The commonest vector used to carry a gene into a bacterial cell is

Explanation

A plasmid is a small circular piece of DNA separate from the bacterial chromosome that replicates on its own and can be taken up from the surrounding medium, which makes it an ideal carrier. Engineered plasmids also carry a marker gene, often for antibiotic resistance, so that transformed cells can be picked out. Viruses are the other common class of vector, especially for animal cells.