American Board of Internal Medicine (ABIM) Certification Practice Exam

Shigella isolates with a ciprofloxacin minimum inhibitory concentration of 0.12 μg/mL or greater are considered what?

1. Resistant to this agent

2. Sensitive to this agent

3. Intermediate resistance

4. Highly susceptible

The correct answer is: Resistant to this agent

Shigella isolates with a minimum inhibitory concentration (MIC) of ciprofloxacin at 0.12 μg/mL or greater indicate resistance to this antibiotic. In microbiology, the MIC is used to determine the susceptibility of bacteria to specific antibiotics. For a drug like ciprofloxacin, established breakpoints guide the interpretation of these MIC values. Typically, an MIC value of less than the established breakpoint suggests that the organism is sensitive to the antibiotic, meaning it can be effectively inhibited by the drug. Conversely, values at or above this threshold indicate that the organism has acquired resistance mechanisms that compromise the effectiveness of ciprofloxacin. Since the value given is 0.12 μg/mL, which meets or exceeds the resistance threshold for ciprofloxacin in Shigella, those isolates would therefore be classified as resistant. This classification is crucial for guiding treatment options since resistant strains may not respond to standard ciprofloxacin therapy, necessitating alternative treatment strategies.