Bacillus (shape)

Bacteria display a large diversity of cell morphologies and arrangements.

A bacillus (plural bacilli) is a rod-shaped bacterium. Bacilli are found in many different taxonomic groups of bacteria. However, the name Bacillus, capitalized and italicized, refers to a specific genus of bacteria. The name Bacilli, capitalized but not italicized, can also refer to a less specific taxonomic group of bacteria that includes two orders, one of which contains the genus Bacillus. When the word is formatted with a lowercase and not italicized, 'bacillus', it will most likely be referring to shape and not to the genus at all. The shape bacillus can also be called rods. ^[1]

Bacilli usually divide in the same plane and are solitary, but can combine to form diplobacilli, streptobacilli, and palisades.^[2]

• Diplobacilli: Two bacilli arranged side by side with each other.
• Streptobacilli: Bacilli arranged in chains.
• Coccobacillus: Oval and similar to coccus (circular shaped bacterium).

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There is no connection between the shape of a bacterium and its color in the Gram staining. MacConkey agar can be used to distinguish among Gram negative bacilli such as E. coli and salmonella.^[4]

Examples of Gram positive bacilli

• Bacillus genus
• Clostridium genus
• Corynebacterium genus
• Listeria genus
• Propionibacterium genus

Examples of Gram negative bacilli

• Bacteroides genus
• Citrobacter genus
• Enterobacter genus
• Escherichia genus
• Pseudomonas genus
• Proteus genus
• Salmonella genus
• Serratia genus
• Shigella genus
• Yersinia genus

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References