Do all bacteria have plasmids?

Yes, all bacteria have plasmids. 

Plasmids are a small, circular, double-stranded DNA molecule that naturally exist in all bacterial cells. Every bacterial cell has its own plasmid which is separated from chromosomal DNA. These plasmids have the ability to replicate independently and are transferred during the conjugation process. Plasmids perform multiple vital functions in a bacterial cell including production of toxins, which help the bacteria to survive and facilitation of the replication process. Few plasmids contain genes that assist in digestion. Although plasmids exist naturally in all bacterial cells, they are not necessary for the bacteria to survive.