Fosfomycin causes transient lysis in Escherichia coli strains carrying fosfomycin-resistance plasmids

Escherichia coli cells carrying fosfomycin-resistance plasmids show high levels of resistance towards this drug. However, the plasmid-carrying strains exhibited a transient lytic phase induced by fosfomycin when grown in rich liquid media. This lytic phase was not observed if the cells were grown in liquid minimal media. Fosfomycin-induced lysis depended on the accumulation of drug inside the bacteria, presumably as a result of the saturation of the fosfomycin modification system. Growth recovery after lysis was not due to drug inactivation in the culture medium and could be explained by selection of mutants showing impaired fosfomycin transport when high concentrations of fosfomycin were used. However, there was no selection of mutants with low drug concentrations.     

Identification of a Second Two-Component Signal Transduction System That Controls Fosfomycin Tolerance and Glycerol-3-Phosphate Uptake

Particular interest in fosfomycin has resurfaced because it is a highly beneficial antibiotic for the treatment of refractory infectious diseases caused by pathogens that are resistant to other commonly used antibiotics. The biological cost to cells of resistance to fosfomycin because of chromosomal mutation is high. We previously found that a bacterial two-component system, CpxAR, induces fosfomycin tolerance in enterohemorrhagic Escherichia coli (EHEC) O157:H7. This mechanism does not rely on irreversible genetic modification and allows EHEC to relieve the fitness burden that results from fosfomycin resistance in the absence of fosfomycin. Here we show that another two-component system, TorSRT, which was originally characterized as a regulatory system for anaerobic respiration utilizing trimethylamine-N-oxide (TMAO), also induces fosfomycin tolerance. Activation of the Tor regulatory pathway by overexpression of torR, which encodes the response regulator, or addition of TMAO increased fosfomycin tolerance in EHEC. We also show that phosphorylated TorR directly represses the expression of glpT, a gene that encodes a symporter of fosfomycin and glycerol-3-phosphate, and activation of the TorR protein results in the reduced uptake of fosfomycin by cells. However, cells in which the Tor pathway was activated had an impaired growth phenotype when cultured with glycerol-3-phosphate as a carbon substrate. These observations suggest that the TorSRT pathway is the second two-component system to reversibly control fosfomycin tolerance and glycerol-3-phosphate uptake in EHEC, and this may be beneficial for bacteria by alleviating the biological cost. We expect that this mechanism could be a potential target to enhance the utility of fosfomycin as chemotherapy against multidrug-resistant pathogens.     

Characterization of Fosfomycin Resistant Extended-Spectrum β-Lactamase-Producing Escherichia coli Isolates from Human and Pig in Taiwan

To investigate the efficacy of fosfomycin against extended-spectrum β-lactamases (ESBL) producing Escherichia coli in Taiwan and the resistance mechanisms and characterization of human and pig isolates, we analyzed 145 ESBL-producing isolates collected from two hospitals (n = 123) and five farms (n = 22) in Taiwan from February to May, 2013. Antimicrobial susceptibilities were determined. Clonal relatedness was determined by PFGE and multi-locus sequence typing. ESBLs, ampC, and fosfomycin resistant genes were detected by PCR, and their flanking regions were determined by PCR mapping and sequencing. The fosfomycin resistant mechanisms, including modification of the antibiotic target (MurA), functionless transporters (GlpT and UhpT) and their regulating genes such as uhpA, cyaA, and ptsI, and antibiotic inactivation by enzymes (FosA and FosC), were examined. The size and replicon type of plasmids carrying fosfomycin resistant genes were analyzed. Our results revealed the susceptibility rates of fosfomycin were 94% for human ESBL-producing E. coli isolates and 77% for pig isolates. The PFGE analysis revealed 79 pulsotypes. No pulsotype was found existing in both human and pig isolates. Three pulsotypes were distributed among isolates from two hospitals. ISEcp1 carrying bla _{CTX-M-group 9} was the predominant transposable elements of the ESBL genes. Among the thirteen fosfomycin resistant isolates, functionless transporters were identified in 9 isolates. Three isolates contained novel amino acid substitutions (Asn67Ile, Phe151Ser and Trp164Ser, Val146Ala and His159Tyr, respectively) in MurA (the target of fosfomycin). Four isolates had fosfomycin modified enzyme (fosA3) in their plasmids. The fosA3 gene was harboured in an IncN-type plasmid (101 kbp) in the three pig isolates and an IncB/O-type plasmid (113 kbp) in the human isolate. In conclusion, we identified that 6% and 23% of the ESBL-producing E. coli from human and pigs were resistant to fosfomycin, respectively, in Taiwan. No clonal spread was found between human and pig isolates. Functionless transporters were the major cause of fosfomycin resistance, and the fosA3-transferring plasmid between isolates warrants further monitoring.     

