What best explains the reduced effectiveness of chloramphenicol in treating typhoid fever?

• A. Selection against chloramphenicol in ΔF508 heterozygotes
• B. Selection against chloramphenicol in wild-type homozygotes
• C. Selection for chloramphenicol resistance in populations of S. typhi
• D. Selection for ΔF508 CFTR, which cannot bind chloramphenicol

Answer: (C)

The reduced effectiveness of chloramphenicol in treating typhoid fever can be best explained by the selection for chloramphenicol resistance in populations of Salmonella typhi. Over time, as chloramphenicol has been used to treat infections, some strains of S. typhi have acquired genetic mutations that confer resistance to the drug. This resistance can arise from various mechanisms, such as mutations in the target site of the antibiotic, enzymatic inactivation of the drug, or changes in permeability that prevent the drug from entering the bacterial cell.

As these resistant strains survive and proliferate in the presence of chloramphenicol, the overall effectiveness of the antibiotic diminishes, leading to treatment failures and the need for alternative therapies. This phenomenon is a clear example of evolutionary pressure where the use of an antibiotic selects for bacteria that can withstand its effects, shaping the bacterial population towards resistance.

In contrast, the other choices do not address the direct relationship between the administration of chloramphenicol and the selection of resistant bacterial strains in S. typhi, making them less relevant to explaining the reduced effectiveness of this treatment.