Jacob Birdsall¹*, Steven Siarakas^1,2 and John Merlino^1,2

*Corresponding Author: Jacob Birdsall, Department of Microbiology and Infectious Diseases, Concord Repatriation General Hospital, NSW Health Pathology, Australia.

Received: March 03, 2021; Published: April 07, 2021

Abstract

Objectives: To compare the fosfomycin minimum inhibitory concentrations of Enterobacterales and non-Enterobacterales, as determined by the BD Phoenix M50, to EUCAST and CLSI clinical breakpoints.

Methods: 158 Gram-negative organisms isolated from clinical samples underwent susceptibility testing using the BD Phoenix M50, NMIC-404 panel. The fosfomycin MICs of both Enterobacterales and non-Enterobacterales were compared to EUCAST and CLSI clinical breakpoints.

Results: A total of 138 Enterobacterales were tested, including 81 E. coli isolates. 131 (95.0%) were considered susceptible using EUCAST breakpoints and 134 (97.1%) were considered susceptible using CLSI breakpoints. Of the 20 non-Enterobacterales tested, 5 isolates (25.0%) showed an MIC below ≤32 mg/L (EUCAST breakpoint for Enterobacterales), and 16 isolates (80.0%) had an MIC of ≤64 mg/L (CLSI breakpoint for E. coli isolates).

Conclusions: This study showed high susceptibility rates of most Enterobacterales to fosfomycin, even in the presence of resistance mechanisms (ESBL and CPO), while most of the non-Enterobacterales tested showed elevated MICs to fosfomycin.

Keywords: coli; Enterobacterales; EUCAST; Fosfomycin

Introduction

  Fosfomycin is a broad-spectrum antibiotic commonly used in UTIs. It has activity against most Enterobacterales, notably Escherichia coli, Proteus mirabilis and Citrobacter spp., while more varied activity has been described against non-Enterobacterales [1]. Mowlaboccus., et al. performed a study which showed low incidence of fosfomycin resistance in Escherichia coli urinary tract infection isolates in Australia (2 of 1033 isolates resistant) [2].

  Fosfomycin acts by inhibiting the MurA enzyme, which catalyses the first stage in peptidoglycan synthesis [3]. EUCAST clinical breakpoints for Enterobacterales in uncomplicated urinary tract infection (UTI), are MIC of ≤32 mg/L (susceptible) and MIC of >32 mg/L (resistant), with no other MIC breakpoints or zone diameters set for non-Enterobacterales [4,5]. CLSI clinical breakpoints, which apply only to E. coli urinary tract isolates, are MIC of ≤64 mg/L (susceptible); MIC of 128 mg/L (intermediate); MIC of ≥256 mg/L (resistant) [8].

  A previous study examined the accuracy of the Phoenix in detecting fosfomycin resistance compared to other methods (i.e. disc diffusion, Etest, Vitek). Despite the current gold standard for susceptibility testing being agar dilution and disc diffusion, the Phoenix showed 99.5% categorical agreement for E. coli, and 93% for K. pneumoniae, with significantly lower major/very major errors compared to the other methods [7].

Aim of the Study

  Over a three-month period, a total of 158 Gram-negative organisms isolated from clinical samples, were collected. This included 23 extended-spectrum β-lactamase (ESBL) producing organisms and 5 carbapenemase-producing organisms (CPO) (IMP4: n = 2, KPC: n = 1, oxa48: n = 1, AIM: n = 1). Each isolate had susceptibility testing performed on a pure culture using the BD Phoenix M50, NMIC-404 panel. This data was correlated and fosfomycin MICs of both Enterobacterales and non-Enterobacterales were compared to EUCAST and CLSI clinical breakpoints.

Results and Discussion

  A total of 138 Enterobacterales were tested, 81 of which were E. coli. Of the E. coli isolates, 81 (100.0%) were classified as susceptible, including 18 ESBL positive isolates, when EUCAST and CLSI breakpoints were used.

  Of the other Enterobacterales (n = 57), 50 (87.7%) were classified as susceptible (MIC of ≤ 32 mg/L) including 5 ESBL producing organisms and 4 CPO, while 7 (12.3%) were classified as resistant using EUCAST breakpoints (MIC of > 32 mg/L); the majority of these were Morganella morganii. Using CLSI breakpoints, 53 isolates (93.0%) were classified as susceptible (MIC of ≤64 mg/L), while 4 isolates (7.0%) showed an MIC of >64 mg/L, which were classified as either intermediate (MIC of 128 mg/L) or resistant (MIC of ≥256 μg/mL); all of which were M. morganii (Figure 1).

Figure 1: Fosfomycin MIC distribution of tested isolates.

  A total of 20 non-Enterobacterales were tested, comprised of Pseudomonas aeruginosa (n = 19) and Pseudomonas putida (n = 1). Using EUCAST breakpoints set for Enterobacterales, only 5 isolates (25.0%) showed an MIC below the breakpoint of ≤ 32 mg/L, while 15 isolates (75.0%) had an MIC > 32 mg/L. Using CLSI breakpoints, 16 isolates (80.0%) had an MIC of ≤ 64 mg/L, including 1 CPO, while 4 isolates (20.0%) had an MIC > 64 mg/L (Figure 1).

  Of the 23 ESBL producing organisms tested, 23 (100.0%) had an MIC of ≤ 32 mg/L, classifying them as sensitive using both EUCAST and CLSI, with 22 (95.6%) having an MIC of ≤ 16 mg/L. Of the 4 CPO positive Enterobacterales, 4 (100.0%) had an MIC ≤ 32 mg/L classifying them as susceptible according to EUCAST and CLSI (Table 1).

Table 1: Breakdown of tested isolates and respective fosfomycin MIC distribution

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  Although 100% of the Klebsiella isolates which were tested had an MIC of ≤ 32 mg/L, indicating in vitro susceptibility, reduced clinical activity, emergence of resistance and failure in treatment has been shown to be significant among Klebsiella spp [6,7].

Conclusion

  The results of this study showed high susceptibility rates of most Enterobacterales to fosfomycin (excluding Morganella spp.), even in the presence of presumed resistance mechanisms, while most of the Pseudomonas spp. tested showed elevated MICs to fosfomycin. EUCAST and CLSI have not set clinical breakpoints for Pseudomonas spp. Fosfomycin may be a useful therapeutic option for most Enterobacterales, including ESBL and CPO positive isolates, however further study is required to ensure adequate clinical activity. Fosfomycin is unlikely to be beneficial in the treatment of non-Enterobacterales.

Transparency Declarations

None to declare.

Conflicts of Interest and Sources of Funding

The authors state that there are no conflicts of interest to disclose. No funding was received for this study.

References

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8. Performance Standards for Antimicrobial Susceptibility Testing. 30^th edition. CLSI supplement M100. Wayne, PA: Clinical and Laboratory Standards Institute (2020).