Role of the CpxAR Two-Component Signal Transduction System in Control of Fosfomycin Resistance and Carbon Substrate Uptake

Although fosfomycin is an old antibiotic, it has resurfaced with particular interest. The antibiotic is still effective against many pathogens that are resistant to other commonly used antibiotics. We have found that fosfomycin resistance of enterohemorrhagic Escherichia coli (EHEC) O157:H7 is controlled by the bacterial two-component signal transduction system CpxAR. A cpxA mutant lacking its phosphatase activity results in constitutive activation of its cognate response regulator, CpxR, and fosfomycin resistance. We have shown that fosfomycin resistance requires CpxR because deletion of the cpxR gene in the cpxA mutant restores fosfomycin sensitivity. We have also shown that CpxR directly represses the expression of two genes, glpT and uhpT, which encode transporters that cotransport fosfomycin with their native substrates glycerol-3-phosphate and glucose-6-phosphate, and repression of these genes leads to a decrease in fosfomycin transport into the cpxA mutant. However, the cpxA mutant had an impaired growth phenotype when cultured with glycerol-3-phosphate or glucose-6-phosphate as a sole carbon substrate and was outcompeted by the parent strain, even in nutrient-rich medium. This suggests a trade-off between fosfomycin resistance and the biological fitness associated with carbon substrate uptake. We propose a role for the CpxAR system in the reversible control of fosfomycin resistance. This may be a beneficial strategy for bacteria to relieve the fitness burden that results from fosfomycin resistance in the absence of fosfomycin.     

In vitro activity of fosfomycin against 'problem' gram-positive cocci.

Fosfomycin was active in vitro against 54 of 60 'problem' Gram-positive cocci (20 methicillin-resistant Staphylococcus aureus, 20 coagulase-negative staphylococci and 20 enterococci). Its activity was significantly greater under anaerobic conditions, especially against coagulase-negative staphylococci. Mutants resistant to fosfomycin were readily demonstrated, but their growth was prevented by rifampicin or ciprofloxacin. The combinations rifampicin+fosfomycin and ciprofloxacin+fosfomycin showed MIC synergy. It is concluded that fosfomycin in an appropriate combination would be a valuable addition to the small and dwindling range of antibiotics active against problem Gram-positive cocci.     

Rapid and selective liquid chromatographic/tandem mass spectrometric method for the determination of fosfomycin in human plasma

A rapid and selective liquid chromatographic/tandem mass spectrometric method for determination of fosfomycin was developed and validated. Following protein-precipitation, the analyte and internal standard (fudosteine) were separated from human plasma using an isocratic mobile phase on an Ultimate XB-CN column. An API 4000 tandem mass spectrometer equipped with Turbo IonSpray ionization source was used as detector and was operated in the negative ion mode. Multiple reaction monitoring using the precursor to product ion combinations of m/z 137-->79 and m/z 178-->91 was performed to quantify fosfomycin and fudosteine, respectively. The method was linear in the concentration range of 0.10-12.0 microg/mL using 50 microL of plasma. The lower limit of quantification was 0.10 microg/mL. The intra- and inter-day relative standard deviation over the entire concentration range was less than 10.6%. Accuracy determined at three concentrations (0.25, 1.00 and 8.00 microg/mL for fosfomycin) ranged from -1.0% to -4.2% in terms of relative error. Each plasma sample was chromatographed within 5.0 min. The method was successfully used in a bioequivalence study of fosfomycin in human plasma after an oral administration of capsules containing 1.0 g fosfomycin (approximately 1.3g calcium fosfomycin).     

