Bacteria are potentially pathogenic single-celled microorganisms that can also be found in forms that are free living as well as some that are of positive benefit to other organisms or at least benign in their interactions. They come in a variety of morphologies or shapes. One of the common forms, the cocci, are spherical, for example. The baccilli are the other common shape of bacteria, appearing as rod-like entities.
The bacilli are a type of bacteria that includes a variety of different species. What they have in common is that they are all rod-shaped. However, there are sub-types within this classification. The vibrio type of bacilli have a curve in them that can make them look like a comma, for example. Another type of bacilli is the spirilla version which appear as spirals. There are also spirochaetes, which appear as tight coils. But there can even be cuboidal and tetrahedral variations and rod-shaped bacteria with star-shapes in their cross-section.
The shape of a bacterium is very much related to its cell wall and cytoskeleton. In giving the bacterium its particular shape these structures fit the species for a particular environment and role in that environment. It determines what surfaces on other objects and creatures it can attach to, what nutrients it can process, and how it can move through the environment, avoiding predators and perhaps pursuing prey, if it is that kind of bacterium.
In some cases the bacilli can be found very much pathogenic forms. One example of a pathogenic bacillus species is that of Bacillus anthracis. This is responsible for cutaneous, pulmonary, and gastrointestinal anthrax. This disease is spread by contact with animals such as sheep and goats, for example. The microorganism can be inhaled but it can also find its way into a victim through abrasions on their skin. Prevention of this requires the anthrax vaccine and treatment, needed early after infection could involve doxycycline or penicillin, for example.
But the bacilli can also be useful microorganisms as well. One example of a use for them is in the industrial production of enzymes. Bacillus amyloliquefaciens produces barnase, amongst other substances, for example. Another use of bacilli is in biological studies as a model organism. In particular the species Bacillus subtilis is used because it is large and its genetics are easy to get at. So the bacilli can be seen as a mixed bag of bacteria, some with uses for us and some that are harmful to us, and perhaps even fatal.