Biology Now, 2e

274 ■ CHAPTER 15 Bacteria and Archaea

BIODIVERSITY

the microbes and chemically dissolving their cell

membranes with detergent, similar to how dish

soap dissolves grease clinging to a dirty dish.

Next, they ran the sample through a silica

strainer, called a column, to capture the DNA

while allowing other debris to pass through.

Once the DNA was isolated, the team amplified

it using the polymerase chain reaction tech-

nique (see Figure 9.9) and then sequenced one

short section of it, about 250 nucleotides of a

well-studied gene called 16S ribosomal RNA

(16S rRNA). Although 16S rRNA is found in all

bacteria and archaeans, it is highly variable from

one species to another, providing an identifying

tag, like a fingerprint or barcode, to distinguish

one species of microbe from another.

In 2012, the team published results from the

initial set of swabs: among 92 belly buttons,

the team identified 1,400 species of bacteria.

“About 600 or so don’t match up in obvious ways

with known species, which is to say either they

are new to science or we don’t know them well

enough,” Dunn said in a media interview after

the paper was published.

feature. Bacteria use pili to link together to form

bacterial mats, or to attach to surfaces in their

environment, such as the cells of the human

intestine. Some bacteria have one or more long,

whiplike structures called flagella (singular

“flagellum”), which spin like a propeller to push

the bacterium through liquid.

Under the microscope, one obvious cellular

feature is missing: prokaryotes do not have a

true nucleus. They typically have much less DNA

than eukaryotic cells have. Prokaryotes have

less DNA because they have far fewer genes and

because they contain relatively little non-coding

DNA (DNA that is not used to construct proteins

but may have regulatory or other functions). In

contrast, eukaryotes generally have many more

genes and far more non-coding DNA.

Dunn’s lab partnered with Noah Fierer,

an ecologist at the University of Colorado, to

sequence the DNA from each belly button

sample to determine the identity of the resident

microbes, especially those that don’t grow in a

laboratory culture. To do so, lab members first

had to access that DNA by mechanically crushing

Rod
(bacillus) Capsule Pili Flagellum

Sphere
(coccus)

Spiral
(spirillum)

Corkscrew

(spirochete)

Comma

(vibrio)

Figure 15.7

A simple structure, a diversity of forms

Bacteria and Archaea share a simple cell structure relative to eukaryotes, but they display a striking diversity of shapes and may

have additional structures that perform special functions.

Q1: Which of these shapes do you think Streptococcus would take?

Q2: From the micrographs here, does it appear that all prokaryotes have a flagellum?

Q3: Which one of these shapes is most clearly capable of self-motility? Why?