Abstract
Background
Fosfomycin (FO) is a bactericidal antibiotic with a broad spectrum of activity against a wide range of Gram-positive and Gram-negative bacteria. Oral FO is mainly used in the treatment of urinary tract infections, particularly those caused by E. coli and E. faecalis. In order to determine MICs to FO, the ETEST® FM is already available but the reading can be difficult especially with E. coli. To resolve this issue, a new ETEST® with FO, called ETEST® FO, has been developed (not FDA cleared, yet). The purpose of this study is to compare this new strip to the agar dilution reference method (AD) on a panel of E. coli and E. faecalis.
Methods
A total of 39 isolates comprising 20 E. coli (ESBL or CPE) and 19 E. faecalis (VRE or VSE) were tested by ETEST® FO and Agar dilution. The isolates were sub-cultured on Columbia agar plates supplemented with 5% sheep blood before testing. After incubation, suspensions of the isolates were prepared in 0.85% saline. These suspensions were used to inoculate both AD and ETEST® plates. Results were read after 16-20 hours incubation at 35°C +2°C in ambient air. Following CLSI QC guideline, 4 QC organisms were tested. Results were analyzed using the FDA/CLSI breakpoints for FO (S < 64µg/mL, I=128 µg/mL, R> 256 µg/mL). Performance was evaluated using FDA performance criteria, essential agreement (EA, ≥ 90%), category agreement (CA, ≥ 90%), major error rate (ME, ≤3.0%) and very major error rate (VME, ≤2.0%).
Results
All the QC strains MICs were within the CLSI ranges. For the panel results, see the table below:
Performance for ETEST® FO on E. coli and E. faecalis
| Species | EA | CA | Very Major Error Rate | Major Error Rate | Minor Error Rate |
| Overall (39) | 97.4% (38/39) | 97.4% (38/39) | 0% (0/2) | 0.0% (0/37) | 2.6 % (1/39) |
| E. faecalis (19) | 100% (19/19) | 94.7% (18/19) | NA (No R) | 0.0% (0/19) | 5.2% (1/19) |
| E. coli (20) | 95.0% (19/20) | 100% (20/20) | 0% (0/2) | 0.0% (0/18) | 0% (0/20) |
Conclusion
This first and preliminary study shows that ETEST® FO can potentially meet the FDA acceptance criteria and could be a valuable tool for determining FO MIC for E. coli harboring various resistance mechanisms and E. faecalis including VRE. Moreover, in comparison with the current ETEST FM strip, this new strip brings a real reading improvement and resolve the issue for E. coli. The clinical study phase will determine the product’s performance.
Disclosures
Marion Pompilio, BioMérieux (Employee) Gilles Zambardi, biomerieux (Employee)