An in vitro resistance study of levofloxacin, ciprofloxacin, and ofloxacin using keratitis isolates of Staphylococcus aureus and Pseudomonas aeruginosa.
OBJECTIVE
We compared levofloxacin with ciprofloxacin and ofloxacin using the in vitro susceptibilities of Staphylococcus aureus (SA) and Pseudomonas aeruginosa (PA) keratitis isolates.


METHODS
Retrospective, clinical laboratory study of antibiotic susceptibility among keratitis isolates.


METHODS
Keratitis isolates from 200 patients with either SA or PA keratitis.


METHODS
Minimum inhibitory concentrations (MICs) were determined for levofloxacin, ofloxacin, and ciprofloxacin for 93 SA keratitis isolates (68 fluoroquinolone-resistant and 25 susceptible, as determined by disk diffusion) and 107 PA keratitis isolates (13 fluoroquinolone-resistant and 94 susceptible). National Committee for Clinical Laboratory Standards susceptibilities were determined and analyzed statistically. Time kill studies were determined for fluoroquinolone-susceptible and -resistant isolates to all antibiotics at 8 microg/ml. The killing rates were determined by regression, and the colony count decreases were analyzed.


METHODS
The susceptibilities and potencies of levofloxacin, ciprofloxacin, and ofloxacin to SA and PA were determined from the MICs. Time kill studies determined the killing rates and decreases in colony counts.


RESULTS
The fluoroquinolone-resistant SA susceptibilities to levofloxacin, ofloxacin, and ciprofloxacin were only 22%, 10%, and 3%, respectively. The fluoroquinolone-susceptible SA were 100% susceptible to all antibiotics, with levofloxacin demonstrating the best potency. The fluoroquinolone-resistant PA were resistant to all antibiotics. The fluoroquinolone-susceptible PA isolates were highly susceptible to levofloxacin, ofloxacin, and ciprofloxacin, with ciprofloxacin demonstrating the highest potency. For fluoroquinolone-susceptible SA and PA, the time kill studies determined that the killing rates and decreases in colony counts were equivalent for all three antibiotics tested. The time kill studies demonstrated no colony count decreases for the fluoroquinolone-resistant SA and PA.


CONCLUSIONS
Taken together, our susceptibility and time kill data failed to demonstrate convincing differences in the susceptibility of SA and PA keratitis isolates to levofloxacin, ciprofloxacin, and ofloxacin. In general, bacterial isolates that were resistant to ciprofloxacin and ofloxacin were also resistant to levofloxacin.
Kowalski
R P
RP
The Charles T. Campbell Ophthalmic Microbiology Laboratory, University of Pittsburgh, Pittsburgh, Pennsylvania, USA.
Pandya
A N
AN
Karenchak
L M
LM
Romanowski
E G
EG
Husted
R C
RC
Ritterband
D C
DC
Shah
M K
MK
Gordon
Y J
YJ
D000890:Anti-Infective Agents / Q000494:pharmacology*; D002939:Ciprofloxacin / Q000494:pharmacology*; D015169:Colony Count, Microbial; D004352:Drug Resistance, Microbial; D015818:Eye Infections, Bacterial / Q000382:microbiology*; D006801:Humans; D007634:Keratitis / Q000382:microbiology*; D064704:Levofloxacin; D008826:Microbial Sensitivity Tests; D015242:Ofloxacin / Q000494:pharmacology*; D011550:Pseudomonas aeruginosa / Q000187:drug effects* / Q000302:isolation & purification; D012189:Retrospective Studies; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000302:isolation & purification; D013997:Time Factors
D000890:Anti-Infective Agents; D002939:Ciprofloxacin; D064704:Levofloxacin; D015242:Ofloxacin