Two different types of fosfomycin resistance in clinical isolates of Klebsiella pneumoniae

Abstract The fosfomycin susceptibility of 100 clinical isolates of Klebsiella pneumoniae and the resistance mechanisms utilized by resistant strains were examined. Washed cells prepared from the strains demonstrating MICs of more than 8 μg ml⁻¹ of fosfomycin inactivated the drug. A crude extract from strain Tf129B, highly resistant to fosfomycin, was used to study the enzymatic properties of the drug-inactivating enzyme. The optimum pH for inactivation was 7.8 and the optimum temperature of the reaction was 37°C. Glutathione was shown to be effective as a cofactor in the inactivation. It was suggested that the inactivating enzyme of Klebsiella pneumoniae was fosfomycin: glutathione-S-transferase, a constitutive enzyme located in the periplasmic space. A good correlation was found between the specific activities of this enzyme and the MIC levels; however, certain strains showed a low level of fosfomycin: glutathione-S-transferase activity which could not account for the increased MIC. Strains Tf129B and Tf408E, both demonstrating MICs of more than 1024 μg ml⁻¹ of fosfomycin carried a transferable resistance plasmid. In strain Tf129B, the mechanism of fosfomycin resistance was due to a high level of enzymic activity. In strain Tf408E, it was determined to be mainly due to the reduced permeability of the cell membrane.     

Effect of antibiotics on immediate hypersensitivity reactions in vitro: suppression of IgE-mediated histamine release from peripheral blood basophils by fosfomycin

The effect of antibiotics on allergic reactions was studied in vitro using the release of histamine from human peripheral blood leukocytes (basophils) after incubation with anti-IgE. For the several antibiotics we tested, including beta-lactams and aminoglycosides, none had the capacity to enhance antigen-induced histamine release, but some of them (minocycline, polymyxin B, and fosfomycin) suppressed the release of histamine in a dose-dependent manner. Since fosfomycin has proved to be capable of suppressing IgE-mediated histamine release non-cytotoxically, the effect of fosfomycin on histamine release induced by other secretagogues was further studied. The suppression of histamine release was also demonstrated when the leukocytes, preincubated with fosfomycin, were challenged with either Ca ionophore A 23187 or a synthetic peptide, formyl-methionyl-leucyl-phenylalanine (FMLP). We concluded that some antibiotics, particularly fosfomycin, have the capacity to suppress histamine release mediated by various secretagogues, suggesting they may possess an anti-allergic property as well as a bactericidal activity.     

The Fosfomycin Resistance Gene fosB3 Is Located on a Transferable,Extrachromosomal Circular Intermediate in Clinical Enterococcus faecium Isolates

Some VanM-type vancomycin-resistant Enterococcus faecium isolates from China are also resistant to fosfomycin. To investigate the mechanism of fosfomycin resistance in these clinical isolates, antimicrobial susceptibility testing, filter-mating, Illumina/Solexa sequencing, inverse PCR and fosfomycin resistance gene cloning were performed. Three E. faecium clinical isolates were highly resistant to fosfomycin and vancomycin with minimal inhibitory concentrations (MICs) >1024 µg/ml and >256 µg/ml, respectively. The fosfomycin and vancomycin resistance of these strains could be co-transferred by conjugation. They carried a fosfomycin resistance gene fosB encoding a protein differing by one or two amino acids from FosB, which is encoded on staphylococcal plasmids. Accordingly, the gene was designated fosB3. The fosB3 gene was cloned into pMD19-T, and transformed into E. coli DH5α. The fosfomycin MIC for transformants with fosB3 was 750-fold higher than transformants without fosB3. The fosB3 gene could be transferred by an extrachromosomal circular intermediate. The results indicate that the fosB3 gene is transferable, can mediate high level fosfomycin resistance in both Gram-positive and Gram-negative bacteria, and can be located on a circular intermediate.     

Tn292l, a transposon encoding fosfomycin resistance.

The determinant of resistance to fosfomycin of the Serratia marcescens plasmid pOU900 was transposed into the plasmid ColE1 and into the plasmid RP4 in the absence of the RecA function of the host. In each case, the acquisition of fosfomycin resistance was correlated with the presence of a discrete piece of DNA, uniform in size and in restriction pattern, This new, 7.5-megadalton transposable element encoding resistance to fosfomycin has been called Tn2921. A preliminary map of the transposon is presented.     

