In vitro activity of fosfomycin against mucoid and non-mucoid Pseudomonas aeruginosa strains

Intravenous Fosfomycin for Systemic Multidrug-Resistant Pseudomonas aeruginosa Infections

Human Pseudomonas infections have high morbidity and mortality rates. Pseudomonas bacteria can cause sepsis or septic shock; they produce biofilm and commonly exhibit a multidrug-resistant phenotype. The choice of antimicrobial therapy in many cases is challenging, and deep knowledge of clinical, microbiological, and pharmacological issues is required. Intravenous fosfomycin is being repurposed in a combination given its favorable pharmacokinetic/pharmacodynamic properties (a small molecule with favorable kinetic both in bloodstream infection and in deep-seated infections), antibiofilm activity, and its interesting synergistic effects with other antimicrobials. Recent literature on epidemiological, microbiological, pharmacological, and clinical data on intravenous fosfomycin therapy against Pseudomonas is herein reviewed and discussed.

Performance Evaluation of BD Phoenix and MicroScan WalkAway Plus for Determination of Fosfomycin Susceptibility in Enterobacterales

BACKGROUND

Fosfomycin is an old bactericidal drug that has gained increasing interest in the last decade for its potential use in multi-drug resistant gram-negative infections. However, evidence on fosfomycin susceptibility testing reports a poor correlation between commercial methods vs. reference agar dilution (AD) for Enterobacterales (EB). The study aimed at assessing the performance of two automated systems for the determination of fosfomycin susceptibility in EB clinical isolates.

METHODS

Fosfomycin susceptibility testing results of two collections of 100 non-duplicate clinical EB strains obtained using two different platforms (BD Phoenix and MicroScan WalkAway Plus) were compared with those obtained by AD. Categorical agreement (CA), major error (ME) and very major error (VME) rates were calculated.

RESULTS

BD Phoenix exhibited a 6.9% rate of false-resistant results and achieved a CA of 69%, whereas MicroScan WalkAway Plus achieved 3.7% of false-resistant results and 72% of CA. Both automated systems showed poor detection of resistant isolates, with 49.1% and 56.2% of false-susceptible results for BD Phoenix and Microscan WalkAway Plus, respectively.

CONCLUSIONS

Overall, agar dilution remains the most suitable method for routine laboratory antimicrobial susceptibility testing of fosfomycin on Enterobacterales strains, given the poor performance of automated systems. The application of both automated systems, in the clinical laboratories reporting of fosfomycin, should be reviewed in light of the accuracy results falling below the acceptable threshold.

Comparison of Three Different Commercial Methods for Fosfomycin Susceptibility Testing in Pseudomonas aeruginosa

The lack of internationally approved breakpoint and a handy susceptibility testing reduces fosfomycin usefulness against Pseudomonas aeruginosa. Previous works defined low or moderate agreement between commercial methods and the reference method (agar dilution [AD]). In particular, data lack about testing against P. aeruginosa. We compared disk diffusion (DD), E-test (ET), and automated broth microdilution (BMD) to AD by testing 150 P. aeruginosa isolates. We obtained better categorical agreement (CA) for DD and ET for minimal inhibitory concentration >128 mg/L (84.7% and 92.7%, respectively), but with high very major error (VME). BMD had the lowest VME rate (2/42), but with 64% CA and 52/108 major errors. We cannot define a method comparable to AD. Larger studies, as well as the definition of a breakpoint value are needed for fosfomycin against P. aeruginosa.

Evaluation of BD Phoenix and Microscan WalkAway for determination of fosfomycin susceptibility in Staphylococcus aureus

We evaluate the performance of BD Phoenix (BD Diagnostic; Hunt Valley, MD) and MiscroScan WalkAway (Beckman Coulter Inc; Carlsbad, CA) systems versus reference agar dilution method for fosfomycin susceptibility determination in 200 Staphylococcus aureus clinical isolates. Both methods exhibit ≤89.0% of categorica agreement and ≥63.3% of very major error. All commercial antimicrobial susceptibility systems show poor concordance with reference method.