Mechanisms accounting for fluoroquinolone resistance in Escherichia coli clinical isolates

Area of Science:

• Microbiology
• Molecular Biology
• Antimicrobial Resistance

Background:

• Fluoroquinolone antibiotics are crucial for treating bacterial infections, but resistance is a growing public health concern.
• Mechanisms of fluoroquinolone resistance in Escherichia coli include target gene mutations (gyrA, gyrB, parC, parE), increased efflux pump activity (AcrAB), and plasmid-mediated resistance genes (e.g., aac(6')-Ib-cr, qnr variants).
• Previous studies identified high and variable fluoroquinolone minimum inhibitory concentrations (MICs) in resistant E. coli clinical isolates.

Purpose of the Study:

• To investigate the genetic and molecular mechanisms contributing to fluoroquinolone resistance in clinical E. coli isolates.
• To correlate specific resistance mechanisms with patient demographics and infection sites.
• To identify the extent to which known resistance mechanisms account for observed fluoroquinolone MICs.

Main Methods:

• Sequencing of gyrA, gyrB, parC, and parE genes.
• Screening for the presence of plasmid-borne resistance genes aac(6')-Ib-cr and qnrA.
• Quantification of AcrA protein levels in E. coli isolates.
• Correlation of genotypic and phenotypic data with available patient information (sex, age, location, infection site).

Main Results:

• All fluoroquinolone-resistant isolates possessed gyrA mutations; 85% also had parC mutations.
• The aac(6')-Ib-cr gene was present in approximately 23% of isolates, significantly increasing ciprofloxacin and norfloxacin MICs, but not gatifloxacin or levofloxacin MICs.
• No qnrA gene was detected. Approximately 33% of resistant isolates exhibited increased AcrA levels, correlating with fluoroquinolone nonsusceptibility but not other antimicrobials.
• Known mechanisms explained 50-70% of the observed fluoroquinolone resistance, with some isolates showing MICs up to 1,500-fold higher than expected.

Conclusions:

• Chromosomal mutations in gyrA and parC, along with increased AcrA levels, are significant contributors to fluoroquinolone resistance in E. coli.
• The aac(6')-Ib-cr gene plays a role in resistance to certain fluoroquinolones.
• A substantial proportion of fluoroquinolone resistance in clinical E. coli isolates is not explained by currently known mechanisms, highlighting the need for further research into novel resistance determinants.