In Vitro Activity of Fosfomycin against Extended-Spectrum-β-Lactamase- Producing Escherichia coli and Klebsiella pneumoniae : Comparison of Susceptibility Testing Procedures

Abstract

The agar dilution, broth microdilution, and disk diffusion methods were compared to determine the in vitro susceptibility of 428 extended-spectrum-␤-lactamase (ESBL)-producing Escherichia coli and Klebsiella pneumoniae to fosfomycin. Fosfomycin showed very high activity against all ESBL-producing strains. Excellent agreement between the three susceptibility methods was found for E. coli, whereas marked discrepancies were observed for K. pneumoniae.Fosfomycin tromethamine is a stable salt of fosfomycin which is licensed for the single-dose treatment of acute uncomplicated urinary tract infections (UTIs) caused by susceptible organisms (8,12). After many years of fosfomycin use, both in Spain and worldwide, fosfomycin continues to be active against the most common uropathogens, and there is a very low incidence of resistant strains in Escherichia coli (about 2%) (2,12,14).Isolation of extended-spectrum-␤-lactamase (ESBL)-producing E. coli and Klebsiella pneumoniae strains is increasing in Spain, and a high proportion of these isolates are recovered from outpatients with uncomplicated UTIs (4, 7, 11). It is common to find that the same plasmid coding for ESBL also contains genes conferring resistance to several groups of antimicrobial agents, such as aminoglycosides and cotrimoxazole (7, 10). The concurrence of quinolone resistance, particularly in ESBL-producing K. pneumoniae, is frequent (6, 10, 13), there being few alternatives for the appropriate oral treatment of uncomplicated UTIs caused by ESBL-producing microorganisms.The present study was designed to assess the in vitro activity of fosfomycin by three different methods against 428 ESBLproducing strains, made up of 290 (68%) E. coli and 138 (32%) K. pneumoniae isolates.The strains were collected in two Spanish studies: the first, developed in 2000, included 40 hospitals representing different regions of Spain, and the second was carried out in Seville, Spain, at the University Hospital of Virgen Macarena (UHVM), using strains recovered between 1995 and 2001 (7, 11). In both studies ESBL production was determined by broth microdilution according to CLSI guidelines (5).The strains collected from the multicenter study included 170 E. coli and 70 K. pneumoniae strains, while those obtained from the UHVM included 120 E. coli and 68 K. pneumoniae strains. One-hundred thirty-two out of 290 (45.5%) E. coli and 16 out of 138 (11.6%) K. pneumoniae strains were isolated from outpatients with community-acquired infections, and among these isolates found in the community, 66 (22.7%) E. coli and 9 (6.5%) K. pneumoniae strains were isolated from urine samples of women with uncomplicated UTIs.The MICs of fosfomycin (Zambon, Milan, Italy) were determined by the agar dilution and broth microdilution methods in cation adjusted Mueller-Hinton medium, supplemented with 25 mg/liter of G-6-P (glucose-6-phosphate; Sigma Chemical Co.). A twofold dilution across a range of 0.25 to 512 mg of fosfomycin per liter was used. The inocula were prepared to achieve 1 ϫ 10 4 CFU per spot (ag...

Antimicrobial susceptibility of multidrug-resistant (MDR) and extensively drug-resistant (XDR) Enterobacteriaceae isolates to fosfomycin

“…Several studies have noted good activity of fosfomycin against Enterobacteriaceae producing ESBLs or characterised as MDR [9]. However, relatively few studies have evaluated the antimicrobial activity of fosfomycin against Enterobacteriaceae isolates with extensive drug resistance (including carbapenem resistance), providing favourable findings regarding the potential value of fosfomycin in this regard [8].…”

“…Furthermore, fosfomycin, which has been mainly used in the treatment of uncomplicated lower urinary tract infections [7], may exhibit good antimicrobial activity against Gram-negative isolates with multidrug resistance, including contemporary isolates [8,9].…”

In vitro activity of minocycline combined with fosfomycin against clinical isolates of methicillin-resistant Staphylococcus aureus

“…A limitation of our methodology is that broth dilution methods are not considered optimal for the determination of antimicrobial susceptibility to fosfomycin, but this might be species specific. 28 …”

Fosfomycin and Its Application in the Treatment of Multidrug-Resistant Enterobacteriaceae Infections

“…9,142,143 This observation is of concern because meaningful evaluation of clinical studies requires accurate data on in vitro susceptibility.…”

“…In this study, the combination was assessed with the Etest, even though the Etest do not give reliable results for determination of the fosfomycin MIC in extended-spectrum β-lactamase-producing strains of K. pneumoniae. 9,142,143 With regard to multiresistant Enterobacteriaceae, there is a need for additional research because, in the event that a new extended-spectrum β-lactamase appears, the right partner for fosfomycin needs to be identified.…”