Directions: Use the information provided and your knowledge of Life Science to answer the following questions. Show all work where necessary.
Directions: Use the information provided and your knowledge of Life Science to answer the following questions. Show all work where necessary.
Staphylococcus aureus is a common bacterium found on human skin and in nasal passages. While most strains are harmless or cause only mild infections, some have evolved resistance to multiple antibiotics - creating methicillin-resistant Staphylococcus aureus, or MRSA. MRSA is a well-documented example of how natural selection leads to adaptation in populations.
Before the widespread use of antibiotics, MRSA did not exist. Most S. aureus cells were susceptible to drugs such as methicillin and penicillin. However, genetic variation was always present within the population. A small number of individual bacteria carried mutations that altered their cell-wall-producing enzymes, preventing antibiotics from binding effectively. These mutations were rare, but they were heritable, meaning they could be passed on when the bacteria reproduced.
Diagram 1.
Source:
https://fity.club/lists/m/mrsa-bacteria-diagram/
When antibiotics were introduced, they created a powerful selective pressure. Susceptible bacteria were killed, while resistant bacteria survived the treatment. The surviving bacteria then reproduced rapidly, filling the environment with offspring that inherited the resistance trait. Over many bacterial generations - sometimes in just days - the population shifted from mostly susceptible cells to predominantly resistant ones.
Natural selection explains this shift. The four key components of natural selection are all visible in MRSA evolution:
Variation: Some bacteria possessed genetic mutations that made them resistant.
Heritability: These mutations passed from parent cells to daughter cells.
Differential survival: Resistant individuals survived antibiotic exposure; susceptible ones did not.
Differential reproduction: Resistant cells left more offspring, increasing the frequency of the trait.
As resistant bacteria gained an advantage, the population became dominated by cells carrying resistance genes such as
Hospitals and community environments further amplify this process. When antibiotics are overused or improperly prescribed, selective pressure intensifies, accelerating the spread of resistant strains. Over time, MRSA has evolved multiple resistance mechanisms, including the ability to survive desiccation, form protective biofilms, and evade immune responses. These adaptations make MRSA a continuing public health concern.
Diagram 2.
Source: https://www.intechopen.com/chapters/49455
Table 1.
Year | MRSA Isolates (%) | Non-Resistant Isolates (%) |
|---|---|---|
2005 | 22 | 78 |
2008 | 28 | 72 |
2011 | 36 | 64 |
2014 | 42 | 58 |
2017 | 47 | 53 |
2020 | 53 | 47 |
2023 | 61 | 39 |
Graph of Information - Figure 1.
Table 2.
Strain Type | Survival Rate After Methicillin (%) |
|---|---|
Non-Resistant | 5 |
Partially Resistant | 38 |
MRSA | 81 |
Graph of Information - Figure 2.
According to Table 1 and Figure 1, what overall trend occurred in MRSA isolates between 2005 and 2023?
Using Diagram 1, describe how mutations in antibiotic-target molecules can lead to resistance. Cite one specific target shown in the diagram.
Using the reading and Diagram 2, explain how differential survival contributes to the rise of MRSA in hospital environments.
Using Table 2 and Figure 2, compare the survival rates of non-resistant, partially resistant, and MRSA strains after methicillin exposure. What pattern supports natural selection?
Which statement BEST explains why MRSA can continue cell-wall synthesis even in the presence of methicillin?
Construct a CER (Claim, Evidence, Reasoning) explaining how the increase of MRSA populations over time demonstrates natural selection and supports the concept of evolution by common descent.