Bacilli refers to a taxonomic

Taxonomy

Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...

 class

Class (biology)

In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 of bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

. It includes two orders, Bacillales

Bacillales

The Bacillales are an order of Gram-positive bacteria, placed within the Firmicutes. Representative genera include Bacillus, Listeria and Staphylococcus....

 and Lactobacillales

Lactobacillales

The Lactobacillales are an order of Gram-positive bacteria that comprise the lactic acid bacteria. They are widespread in nature, and are found in soil, water, plants and animals. They are widely used in the production of fermented foods, including dairy products such as yogurt, cheese, butter,...

, which contain several well-known pathogens like Bacillus anthracis

Bacillus anthracis

Bacillus anthracis is the pathogen of the Anthrax acute disease. It is a Gram-positive, spore-forming, rod-shaped bacterium, with a width of 1-1.2µm and a length of 3-5µm. It can be grown in an ordinary nutrient medium under aerobic or anaerobic conditions.It is one of few bacteria known to...

(the cause of anthrax

Anthrax

Anthrax is an acute disease caused by the bacterium Bacillus anthracis. Most forms of the disease are lethal, and it affects both humans and other animals...

).

Ambiguity

There are several related concepts that make use of similar words, and the ambiguity can create considerable confusion. The term "Bacillus

Bacillus

Bacillus is a genus of Gram-positive, rod-shaped bacteria and a member of the division Firmicutes. Bacillus species can be obligate aerobes or facultative anaerobes, and test positive for the enzyme catalase. Ubiquitous in nature, Bacillus includes both free-living and pathogenic species...

" (capitalized and italicized) is also the name of a genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 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

Bacillus (shape)

The word bacillus may be used to describe any rod-shaped bacterium, and such bacilli are found in many different taxonomic groups of bacteria. However, the name Bacillus, capitalized and italicized, refers to a specific genus of bacteria...

" (or the plural "bacilli") can be a generic term to describe the morphology

Bacterial cellular morphologies

Bacteria are classified by direct examination with the light microscope through its morphology and aggregation.The basic forms are spheres and round-ended cylinders . But there may be others such as helically-twisted cylinders , cylinders curved in one plane and unusual morphologies...

 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

Staphylococcus

Staphylococcus is a genus of Gram-positive bacteria. Under the microscope they appear round , and form in grape-like clusters....

is spherical), and many other rod-shaped bacteria that do not fall within that class (Clostridium

Clostridium

Clostridium is a genus of Gram-positive bacteria, belonging to the Firmicutes. They are obligate anaerobes capable of producing endospores. Individual cells are rod-shaped, which gives them their name, from the Greek kloster or spindle...

is rod-shaped but very different taxonomically) exist. Moreover, the general term "bacillus" does not necessarily indicate the Gram-positive

Gram-positive

Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...

 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

Gram-negative

Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...

 and does not belong to genus Bacillus or class Bacilli. Some microbiologist

Microbiologist

A microbiologist is a scientist who works in the field of microbiology. Microbiologists study organisms called microbes. Microbes can take the form of bacteria, viruses, fungi, and protists...

s have forsaken the general "bacillus" term because of the confusion it can create.