Bacilli (or bacillus for single-cell) are rod-shaped bacteria or archaea that can divide into one plane producing a diplobacillus, streptobacillus, or coccobacillus arrangement.

Most bacilli appear as single rods. However, Diplobacilli (“diplo” means double) bacteria are the type of bacilli that occur as a pair.  Following cell division, the two cells do not separate and continue existing as a pair. One example of diplobacilli bacteria includes Coxiella burnetii.

Streptobacilli (“strepto” means twisted chain) bacteria occur as elongated chains. As such, they are the result of division on a single plane. An example of streptobacilli bacteria includes Streptobacillus moniliformis.

Compared to other bacilli, Coccibacilli (“cocci” means berry) bacteria are shorter in length and appear oval and somehow similar to a coccus. Examples of Coccibacilli include Chlamydia trachomatis and Haemophilus influenzae.

Key Points

• Diplobacilli bacteria are the type of bacilli that occur as a pair.

• Streptobacilli bacteria occur as elongated chains and are the result of division on a single plane.

• Coccibacilli bacteria are shorter in length and appear oval and somehow similar to a coccus.

Key Terms

diplobacilli: two bacilli arranged side by side with each other

streptobacillus: bacilli arranged in chains

coccobacillus: oval and similar to coccus (circular shaped bacterium)

bacilli: rod-shaped bacteria

archaea: a taxonomic domain of single-celled organisms lacking nuclei, formerly called archaebacteria, but now known to differ fundamentally from bacteria

bacteria: single-celled organisms

coccus: any spherical or roughly spherical bacterium.