Prevalence in the United States of aac(6')-Ib-cr encoding a ciprofloxacin-modifying enzyme

Area of Science:

• Microbiology
• Molecular Biology
• Antimicrobial Resistance

Background:

• Enterobacteriaceae infections pose a significant public health threat.
• Emergence of antimicrobial resistance, particularly to fluoroquinolones like ciprofloxacin, complicates treatment.
• Understanding the genetic basis of resistance is crucial for effective antimicrobial stewardship.

Purpose of the Study:

• To investigate the prevalence and characteristics of the aac(6 ')-Ib-cr gene in Enterobacteriaceae isolates from the United States.
• To determine the association of aac(6 ')-Ib-cr with ciprofloxacin resistance and other resistance genes.
• To assess the geographical distribution and temporal stability of the aac(6 ')-Ib-cr variant.

Main Methods:

• Analysis of 313 Enterobacteriaceae isolates from the United States.
• Determination of ciprofloxacin Minimum Inhibitory Concentrations (MICs).
• Detection of the aac(6 ')-Ib gene and its cr variant using molecular methods.

Main Results:

• The aac(6 ')-Ib gene was found in 50.5% of isolates with reduced susceptibility to ceftazidime and elevated ciprofloxacin MICs.
• The aac(6 ')-Ib-cr variant, responsible for low-level ciprofloxacin resistance, was present in 28% of these isolates.
• aac(6 ')-Ib-cr was geographically widespread, stable over time, most prevalent in Escherichia coli, and equally common in both susceptible and resistant strains, with no association with qnr genes.

Conclusions:

• The aac(6 ')-Ib-cr gene is a significant contributor to low-level ciprofloxacin resistance in Enterobacteriaceae in the US.
• Its widespread prevalence and stability underscore the need for ongoing surveillance and targeted interventions.
• This finding has implications for antimicrobial susceptibility testing interpretation and treatment strategies.