Quinolone resistance due to reduced target enzyme expression

Area of Science:

• Microbiology
• Molecular Biology
• Genetics

Background:

• Quinolone antibiotics are crucial for treating Staphylococcus aureus infections.
• Understanding resistance mechanisms is vital for effective treatment strategies.
• Topoisomerase IV is a primary target for quinolone action in S. aureus.

Purpose of the Study:

• To investigate the molecular basis of low-level quinolone resistance in Staphylococcus aureus.
• To identify genetic mutations and their impact on topoisomerase IV expression and function.
• To explore the fitness implications of quinolone resistance mechanisms.

Main Methods:

• Isolation and characterization of a single-step quinolone-resistant mutant (P18) of S. aureus.
• DNA sequencing of topoisomerase IV genes (parEC, gyrBA) and their promoter regions.
• Gene expression analysis of parEC, gyrBA, topB, and topA transcripts.
• Allelic exchange experiments to confirm the role of identified mutations.
• Growth competition assays at various temperatures.

Main Results:

• A point mutation (G-->A) upstream of the parE start codon was identified in the resistant mutant P18.
• This mutation correlated with decreased expression of ParE and reduced parEC transcript levels.
• Resistant mutants exhibited increased MICs for premafloxacin and ciprofloxacin.
• While reduced ParE levels may incur a fitness cost at higher temperatures, compensatory mechanisms involving gyrBA and topB expression were observed.

Conclusions:

• Decreased topoisomerase IV expression, mediated by a specific parE mutation, is a novel mechanism for low-level quinolone resistance in S. aureus.
• Compensatory upregulation of other genes can mitigate potential fitness costs associated with this resistance.
• This finding provides new insights into the evolution of antibiotic resistance in S. aureus.