What are the characteristics shared by all bacteria?

The four main characteristics shared by all bacteria include: lack of membrane-bound organelles, small size, unicellular, and having a plasma membrane. 

1. Bacteria typically don't have membrane-bound compartments within their cells, meaning they have fewer large structures inside. As a result, they don't possess a defined nucleus like eukaryotic cells do, nor do they have chloroplasts, mitochondria, or other specialized organelles found in eukaryotes (e.g. Golgi apparatus). 
2. In general, bacteria are notably small in size (microscopic), with some exceptions; they often range in size from 0.5 to 5 micrometers in diameter. The small size of bacteria is crucial because it provides a high surface area-to-volume ratio. This ratio enables rapid absorption of nutrients and efficient disposal of waste within the cell. 
3. In unicellular organisms like bacteria, the growth of individual cells and the process of reproduction through cell division are closely connected. Bacteria typically grow to a specific size and then undergo binary fission, a method of asexual reproduction, to produce two identical daughter cells.
4. Bacteria are predominantly unicellular organisms. This single cell is capable of performing all necessary life functions, including metabolism, reproduction, and response to stimuli. 
5. Lastly, the plasma membrane is a vital structure present in all bacteria. It serves as a barrier that separates the interior of the cell from its external environment and regulates the passage of molecules in and out of the cell.