Their size.
This structure is called as Cell Wall that gives shape to the bacteria. This wall gives protection to the bacteria and prevents it from exploding because of osmotic lysis.
MreB is the most common one, and gives bacteria their tubular form. Then there is Crescentin, which is involved in formation of the crescent shape of some bacteria.
It protects and supports the cell.It also gives shape.
vacuole
The bacteria that can change shape call pleomorphic
Coccus(Round shape), Bacillus(Rod shape), Spiral bacteria(Motile bacteria having a helical or spiral shape) By Arcangel Meynard
cell wall
Cell wall is the firm protective structure that provides shape and support to plant cells, fungi, most bacteria, and some protists. It is made up of cellulose in plants, chitin in fungi, and peptidoglycan in bacteria.
cocci bacteria are round in shape
Streptococcus, the bacteria that causes strep throat, is spherical (cocci) in shape.
The shape of a virus that attacks bacteria is typically cylindrical or polyhedral. These viruses that infect bacteria are known as bacteriophages and come in various shapes and sizes.
Bacilli are rod-shaped bacteria.