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 drug- resistant 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.

Verified step by step guidance

1. Understand the concept of natural selection: Natural selection is a process where organisms better adapted to their environment tend to survive and produce more offspring. In the context of antibiotic resistance, bacteria that have mutations allowing them to survive the presence of an antibiotic will reproduce more successfully than those that do not.
2. Consider the role of pre-existing genetic variation: Before the introduction of methicillin, there may have been a small number of S. aureus bacteria with mutations that conferred resistance to methicillin. These bacteria would have had a survival advantage once methicillin was used.
3. Analyze the impact of antibiotic use: When methicillin is used to treat infections, it kills susceptible bacteria, but resistant bacteria survive and continue to reproduce. Over time, the proportion of resistant bacteria increases in the population.
4. Evaluate the options given: Option c suggests that some drug-resistant bacteria were present at the start of treatment, and natural selection increased their frequency. This aligns with the concept of natural selection and the impact of antibiotic use.
5. Conclude with the most plausible explanation: Based on the understanding of natural selection and antibiotic resistance, option c is the best explanation for the emergence of MRSA in the community after methicillin use.

Key Concepts

Antibiotic Resistance

Antibiotic resistance occurs when bacteria evolve mechanisms to withstand the effects of drugs designed to kill them. This can happen through genetic mutations or acquiring resistance genes from other bacteria. Overuse or misuse of antibiotics accelerates this process, leading to strains like MRSA that are resistant to multiple drugs.

Natural Selection

Natural selection is a process where organisms better adapted to their environment tend to survive and reproduce more than others. In the context of antibiotic resistance, bacteria with mutations that confer drug resistance survive antibiotic treatment, while susceptible ones die off, increasing the frequency of resistant strains in the population.

Genetic Variation in Bacteria

Genetic variation in bacteria is crucial for evolution and adaptation. Bacteria can acquire genetic changes through mutations or horizontal gene transfer, which can lead to traits like antibiotic resistance. This variation allows some bacteria to survive antibiotic treatment, leading to the emergence of resistant populations like MRSA.