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Back to Bacteria Index
Bacillus
The word bacillus is a descriptive term for the appearance of certain
bacteria when viewed microscopically. It derives from the Latin for
"staff" and means "rod-shaped". A Gram stain allows one to distinguish
between cocci (round) and bacilli (rod-shaped) bacteria, as well as
between Gram-positive and Gram-negative bacteria (the former are
stained red, the latter purple). In this sense, bacilli can be either
Gram-positive or Gram-negative.
Bacillus bacteria are ubiquitous in nature and can form roughly
spherical spores when conditions are stressful in order to survive in
a dormant state for extended periods. There exist a huge number of
species in this genus, most of which are harmless.
The two species considered medically significant are
Bacillus anthracis (which causes anthrax) and
Bacillus cereus (which can cause
a form of foodborne illness similar to that of
Staphylococcus). Two
species that are notable food spoilers are
Bacillus subtilis and
Bacillus coagulans.
B. subtilis is an obligate aerobe whose spores can
survive the extreme heating which is often used to cook food. It is
responsible for causing ropiness in spoiled bread. B. coagulans can
grow all the way down to pH 4.2 and causes a flat sour taste in
contaminated canned food (including acidic foods which normally keeps
most bacterial growth to a minimum).
Bacilli are gram and catalase positive bacteria who use oxygen as
their terminal electron acceptor in their energy metabolic pathway.
Individual Bacillus appear as rods under the microscope with usually a
substantial number of the rods having an oval endospore which tends to
bulge the bacterium at one end. Colonies of this genus are usually are
observed as large, spreading and irregularly-shaped.
An easy way to isolate a Bacillus species is by placing non-sterile
soil in a test tube with water, shake, place in melted mannitol salts
agar, and incubate at room temperature for at least a day.
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