Biology Now, 2e

Battling Resistance ■ 221

The same does not hold true for bacteria,

which are single cells and reproduce via asexual

reproduction. All genetic mutations in a bacterial

cell are passed on to the offspring. (See Chapter 6

for a review of cellular replication.) If a mutation

passed to an offspring increases the individual’s

ability to reproduce, that mutation is favored by

natural selection. These favored mutated genes

are passed from parent to offspring, spreading

through future generations in a way that alters

the population as a whole.

But bacteria don’t always wait for the right

random mutations to pop up in their genomes.

Sometimes they simply borrow new alleles from

other organisms. That appeared to be the case

with VRSA, researchers found. “In most cases,

VRSA has developed in a perfect storm,” says

Sievert. It emerges within “a very bad wound

that’s not healing and is a soup mix of organisms

coming together and sharing genes.”

Horizontal gene transfer is the process

by which bacteria pass genes to one another

(Figure 12.12). Bacteria store these genes on

small, circular pieces of DNA called plasmids.

Some bacteria send these plasmids to each other

through small tunnels, like dropping a package

down a trash chute.

Horizontal gene transfer is one example of

another mechanism by which evolution occurs:

gene flow. Gene flow is the exchange of alleles

between populations. Gene flow can occur

between two different species—in this case,

between a population of staph and a population

of Enterococcus—or between two populations

of the same species, such as strains of staph

passing methicillin resistance among them-

selves in the community. An individual that

migrates between two otherwise isolated popu-

lations of a species may facilitate gene flow as

well (Figure 12.13). Gene flow can also occur

when only gametes move from one population

to another, as happens when wind or pollinators

transport pollen from one population of plants

to another.

The introduction of new alleles via gene flow

can have dramatic effects. Two-way gene flow

consists of an exchange of alleles between one

population and another, so it tends to make

the genetic composition of different popula-

tions more similar. If one strain of staph shares

the methicillin-resistance allele with another

strain of staph, for example, the two populations

These bacteria are connected

by a conjugation tube and are in

the process of passing plasmid

DNA from one to another.

The donor bacterium

attaches to a recipient.

DNA is transferred to the

recipient through the tube.

The membranes of the two cells

fuse to form a conjugation tube.

Donor bacterium Recipient bacterium

Cytoplasm

Plasma

membrane

Cell wall

Chromosomal

Plasmid DNA DNA

Conjugation

tube

Figure 12.12

Horizontal gene transfer accelerates the rate of evolution

in prokaryotes

The diagram depicts the horizontal gene transfer of plasmid DNA through

conjugation, the physical process of transferring genetic material through

direct contact. In the case of VRSA, Staphylococcus aureus acquired

the vancomycin-resistance gene through horizontal gene transfer from

Enterococcus. M