MicrobiologyBytes: MicrobiologyBytes Video Library: Bacillus FAQSearch


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. This is a large genus with many members. Here are just a few examples:

Bacillus cereus

B. cereus is an important food-poisoning organism, so we've given it it's very own page.

Bacillus clausii

B. clausii is an alkalitolerant species of bacillus. Alkaliphilic microorganisms are widely distributed in nature, but humans have increased the number of alkaline environments artifically in the last century through the widespread use of concrete, a relatively alkaline substance. Typical of most Bacillus species grown in liquid culture, the cells shown in this video are highly motile (other examples of bacterial motility are shown here). Gram-stained cells: 1 µm wide, 5-10 µm long:

Bacillus halodenitrificans

B. halodenitrificans is a facultatively anaerobic halotolerant (salt-tolerant) denitrifier - i.e. it can obtain energy by reducing nitrate to nitrogen. A broad range of bacteria can carry out this process, but most denitrifying bacteria are Gram-negative, so B. halodenitrificans is somewhat unusual in this respect. Typical of most Bacillus species grown in liquid culture, the cells shown in this video are highly motile (other examples of bacterial motility are shown here). Paired cells in the video are undergoing cell division by binary fission but have not yet completely separated. Gram-stained cells: 1 µm wide, 5-10 µm long:


Bacterial endospores are resistant to hostile physical and chemical conditions. 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. You can see the endospores as bright refractile dots inside the cells in the video of B. halodenitrificans above, but they can be seen more clearly using darkfield microscopy. At the start of this video, Bacillus cereus 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.


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