Spreading Resistance Genes
Bacteria that flourish in the presence of antibiotics can spread resistance in two ways. One way is simple reproduction. Some species also possess an additional evolutionary advantage. These bacteria share genes with other members of a population, both within and beyond the boundaries of their own species. The gene exchange described in this diagram spreads antibiotic resistance faster than the rate of reproduction.
Each bacterium carries its essential genes on circular chromosome(s), but additional genes are also carried on smaller, accessory pieces of circular DNA called plasmids. Genes that help a bacterium survive an antibiotic are often found on a plasmid.
Some bacteria can connect with others using a tube that extends from the surface of the cell wall. Plasmids are exchanged through this tube, which is called a pilus.
The donor bacterium makes a copy of the plasmid that will be exchanged. The copy travels through the pilus from the donor cell to the recipient.
Once the recipient bacterium has a copy of the plasmid, it also becomes resistant to the antibiotic. Both bacteria will pass the resistance gene to their daughter cells when they divide, and both can also continue passing the gene along to other cells in the process shown here.