Protective effect of fosfomycin on gentamicin-induced lipid peroxidation of rat renal tissue

Fosfomycin is clinically recognized to reduce the aminoglycoside antibiotics-induced nephrotoxicity. However, little has been clarified why fosfomycin protects the kidney from the aminoglycosides-induced nephrotoxicity. Gentamicin, a typical aminoglycoside, is reported to cause lipid peroxidation. We focused on lipid peroxidation induced by gentamicin as a mechanism for the aminoglycosides-induced nephrotoxicity. The aim of this study is to investigate the effect of fosfomycin on the gentamicin-induced lipid peroxidation. In rat renal cortex mitochondria, fosfomycin was shown to depress the gentamicin-induced lipid peroxidation, which was evaluated by formation of thiobarbituric acid reactive substances (TBARS). Interestingly, this effect was observed in rat renal cortex mitochondria, but not in rat liver microsomes. However, fosfomycin did not affect lipid peroxidation of arachidonic acid caused by gentamicin with iron. Fosfomycin inhibited the gentamicin-induced iron release from rat renal cortex mitochondria. These results indicated that fosfomycin inhibited the gentamicin-induced lipid peroxidation by depressing the iron release from mitochondria. This may possibly be one mechanism for the protection of fosfomycin against the gentamicin-induced nephrotoxicity.     

Assessing the Emergence of Resistance: The Absence of Biological Cost In Vivo May Compromise Fosfomycin Treatments for P. aeruginosa Infections

Background

Fosfomycin is a cell wall inhibitor used efficiently to treat uncomplicated urinary tract and gastrointestinal infections. A very convenient feature of fosfomycin, among others, is that although the expected frequency of resistant mutants is high, the biological cost associated with mutation impedes an effective growth rate, and bacteria cannot offset the obstacles posed by host defenses or compete with sensitive bacteria. Due to the current scarcity of new antibiotics, fosfomycin has been proposed as an alternative treatment for other infections caused by a wide variety of bacteria, particularly Pseudomonas aeruginosa. However, whether fosfomycin resistance in P. aeruginosa provides a fitness cost still remains unknown.

Principal Findings

We herein present experimental evidence to show that fosfomycin resistance cannot only emerge easily during treatment, but that it is also cost-free for P. aeruginosa. We also tested if, as has been reported for other species such as Escherichia coli, Klebsiella pneumoniae and Proteus mirabilis, fosfomycin resistant strains are somewhat compromised in their virulence. As concerns colonization, persistence, lung damage, and lethality, we found no differences between the fosfomycin resistant mutant and its sensitive parental strain. The probability of acquisition in vitro of resistance to the combination of fosfomycin with other antibiotics (tobramycin and imipenem) has also been studied. While the combination of fosfomycin with tobramycin makes improbable the emergence of resistance to both antibiotics when administered together, the combination of fosfomycin plus imipenem does not avoid the appearance of mutants resistant to both antibiotics.

Conclusions

We have reached the conclusion that the use of fosfomycin for P. aeruginosa infections, even in combined therapy, might not be as promising as expected. This study should encourage the scientific community to assess the in vivo cost of resistance for specific antibiotic-bacterial species combinations, and therefore avoid reaching universal conclusions from single model organisms.     

Fosfomycin Permeation through the Outer Membrane Porin OmpF

Fosfomycin is a frequently prescribed drug in the treatment of acute urinary tract infections. It enters the bacterial cytoplasm and inhibits the biosynthesis of peptidoglycans by targeting the MurA enzyme. Despite extensive pharmacological studies and clinical use, the permeability of fosfomycin across the bacterial outer membrane is largely unexplored. Here, we investigate the fosfomycin permeability across the outer membrane of Gram-negative bacteria by electrophysiology experiments as well as by all-atom molecular dynamics simulations including free-energy and applied-field techniques. Notably, in an electrophysiological zero-current assay as well as in the molecular simulations, we found that fosfomycin can rapidly permeate the abundant Escherichia coli porin OmpF. Furthermore, two triple mutants in the constriction region of the porin have been investigated. The permeation rates through these mutants are slightly lower than that of the wild type but fosfomycin can still permeate. Altogether, this work unravels molecular details of fosfomycin permeation through the outer membrane porin OmpF of E. coli and moreover provides hints for understanding the translocation of phosphonic acid antibiotics through other outer membrane pores.     

