A multicentre agar and broth dilution susceptibility testing study of fluoroquinolones against the Bacteroides fragilis group

Abstract number: 1134_02_402

Aldridge K.E., Snydman D., Hecht D., Edlund C., Herrington J.

Objectives:

The development of fluoroquinolones (FQ) antimicrobials has resulted in effective therapy for a variety of infections due to aerobic and facultatively anaerobic gram-positive and -negative pathogens. FQ activity against anaerobes has not been as promising with only trovafloxacin receiving FDA approval for anaerobic infections. Moreover, more recent reports of increased resistance to FQ among the B. fragilis group has prompted interested to determine if these increases are due to susceptibility testing methodology problems. This study was designed to compare the susceptibility results from five laboratories that tested the same 70 isolates against FQ using agar dilution(AD) and broth microdilution (BMD) testing.

Methods:

Seventy clinical isolates of the B. fragilis group identified by only a sequential number were distributed to each of the five participating laboratories. Four of the laboratories performed AD and one laboratory performed BMD susceptibility testing using NCCLS recommendations including breakpoints where appropriate. Serial two-fold dilutions of moxifloxacin (MOX), gatifloxacin (GATE), trovafloxacin (TROV), and metronidzaole (METRO) were prepared in the test medium within a range of 0.03 to 64 mg/L. Plates were inoculated with 1 × 10⁵ CFU per spot or well, incubated anaerobically for 48 hours and the MIC read. QC was performed using NCCLS-recommended ATCC strains.

Results:

MIC values for each antimicrobial were collated as MIC90 and per cent (%) of isolates inhibited at <=2 and <=4 mg/L:

Conclusions:

These data indicate that inter-laboratory testing of the same test isolates gave very similar results for each antimicrobial. BMD gave slightly lower MIC results than AD but was not significantly different. We conclude that susceptibility methodology is not responsible for unusually high resistance rates.