Why horizontal gene transfer (and mobile genetic elements) are clinically important
Antibiotic resistance is conferred to bacteria cells from genes (called antibiotic resistance genes, or sometimes just resistance genes) they carry in their DNA. Darwinian evolution (spontaneous random mutation and selection) can account for some of the resistances we see, but does not explain how bacteria can simultaneously resist a number of man-made antibiotics. Bacteria’s ability to share genes between cells (“horizontally”) makes it possible for organisms to develop resistance much faster and is the reason antibiotic resistance is on the rise.
Strictly speaking, horizontal gene transfer is the movement of DNA between bacteria cells, even of different species. There are several biochemical processes by which this can occur but the result is that phenotypes (including resistance to antibiotics) can spread even if the infection that originally carried them is wiped out.
Antibiotics can accelerate the emergence of resistance genes by killing off the susceptible cells that compete for food. Over time, and increased exposure to multiple antibiotics, a microbial ecosystem becomes richer in “superbugs” – bacteria that resist “last resort” antibiotics such as carbapenems and methicillin.