a Institute of Hygiene, Fritz Pregl Strasse 3, A-6020 Innsbruck, Austria; b Robert Koch Institute, Burgstrasse 37, D-38855 Wernigerode, Germany
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Abstract |
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Introduction |
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Materials and methods |
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One hundred and eighteen enterococcal strains (with and without different types of resistance to glycopeptides) of the following species were investigated for their in-vitro susceptibility to fosfomycin: E. faecium (VanA, n = 69; VanB, n = 5; glycopeptide-sensitive, n = 8), Enterococcus faecalis (VanA, n = 11; VanB, n = 3; glycopeptide sensitive, n = 10), Enterococcus gallinarum (VanC1, n = 10), and Enterococcus casseliflavus (VanC2n = 2). Isolates originated from the strain collection of the Robert Koch Institute, Wernigerode, Germany.
Susceptibility testing
Minimal inhibitory concentrations (MICs) of fosfomycin were determined by agar dilution test
(inoculum 5 x 104 cfu/spot)3 and broth
microdilution test (inoculum 5 x 105 cells/mL).4 In all susceptibility test methods, MuellerHinton
(MH) agar or broth were used as nutrient media (Oxoid, Basingstoke, UK). As recommended by
the producer of fosfomycin (Sigma Chemical Co., St Louis, MO, USA), the liquid and the solid
MH media were supple?mented with -D-glucose-6-phosphate (Sigma; final
concentration 25 mg/L). An MIC of
128 mg/L characterizes a strain as resistant, 32
64 mg/L as intermediate, and
16 mg/L as susceptible.
5,6 Isolates were also
tested by the
disc diffusion test (according to the NCCLS method
7 with confluent bacterial growth on the agar plates and
according to the DIN 58940 technique
8 with a lighter inoculum resulting in a semiconfluent
growth). For preparing the inocula the same cultures of the corresponding strains in nutrient
broth were used in all four susceptibility tests. Oxoid discs loaded with fosfomycin (50 µg)
-D-glucose-6-phosphate (50 µg) were used for disc susceptibility testing.
Andrews et al.
6 proposed breakpoints for fosfomycin (corresponding to
intravenous administration of 2 or 4 g), of 16 and 64 mg/L, corresponding to zone sizes (NCCLS
method) of
11 and
18 mm, respectively. No official NCCLS zone sizes exist for
fosfomycin. According to DIN 58940, zone diameters of
13 mm are read as resistant, and
those of
20 mm as susceptible.
5 The recommended daily dosage of fosfomycin for
severely ill patients is 816 g.9
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Results |
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Inoculating plates for disc diffusion testing according to the NCCLS method, with the breakpoints stated by Andrews et al., 6 gave rates of resistance comparable with those of the other three methods (Table IV). Of 74 VR E. faecium isolates, 6 were found to be fosfomycin resistant. Finally, full resistance to fosfomycin could only be detected in VanA and VanB type E. faecium.
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Discussion |
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Vancomycin-resistant E. faecium are often highly resistant to ß-lactams and
combinations of other antimicrobials. For infections due to strains for which MICs of ampicillin
are 64 mg/L, high-dose ampicillin may be tried, in combination with an aminoglycoside if
there is no high-level resistance to the corresponding aminoglycoside.
10
A variety of other regimens have been utilized but there are no published data on the effectiveness of these regimens in humans. Teicoplanin has been used in cases of infection due to VR enterococci exhibiting the VanB phenotype. The investigational drugs quinupristin/dalfopristin, oxazolidinone, everninomycin, a new glycopeptide (LY 333328), and several fluoroquinolones including trovafloxacin and clinafloxacin with activity against VR enterococci are presently undergoing clinical evaluation. 10
Fosfomycin inhibited 97, 94 and 96% of the VR strains tested in our study by means of agar dilution, broth microdilution, and disc diffusion by DIN 58940, respectively. Using zone size diameters suggested by Andrews et al., 6 the disc diffusion test (according to the NCCLS method) gave susceptibility results concordant with those of the three other methods tested here. NCCLS criteria do not exist for disc diffusion testing of fosfomycin.
Recently Barry et al. 11 and Fuchs et al. 12 used NCCLS methods for testing susceptibility to fosfomycin tromethamine, a drug to be used for single-dose treatment of uncomplicated lower urinary tract infections, but they abstained from defining breakpoints for enterococci.
Hamilton-Miller13 found that fosfomycin could be a useful addition to the limited number of antibiotics currently available that are effective against infections caused by problematic Gram-positive cocci but no detailed information on species results or glycopeptide susceptibility was given for the 20 enterococci tested in his study, 19 of which were found to be susceptible to fosfomycin (MIC range 8128 mg/L, MIC50 22 mg/L, MIC90 50 mg/L).
Kayser 14tested 50 VS E. faecalis and 31 VS E. faecium isolates and concluded that with mode MICs of 64 mg/L this drug exhibits only very limited activity against these enterococci (MIC range 32512 mg/L, MIC50 64 mg/L, MIC90 128 mg/L).
Fosfomycin may be especially useful for treatment of enterococcal infection in combination. For example, Caron et al. 15 reported that in an animal endocarditis study using a VR E. faecium strain (inhibited by fosfomycin plus ceftriaxone in vitro), most vegetations were sterilized. Landman & Quale 16 found that this combination showed bactericidal synergy against some VRE.
Screening for the ß-lactam with the strongest beneficial effect in combination with
fosfomycin, Pestel et al.
17 showed that cefotaxime (at 64 mg/L) combined with
fosfomycin (at 64 mg/L) was bactericidal against six of ten bloodstream isolates of VS
enterococci tested. Studying the in-vivo activity of the combination of daptomycin and
fosfomycin against a VS E. faecalis strain in a relapse model of rat endocarditis, Rice et al.
18 in contrast found that there was no difference between
the number of valves sterilized by daptomycin alone and by daptomycin plus fosfomycin, despite
synergic bactericidal activity found in in-vitro timekill studies. In-vitro synergy does not
necessarily translate into greater therapeutic efficacy against clinical infection.
In conclusion, the MICs of fosfomycin for most VR isolates were in the intermediate range, yielding an MIC50 of 32 mg/L and an MIC90 of 64 mg/L. The majority of zone sizes corresponded to intermediate susceptibility. Fosfomycin as a single agent at a low dosage therefore seems unsuitable for the treatment of infections due to VR enterococci. Fosfomycin at a high dosage could have a potential for the treatment of infections due to VR enterococci, especially if used in combination with other drugs. However, in-vitro activity does not guarantee in-vivo success. Until more effective agents are available, cure of some deep-seated infections due to VR enterococci may even require surgical intervention.16
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Acknowledgments |
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Notes |
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References |
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18 . Rice, L. B., Eliopoulos, C. T., Yao, J. D., Eliopoulos, G. M., Moellering, R. C. (1992).In vivo activity of the combination of daptomycin and fosfomycin compared with daptomycin alone against a strain of Enterococcus faecalis with high-level gentamicin resistance in the rat endocarditis model.Diagnostic Microbiology and Infectious Disease 15, 1736.[ISI][Medline]
Received 26 January 1998; returned 17 March 1998; revised 6 April 1998; accepted 6 May 1998