Bacilli

This page is about the bacterial class. For the genus, see Bacillus. For rod-shaped bacteria in general, see Bacillus (shape).

Bacillus
Bacillus subtilis, Gram stained
Scientific classification
Domain:	Bacteria
Phylum:	Firmicutes
Class:	Bacilli
Orders
Bacillales Lactobacillales

Bacilli refers to a taxonomic class of bacteria. It includes two orders, Bacillales and Lactobacillales, which contain several well-known pathogens like Bacillus anthracis (the cause of anthrax).

Ambiguity

There are several related concepts that make use of similar words, and the ambiguity can create considerable confusion. The term "Bacillus" (capitalized and italicized) is also the name of a genus that, among many other genera, falls within the class Bacilli. All types of bacteria are one-celled organisms that live alone or in chains or groups.

Also, "bacillus" (or the plural "bacilli") can be a generic term to describe the morphology of any rod-shaped bacterium. This general term does not mean that the subject is a member of class Bacilli or genus Bacillus. Thus, it does not necessarily imply a similar group of characteristics. Not all members of class Bacilli are rod-shaped (Staphylococcus is spherical), and many other rod-shaped bacteria that do not fall within that class (Clostridium is rod-shaped but very different taxonomically) exist. Moreover, the general term "bacillus" does not necessarily indicate the Gram-positive staining common to class Bacilli. For example, E. coli is a rod-shaped bacterium that could, therefore, be described as "a bacillus," but it stains Gram-negative and does not belong to genus Bacillus or class Bacilli. Some microbiologists have forsaken the general "bacillus" term because of the confusion it can create.