Biology Now, 2e

Battling Resistance ■ 213

control any potential for panic,” says Sievert. Her

team asked the local lab to run its own test again

and to send the health department a sample to

independently test. Both teams waited.

The tests came back from each lab. Both were

positive; the woman’s foot was infected with the

first reported vancomycin-resistant strain of

S. aureus. VRSA had arrived.

Birth of a Superbug

“At that point, it was the first-ever VRSA in the

world,” says Sievert. Unfortunately, it was not

the last.

Through evolution, staph first survived

penicillin, then methicillin, then vancomycin

(Figure 12.4). The evolution of Staphylococcus

Yellow bacteria are

killed by methicillin,

while red bacteria

are genetically

resistant.

Methicillin-resistant

bacteria S. aureus

(MRSA) survives and

reproduces over time.

Treatment with the

antibiotic methicillin

is represented by a

Many kitchen strainer.

generations

later

A population of

Staphylococcus

aureus bacteria

Figure 12.2

Natural selection results in resistance

to antibiotics

The use of antibiotics allows any bacteria (in

this case, S. aureus) that are resistant to the

antibiotic to survive and reproduce. Over time,

the frequency of resistant bacteria increases in

the surviving populations.

Q1: What is natural selection selecting for

here?

Q2: Why do bacteria that are not

genetically resistant to antibiotics die out

when exposed to antibiotics?

Q3: Why is the antibiotic represented by a

kitchen strainer in this figure?

Figure 12.3

MRSA infection in the foot of a patient

with diabetes

MRSA (methicillin-resistant Staphylococcus

aureus) bacteria growing on an agar plate

VRSA (vancomycin-resistant Staphylococcus

aureus) bacteria growing on an agar plate

Paper tab infused with vancomycin. The

surrounding area it has diffused into, killing

off all bacteria in the area, is called the

zone of inhibition.

Paper tab infused with vancomycin. Note

that the zone of inhibition is much smaller,

demonstrating that the bacteria can grow in

the presence of the antibiotic.

Figure 12.4

MRSA versus VRSA in a vancomycin-resistance test

In the presence of vancomycin, MRSA (top) does not grow, while VRSA

(bottom) grows relatively well.

Q1: What is the difference between MRSA and VRSA?

Q2: Why is there a clear zone (the “zone of inhibition”) around the

paper disk in the top dish but not in the bottom dish?

Q3: Why is the lack of a clear zone around the paper disk in the

bottom dish so alarming?