Several related concepts 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 (
Bacillus anthracis) that, among many other genera, falls within the class Bacilli. The word "
bacillus" (or its plural "bacilli", with a small b) is also 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 exist (e.g.,
Clostridium is rod-shaped but very different taxonomically). 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 can be described as "a bacillus", but it stains
Gram-negative and does not belong to the genus
Bacillus or the class Bacilli. Some
microbiologists have forsaken the general "bacillus" term because of the confusion it can create. ==Phylogeny==