Within six months of effectively using methicillin to treat...

Within six months of effectively using methicillin to treat S. aureus infections in a community, all new $S$. aureus infections were caused by MRSA. How can this best be explained? a. A patient must have become infected with MRSA from another community. b. In response to the drug, $S$. aureus began making drugresistant versions of the protein targeted by the drug. c. Some drug-resistant bacteria were present at the start of treatment, and natural selection increased their frequency. d. S. aureus evolved to resist vaccines.

Within six months of effectively using methicillin to treat
S. aureus infections in a community, all new $S$. aureus infections were caused by MRSA. How can this best be explained?
a. A patient must have become infected with MRSA from another community.
b. In response to the drug, $S$. aureus began making drugresistant versions of the protein targeted by the drug.
c. Some drug-resistant bacteria were present at the start of treatment, and natural selection increased their frequency.
d. S. aureus evolved to resist vaccines.

Key Concepts

Natural Selection

Natural selection is the process by which individuals with traits that better enable them to adapt to a given environment tend to survive and reproduce more than those without such traits. This concept explains how populations evolve over time, particularly when environmental pressures, such as exposure to antibiotics, favor organisms with resistance traits.

Antibiotic Resistance

Antibiotic resistance refers to the ability of bacteria to survive and multiply despite the presence of drugs designed to kill them. It is a significant phenomenon in microbiology and public health, as it results from the selection of resistant strains in environments where antibiotics are used extensively, leading to treatment failures.

Selective Pressure

Selective pressure is an environmental challenge that influences which individuals in a population are more likely to survive and reproduce. In the context of antibiotic use, the drug acts as a selective pressure by killing susceptible bacteria and allowing those with resistance mechanisms to thrive, ultimately altering the population's genetic makeup.

Pre-existing Genetic Variation

Pre-existing genetic variation within a population refers to the differences in genetic makeup among individuals before any external pressure is applied. In bacterial populations, some individuals may already possess mutations or resistance genes that enable them to survive antibiotic treatment, and these traits become more common when the population is exposed to the drug.

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