Stereoselective epoxidation of <Emphasis Type="Italic">cis</Emphasis>-propenylphosphonic acid to fosfomycin by a newly isolated bacterium <Emphasis Type="Italic">Bacillus</Emphasis><Emphasis Type="Italic">simplex</Emphasis> strain S101

In industry, fosfomycin is mainly prepared via chemical epoxidation of cis-propenylphosphonic acid (cPPA). The conversion yield of fosfomycin is less than 50% in the whole process and a large quantity of waste is produced. Biotransformation by microorganisms is an alternative method of preparation. This kind of conversion is more delicate, environmentally friendly, and the conversion yield of fosfomycin would be higher. In this work, an aerobic bacterium capable of transforming cPPA to fosfomycin was isolated. The organism, designated as strain S101, was identified as Bacillus simplex by morphological and physiological characteristics as well as by analysis of the gene encoding the 16S rRNA. Fosfomycin was assayed by two means, bioassay and gas chromatography (GC). Glycerol was a good carbon source for growth and cPPA conversion of strain S101. When cPPA was used as the sole carbon source, neither growth nor conversion to fosfomycin occurred. The optimum cPPA concentration in the conversion medium was 2,000 μg ml⁻¹. After 6 days of incubation, the concentration of fosfomycin reached its maximum level (1,838.2 μg ml⁻¹), with a conversion ratio of 81.3%. Air was indispensable for the growth but not for the conversion to fosfomycin. Furthermore, vanadium ions were found to be essential for the conversion. High concentrations of cPPA had fewer inhibitory effects on the growth of strain S101.     

On the conversion of structural analogues of (S)-2-hydroxypropylphosphonic acid to epoxides by the final enzyme of fosfomycin biosynthesis in S. fradiae

2-Hydroxyethyl- and (S)-2-hydroxybutylphosphonic acid were prepared, starting in the latter case from (S)-2-aminobutyric acid. They were fed to cultures of Streptomyces fradiae producing fosfomycin. Only the latter (150 microg/mL of medium) was converted to the ethyl analogue of fosfomycin, isolated as 2-amino-1-hydroxybutylphosphonic acid (3%) in admixture with 2-amino-1-hydroxypropylphosphonic acid (97%) derived from fosfomycin.     

Structural and biochemical insights into the mechanism of fosfomycin phosphorylation by fosfomycin resistance kinase FomA

We present here the crystal structures of fosfomycin resistance protein (FomA) complexed with MgATP, with ATP and fosfomycin, with MgADP and fosfomycin vanadate, with MgADP and the product of the enzymatic reaction, fosfomycin monophosphate, and with ADP at 1.87, 1.58, 1.85, 1.57, and 1.85 ? resolution, respectively. Structures of these complexes that approximate different reaction steps allowed us to distinguish the catalytically active conformation of ATP and to reconstruct the model of the MgATP·fosfomycin complex. According to the model, the triphosphate tail of the nucleotide is aligned toward the phosphonate moiety of fosfomycin, in contest to the previously published MgAMPPNP complex, with the attacking fosfomycin oxygen positioned 4 ? from the γ-phosphorus of ATP. Site-directed mutagenesis studies and comparison of these structures with that of homologous N-acetyl-l-glutamate and isopentenyl phosphate kinases allowed us to propose a model of phosphorylation of fosfomycin by FomA enzyme. A Mg cation ligates all three phosphate groups of ATP and together with positively charged K216, K9, K18, and H58 participates in the dissipation of negative charge during phosphoryl transfer, indicating that the transferred phosphate group is highly negatively charged, which would be expected for an associative mechanism. K216 polarizes the γ-phosphoryl group of ATP. K9, K18, and H58 participate in stabilization of the transition state. D150 and D208 play organizational roles in catalysis. S148, S149, and T210 participate in fosfomycin binding, with T210 being crucial for catalysis. Hence, it appears that as in the homologous enzymes, FomA-catalyzed phosphoryl transfer takes place by an in-line predominantly associative mechanism.     

Fosfomycin enhances the expression of penicillin-binding protein 2 in methicillin-sensitive and methicillin-resistant Staphylococcus aureus strains

The effects of fosfomycin on penicillin-binding proteins (PBPs) were studied on the methicillin-resistant Staphylococcus aureus strain CIP (Collection de l'Institut Pasteur, Paris, France) 65-25 and on a methicillin-susceptible S. aureus strain CIP 65-6. The combinations of fosfomycin and oxacillin were synergistic, additive or antagonistic, depending on antibiotic concentrations. Fosfomycin induced modifications of the PBP profile of the two strains studied. In particular, it increased the expression of PBP2. This suggested that this protein is inducible; the only PBP not affected by fosfomycin was PBP3.     

