Fluoroquinolones Possess Good Enterobacterales Activity
All fluoroquinolones demonstrate excellent activity against Enterobacterales (formerly Enterobacteriaceae), making this the correct answer (D).
Spectrum of Activity Across All Fluoroquinolones
Enterobacterales Coverage (Answer D - CORRECT)
Fluoroquinolones consistently demonstrate potent activity against Enterobacterales species, including E. coli, Klebsiella pneumoniae, Enterobacter species, Proteus mirabilis, Citrobacter species, and Serratia marcescens 1, 2, 3.
Susceptibility rates for Enterobacterales to fluoroquinolones approach or exceed 86-90% across most species, with the notable exception of Providencia species (≤65% susceptibility) 4, 5.
Both ciprofloxacin and levofloxacin maintain excellent activity against these organisms, with MIC values of ≤1 μg/mL considered susceptible 2, 3.
This broad Enterobacterales coverage is a defining characteristic shared by all fluoroquinolones, distinguishing them from other antibiotic classes 6.
Why Other Options Are Incorrect
Streptococcus pneumoniae Activity (Option A - INCORRECT):
Older fluoroquinolones like ciprofloxacin have poor activity against S. pneumoniae, with AUC-to-MIC ratios of only 10-20 (target is 25-30) 1.
Only newer "respiratory fluoroquinolones" (levofloxacin, moxifloxacin, gatifloxacin) have enhanced pneumococcal activity 1.
Ciprofloxacin requires combination with gram-positive coverage (e.g., clindamycin) for pneumococcal infections 1.
Pseudomonas aeruginosa Activity (Option B - INCORRECT):
While ciprofloxacin has good P. aeruginosa activity (approximately 70-73% susceptibility), this is not universal across all fluoroquinolones 4, 5.
Moxifloxacin and other respiratory fluoroquinolones have limited or no activity against P. aeruginosa 1.
Resistance rates in P. aeruginosa to fluoroquinolones can be significant, particularly in healthcare settings 1.
Bacteroides fragilis Activity (Option C - INCORRECT):
Most fluoroquinolones have moderate to poor anaerobic activity, requiring combination with metronidazole for intra-abdominal infections 1.
Only moxifloxacin has reasonable anaerobic coverage (approximately 90% of Bacteroides species), but this is not a class-wide characteristic 1.
New cephalosporin combinations must be combined with metronidazole due to limited activity against Bacteroides species 1.
Clinical Implications
Fluoroquinolones are commonly used for urinary tract infections precisely because of their excellent Enterobacterales coverage, particularly E. coli 1, 2, 3.
Resistance among Enterobacterales to fluoroquinolones has been increasing, particularly in E. coli, limiting their empiric use in some settings 1.
The association between fluoroquinolone resistance and ESBL-producing Enterobacterales has led to recommendations to limit extended fluoroquinolone use 1.