Biology Now, 2e

276 ■ CHAPTER 15 Bacteria and Archaea

BIODIVERSITY

Talk, but No Sex

Like the plants and animals in a rainforest,

microbes in an ecosystem communicate with each

other, reproduce, and more. Researchers used

to think prokaryotes were the ultimate loners—

self-sufficient and maintaining strict single-celled

lifestyles. Then they discovered that bacteria

physically interact with each other in many ways.

Some bacteria (and Archaea) have a unique

system of cell-to-cell communication called

quorum sensing that enables them to sense

and respond to other bacteria in the area in

accordance with the density of the population.

Disease-causing bacteria, for instance, may

begin to multiply rapidly upon sensing that their

numbers are high enough to overwhelm the

host organism’s immune system. Other bacte-

ria coordinate their behavior by forming tough

aggregates called biofilms that are made up of

the same or different species (Figure 15.9).

It’s strange to think about, but all these activ-

ities are happening on our skin: “Everything

you can imagine life doing, happens in you,”

says Dunn, including reproduction. Prokaryotes

counters, door frames, television screens, and

more. Once again, they found several species of

archaeans. “They are actually a lot more common

than people tend to realize,” says Menninger.

“We’re starting to have the molecular tools that

allow us to find them.” Dunn thinks the discov-

ery is a bit ironic: “We spent so much time find-

ing archaeans in the first place, in hard-to-reach,

faraway places, and somebody could have just had

some introspection with their own belly hole.”

Yet back in the belly button, Dunn was unable

to answer his main question about the bacteria:

Why do individuals have the bacteria they do?

The team looked at gender, ethnicity, how often

participants washed their belly buttons, age, and

more, yet they were unable to attribute any of the

variation to biological or lifestyle factors.

So, as scientists tend to do, Dunn went looking

for more samples. When Menninger joined the

team, she initiated a program to have students

and volunteers from around the country send

in belly button swabs. Thanks to Menninger’s

outreach efforts, volunteers nationwide swabbed,

swirled, and sealed. Soon the lab had over 600

samples to work with. That’s a lot of microbes.

Free-swimming bacteria

attach to a living or

nonliving surface.

Attachment

Slow growth

of colony Quorum sensing Explosive growth

Dispersal from

mature biofilm

Swarm

Surface Mature biofilm

1

Slimy secretions protect

attached bacteria and

anchor them more firmly.

2

Signaling molecules

enable bacteria to

sense their numbers.

3

Bacteria break off

and swim away with

the help of flagella.

4

Recruitment

Q1: How do individual bacteria know that they have a “quorum”?

Q2: There is a well-known biofilm found in your mouth. What is it?

Q3: Under what conditions might bacteria want to coordinate (via quorum sensing) to increase their reproductive rate?

Figure 15.9

Quorum sensing enables coordinated behavior in bacteria

With quorum sensing, a bacterial population can increase its virulence, reproductive rate, or antibiotic resistance under the

appropriate conditions. Biofilms are produced via quorum sensing to protect bacteria from environmental hazards.