcereus.htm) Oct. 25
Summary of Biochemical Activities
Bacillus species are worldwide soil saprophytes, which means they survive on non-living organic material, and are responsible for the recycling of environmental Carbon, Nitrogen, and essential mineral nutrients. As such, they are important scavengers in ecosystems, and their spores can amazingly survive for 60 or more years under harsh natural conditions.
Besides Bacillus anthracis, B. cereus is the only "medically significant" species of Bacillus. Once inside the body of a human being, for instance, B. cereus makes several different biological consequences possible, and this is why this microorganism has been so extensively researched and studied in laboratory conditions. (http://medic.med.uth.tmc.edu/path/00001437.htm) Oct. 24
Bacillus cereus food poisoning results from the ingestion of preformed enterotoxins, producing predominantly vomiting and diarrhea. The vomiting form is most often associated with ingestion of a heat stable toxin from contaminated rice, while the diarrheal form is most often associated with ingestion of a heat labile toxin from contaminated meat or vegetables. Because toxin production usually takes place after the infected foods are cooked, proper cold storage of food is recommended immediately after preparation.
The preformed enterotoxins are made by B. cereus in food that has been contaminated, usually by a food handler, and then left to incubate outside a refrigerator. When the food is ingested, the toxin works on the wall of the gastrointestinal tract and/or on the central nervous system to produce the acute onset of vomiting or diarrhea. The bacilli do not necessarily multiply in the body, so the disease is not really an infection in the strict sense. It is better
described as an intoxication. The incubation period between ingestion of toxin and onset of symptoms is much shorter than occurs in food poisoning (where bacterial multiplication must occur). Intoxications may also be produced by Clostridium botulinum (botulism) or Staphylococcus aureus. (http://www.americanhistology.com/letters/library/currentconcepts.htm) Oct. 25
Bacillus cereus' extracellular products include proteases (sweet curdling of dairy products), phospholipases, hemolysins, and two toxin types:
1) enterotoxins: diarrheal and emetic (already mentioned in this paper)
2) lethal toxin: kills mice injected intravenously
(http://www.foogene.co.uk/features/b.cereus.htm) Oct. 25
Since Bacillus cereus is most closely related to the anthrax bacterium Bacillus anthracis, the following data will show some differences between the two:
Characteristic |
B. cereus | B. anthracis |
growth requirement for thiamin |
- | + |
hemolysis on sheep blood agar |
+ | - |
glutamyl-polypeptide capsule (mucoid colony) |
- | + |
lysis by gamma phage |
- | + |
motility |
+ | - |
growth on chloralhydrate agar |
+ | - |
string-of-pearls test |
- | + |
(http://www.bact.wisc.edu/Bact330/lectureanthrax) Oct. 23
A more complete comparison, including other types of bacilli, is included also:
(http://gsbs.utmb.edu/microbook/ch015.htm) Oct. 24
Genetics
Endospore-forming bacteria produce a unique resting cell called an endospore. As mentioned earlier, they are gram positive and usually rod-shaped, but there are exceptions. One important genera is Bacillus, the members of which are aerobic sporeformers in the soils and some aquatic environments.
Endospores are dehydrated, refractile cells appearing as points of bright light under phase microcsopy. Endospore-forming bacteria are characterized by the location (position) of the endospore in the mother cell (sporangium) before its release. The spore may be central, terminal or subterminal, and the sporangium may or may not be swollen to accomodate the spore. Endospores differ from the vegetative cells that form them in a variety of ways. Several new surface layers develop outside the core (cell) wall, including the cortex and spore coat.
The cytoplasm is dehydrated and contains only the cell genome and a few ribosomes and enzymes. The endospore is cryptobiotic (exhibits no signs of life) and is remarkably resistant to environmental stress such as heat (boiling), acid, irradiation, chemicals and disinfectants. Some endospores have remained dormant for 25 million years preserved in amber, only to be shaken back into life when extricated and introduced into a favourable environment.
The following image is an artist-drawn cross-section of a Bacillus cereus endospore, clearly showing its constituent parts and location of its inner genetic material:
(http://www.bact.wisc.edu/bact303/MajorGroupsOfProkaryotes) Oct. 22
Pathogenicity
Pathogenicity, by definition, is the quality or characteristic of being able to cause disease. The term is frequently given quantitative qualification (high, moderate, low) and used synonymously with virulence. However, it is useful to preserve a distinction between pathogenicity and virulence. Pathogenicity should be regarded as an attribute of a genus, species or some other commonly accepted grouping, irrespective of the fact that particular entities (strains, formae, isolates or individuals) within the group may not be pathogenic in a given host, and virulence as the ability of an individual entity in the group to cause disease under defined conditions. (http://www.bspp.org.uk/fbpp.htm#3) Oct. 24
Possible B. cereus-related infections in humans and animals include:
Early reports of diarrheal food poisoning began to surface in Europe in the early 1900's, but it wasn't until the 1950's that the incidents became linked to foodborne bacteria. The Bacillus cereus microorganism became recognized world-wide by the 1970's, and there were a total of 205 reported outbreaks in North America between 1977 and 1984.
Normally, a high number of these B. cereus bodies (>105 / gram of food ingested) are required for an infection to initiate within a human being.
Typical symptoms of an infected person include any of vomiting, nausea, watery diarrhea, cramps, or abdominal pain; these symptoms can take anywhere from 1 to 15 hours to become evident. Although uncomfortable, the infection is not life-threatening (duration is normally 6 to 24 hours in total).
It is virtually impossible to keep the organism out of food completely, so it is important to destroy vegetative cells during any cooking process using adequate heat. In other words, it is best to prepare food in small quantities for rapid heating and cooling to take place. In this manner, prevention of spore germination, outgrowth, and multiplication can be accomplished.
(http://www.ag.iastate.edu/departments/foodsci/classes/fshn420/bcereus.htm) Oct. 27
References
1. http://www.cehs.siu.edu/fix/medmicro/bacil.htm
(Bacillus cereus Fact Sheet)
2. http://medic.med.uth.tmc.edu/path/00001437.htm
(B. cereus / B. anthracis Comparison Study)
3. http://www.ag.iastate.edu/departments/foodsci/classes/fshn420/bcereus.htm
(Food Microbiology Webpage)
4. http://www.americanhistology.com/letters/library/currentconcepts.htm
(Current Concepts in Pathologic Diagnosis: Bacterial Diseases)
5. http://www.foogene.co.uk/features/b.cereus.htm
("FooGene" Training and Consulting)
6. http://www.bact.wisc.edu/Bact330/lectureanthrax
(Bacteriology 330 Lecture Topics: Anthrax)
7. http://gsbs.utmb.edu/microbook/ch015.htm
(Bacillus General Concepts)
8. http://www.bact.wisc.edu/bact303/MajorGroupsOfProkaryotes
(Major Groups of Prokaryotes; UW-Madison Bacteriology 303 Main Page)
9. http://www.bspp.org.uk/fbpp.htm#3
(A Guide to the Use of Terms in Plant Pathology)