The Clinical Microbiology Institute, 9725 SW Commerce Circle Wilsonville, OR 97070, USA
Abstract
One hundred percent of 1097 Escherichia coli and 97.5% of 157 Enterococcus faecalis isolates from outpatient urine specimens at ten North American medical centres were susceptible to fosfomycin tromethamine. The Etest MICs correlated well with those of agar dilution. Disc diffusion zone diameters correlated well with MICs and supported the previously proposed zone diameter breakpoints for fosfomycin.
Introduction
Fosfomycin tromethamine is a phosphonic acid antibacterial agent that has been approved in the USA for the treatment of uncomplicated urinary tract infections due to Escherichia coli and Enterococcus faecalis. This agent, given as a single 3 g oral dose, has been reported to be comparable with other antimicrobials for the treatment of uncomplicated urinary tract infections in women.1,2,3,4 The present study was designed to assess the in-vitro activity of fosfomycin by three different test methods. Fosfomycin tromethamine and four other antimicrobial agents were tested against consecutive outpatient urinary tract isolates of E. coli and E. faecalis collected from ten geographically separate North American medical centres.
Materials and methods
Microorganisms
During the winter of 1998, approximately 110 consecutive outpatient urinary tract isolates of
E. coli were collected by each of ten medical centres (listed in Acknowledgements), and
were shipped frozen to the Clinical Microbiology Institute for testing. In addition, all urinary
isolates of enterococci encountered during this period were similarly collected and shipped. Only
organisms that were predominant and with colony counts 104 cfu/mL were
included; mixed flora or isolates of questionable significance were excluded as were multiple
isolates from the same patient.
Antimicrobial agents
Fosfomycin tromethamine powder and fosfomycin Etest strips were provided by Forest Laboratories, New York, NY, USA. Ampicillin, ciprofloxacin, nitrofurantoin and trimethoprim/sulphamethoxazole were obtained from their respective manufacturers or from commercial sources. Commercially prepared antimicrobial discs included: fosfomycin, 200 µg + 50 µg of glucose 6-phosphate; ampicillin, 10 µg; ciprofloxacin, 5 µg; nitrofurantoin, 300 µg; and trimethoprim/sulphamethoxazole, 1.25/23.75 µg.
Susceptibility test
Fosfomycin MICs were determined by the agar dilution method as outlined by the National Committee for Clinical Laboratory Standards (NCCLS),5 using MuellerHinton agar supplemented with 25 mg/L of glucose 6-phosphate. Concentrations ranged from 0.06 to 128 mg of fosfomycin per litre (excluding the tromethamine portion of the salt). The inoculum contained approximately 1 x 104 cfu/spot and the results were read after 1820 h incubation at 35°C in air. The other antimicrobials were tested by the broth microdilution method outlined by the NCCLS5 with concentrations of 0.0364 mg/L (ampicillin), 0.0158.0 mg/L (ciprofloxacin), 0.12256 mg/L (nitrofurantoin), and 0.03/0.2764/576 mg/L (trimethoprim/sulphamethoxazole). Disc diffusion tests were performed at the same time by the method outlined by the NCCLS.6 Fosfomycin Etest strips were also applied to the disc diffusion plates and read according to the instructions of the manufacturer. Etest MIC results that fell between even log2 dilution intervals were read as the next highest even log2 concentration.
Quality control
Quality control organisms were tested on each day of testing, and included: E. coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, Staphylococcus aureus ATCC 29213 and ATCC 25923, and E. faecalis ATCC 29212. Of 368 tests with antibiotic/organism combinations that have NCCLS control limits, 98.6% of results fell within these limits. All quality control results with fosfomycin were within previously recommended ranges for this drug.7
Results and discussion
A total of 1097 urinary isolates of E. coli was tested and all (100%) were
susceptible to fosfomycin at 64 mg/L (Table). The proportion
susceptible to the four
comparison drugs ranged from 67.7% (ampicillin) to 99.3% (ciprofloxacin). The
MIC90 of fosfomycin was 1.0 mg/L and >98% of the isolates were
susceptible to
2.0 mg/L of fosfomycin. Only minor differences in MIC distributions of all
five antimicrobials were observed among the ten participating medical centres.
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The comparison of fosfomycin MICs determined by agar dilution and by Etest is also shown inTable. The Etest MICs for E. coli averaged slightly more than half of one log 2 concentration higher than those determined by agar dilution, but 94% of the paired results were within one two-fold dilution of each other. For all enterococci, 99.5% of the paired MIC results by the two methods were within one two-fold dilution of each other with a slight skewing toward lower MICs by the Etest. The Etest appears to be an adequate substitute for agar dilution in determining MICs of fosfomycin tromethamine.
Scattergrams of the fosfomycin agar dilution MICs and the disc diffusion zone diameters are
provided in the Figure. No major or very major discrepancies occurred
with either organism; the minor discrepancy rates were 0.1% for E. coli and
2.5% for E. faecalis. The current susceptible breakpoint for fosfomycin is 64
mg/L, and this breakpoint has been shown to correlate well with clinical efficacy.8 The good correlation between MICs and disc diffusion zone
diameters plus the low discrepancy rates support the previously proposed zone diameter
breakpoints.9
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Acknowledgments
The participating facilities and their respective microbiology director or supervisor were: The Cleveland Clinic, Cleveland, OH (J. Washington); Carolinas Medical Center, Charlotte, NC (S. Jenkins); Good Samaritan Hospital, Phoenix, AZ (M. Saubolle); St Vincent Medical Center, Portland, OR (M. Bauman); University of Alberta Hospital, Edmonton, Alberta, Canada (R. Rennie); UCLA Medical Center, Los Angeles, CA (J. Hindler); University of Iowa College of Medicine, Iowa City, IA (M. Pfaller); University of Massachusetts Medical Center, Worcester, MA (B. Brogden-Torres); University of New Mexico Medical Center, Albuquerque, NM (G. Overturf); and University of Rochester Medical Center, Rochester, NY (D. Hardy). Financial support was provided by Forest Laboratories, New York, NY.
Notes
* Corresponding author. Tel: +1-503-682-3232; Fax:
+1-503-682-2065; E-mail: cmi{at}hevanet.com
References
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Received 20 April 1998; returned 17 June 1998; revised 6 July 1998; accepted 3 August 1998