horizontal gene transfer
/ HGT /
We usually picture inheritance as strictly vertical: genes flow down from parent to offspring, like heirlooms passed from one generation to the next. But imagine if you could also acquire a useful skill simply by picking up a tool a stranger dropped, or by catching it like a cold. For microbes, this is everyday reality. They routinely pick up genes sideways, from neighbors that are not their parents and may not even be the same species. That sideways flow is horizontal gene transfer.
Horizontal gene transfer, also called lateral gene transfer, is the movement of genetic material between organisms by routes other than parent-to-offspring inheritance. In bacteria it happens by three main means: transformation (a cell takes up loose DNA released into the environment by dead cells), conjugation (two cells touch and one passes DNA, often a plasmid, through a bridge), and transduction (a virus accidentally carries DNA from one bacterium to another). The transferred DNA, if it integrates and is useful, becomes part of the recipient's genome and is then inherited normally. This is the dominant way antibiotic-resistance genes spread through bacterial populations — not by slow mutation, but by passing a ready-made resistance gene from cell to cell.
Horizontal transfer is rampant in microbes and is a genuine challenge to the simple picture of a tree of life. A tree assumes genes travel only down branches; but if a gene can jump sideways between distant branches, then different genes in the same organism can tell different histories, and the deepest parts of the microbial tree look more like a tangled web than a clean tree. This is one reason microbial phylogenies are built from many genes and read cautiously. A caveat: horizontal transfer is far more common in bacteria and archaea than in animals and plants, where it occurs but is comparatively rare.
When one bacterium passes a plasmid carrying an antibiotic-resistance gene to another during conjugation, the recipient becomes resistant overnight — which is why resistance can spread between unrelated bacterial species in a hospital.
Microbes acquire genes sideways, not just by inheritance — which tangles the tree of life.
Horizontal transfer is common in bacteria and archaea but comparatively rare in animals and plants. Because different genes can have different histories, a single gene tree can be misleading for microbes — which is why their deep relationships look more like a web than a tree.