A bacterial infection is treated repeatedly with the same

antibiotic. Each episode resolves clinically, but relapse occurs after

treatment cessation. The causative strain shows no increase in MIC and no

detectable resistance mutations. Further investigation reveals a persistent

subpopulation that survives antibiotic exposure through metabolic inactivity.

Assuming repeated antibiotic exposure over time, what type

of selection pressure is most likely acting on the bacterial population?

A Gram‑negative bacterial species

acquires mutations that reduce expression of outer‑membrane

porins. This decreases susceptibility to β‑lactams

and carbapenems, without altering target enzymes or producing β‑lactamases.

The mutations also slow nutrient uptake and reduce growth rate in antibiotic‑free

environments.

A bacterial population contains a small proportion of

antibiotic‑resistant mutants that grow more slowly than

susceptible bacteria. In the absence of antibiotics, bacteria that are susceptible

dominate. During prolonged antibiotic exposure, resistant bacteria rapidly take

over the population.

Two bacterial species isolated from the same patient both

contain the molecular target of a newly developed antibiotic, and sequencing

confirms the target protein has an identical amino acid sequence in both

organisms. Despite this, one species is rapidly killed by the drug, while the

other shows no detectable growth inhibition.

A patient with a bacterial infection is treated with an

antibiotic to which the pathogen is classified as “susceptible.” The infection

resolves slowly, and viable bacteria can still be isolated midway through

treatment. Genomic analysis shows no resistance mutations, and post‑treatment

isolates remain fully susceptible in vitro. Tolerance is identified as the

underlying phenotype.

If this treatment scenario is common in a hospital setting

over many years, which evolutionary outcome is most likely?

Two Gram‑negative bacterial strains differ

only in the expression of a specific outer‑membrane porin. One strain becomes

resistant to multiple antibiotics despite no changes in drug targets or efflux

systems.

Which interpretation best accounts for this resistance

phenotype?

Despite the significant burden of fungal infections, the

range of effective antifungal drugs is much more limited than that of

antibacterial agents. Many candidate compounds that inhibit fungal growth also

show unacceptable toxicity in human cells during development.

β‑lactam antibiotics exert their antibacterial effect by

interfering with cell wall biosynthesis, leading to loss of structural

integrity and cell death in actively dividing bacteria. Structural studies show

that these drugs covalently interact with enzymes responsible for peptidoglycan

cross‑linking.

A pharmaceutical company develops two antibiotics, X and Y,

that inhibit the same essential bacterial enzyme involved in cell wall

metabolism. In biochemical assays, both compounds bind the enzyme with similar

affinity. However, clinical testing reveals that Antibiotic X is effective

against both Gram‑positive and Gram‑negative infections, while

Antibiotic Y is effective only against Gram‑positive bacteria.

Considering known structural differences between Gram‑positive

and Gram‑negative bacteria, which inference best explains the

observed difference in antibacterial spectrum?

A melanoma patient with a B-RAF(V600E)

tumour initially responds well to the BRAF inhibitor vemurafenib, but the

tumour later resumes rapid growth. Genetic analysis reveals the presence of a

p61 B-RAF(V600E) splice variant that lacks part of the regulatory region of the

protein.

Which mechanism best explains why this

variant confers resistance to vemurafenib?