Resistance Development of Cystic Fibrosis Respiratory Pathogens When Exposed to Fosfomycin and Tobramycin Alone and in Combination under Aerobic and Anaerobic Conditions

Although antibiotics from different classes are frequently prescribed in combination to prevent the development of resistance amongst Cystic Fibrosis (CF) respiratory pathogens, there is a lack of data as to the efficacy of this approach. We have previously shown that a 4∶1 (w/w) combination of fosfomycin and tobramycin (F∶T) has excellent activity against CF pathogens with increased activity under physiologically relevant anaerobic conditions. Therefore, the aim of this study was to determine whether F∶T could delay or prevent the onset of resistance compared to either fosfomycin or tobramycin alone under aerobic and anaerobic conditions. The frequency of spontaneous mutants arising following exposure to fosfomycin, tobramycin and F∶T was determined for clinical Pseudomonas aeruginosa and MRSA isolates under aerobic and anaerobic conditions. The effect of sub-inhibitory concentrations of fosfomycin, tobramycin and F∶T on the induction of resistance was also investigated, with the stability of resistance and fitness cost associated with resistance assessed if it developed. P. aeruginosa and MRSA isolates had a lower frequency of spontaneous mutants to F∶T compared to fosfomycin and tobramycin under both aerobic and anaerobic conditions. There was a maximum two-fold increase in F∶T MICs when P. aeruginosa and MRSA isolates were passaged in sub-inhibitory F∶T for 12 days. In contrast, sequential resistance to fosfomycin and tobramycin developed quickly (n = 3 days for both) after passage in sub-inhibitory concentrations. Once developed, both fosfomycin and tobramycin resistance was stable and not associated with a biological fitness cost to either P. aeruginosa or MRSA isolates. The results of this study suggest that F∶T may prevent the development of resistance compared to fosfomycin or tobramycin alone under aerobic and physiologically relevant anaerobic conditions. F∶T may be a potential treatment option in CF patients chronically colonised by MRSA and/or P. aeruginosa.     

Influence of Culture Media on the Antistaphylococcal Activity of Fosfomycin

Antagonism or double zones of inhibition (or both) resulted when Staphylococcus aureus was exposed to fosfomycin in Brain Heart Infusion, tryptic/papaic digest of casein/soybean, and Mueller Hinton culture media; neither occurred with a totally defined, completely synthetic medium. The extent of antagonism was quantitated by determination of the absolute minimal inhibitory concentration of fosfomycin in each medium. By using the synthetic medium and inducers, the hexose-phosphate transport system appeared to be more important than the alpha-glycerophosphate transport system in providing ingress of fosfomycin into staphylococci.     

Interaction of fosfomycin with the Glycerol 3-phosphate Transporter of Escherichia coli

Background

Fosfomycin is widely used to treat urinary tract and pediatric gastrointestinal infections of bacteria. It is supposed that this antibiotic enters cells via two transport systems, including the bacterial Glycerol-3-phosphate Transporter (GlpT). Impaired function of GlpT is one mechanism for fosfomycin resistance.

Methods

The interaction of fosfomycin with the recombinant and purified GlpT of Escherichia coli reconstituted in liposomes has been studied. IC₅₀ and the half-saturation constant of the transporter for external fosfomycin (K_i) were determined by transport assay of [¹⁴C]glycerol-3-phosphate catalyzed by recombinant GlpT. Efficacy of fosfomycin on growth rates of GlpT defective bacteria strains transformed with recombinant GlpT was measured.

Results

Fosfomycin, externally added to the proteoliposomes, poorly inhibited the glycerol-3-phosphate/glycerol-3-phosphate antiport catalyzed by the reconstituted transporter with an IC₅₀ of 6.4 mM. A kinetic analysis revealed that the inhibition was completely competitive, that is, fosfomycin interacted with the substrate-binding site and the K_i measured was 1.65 mM. Transport assays performed with proteoliposomes containing internal fosfomycin indicate that it was not very well transported by GlpT. Complementation study, performed with GlpT defective bacteria strains, indicated that the fosfomycin resistance, beside deficiency in antibiotic transporter, could be due to other gene defects.

Conclusions

The poor transport observed in a reconstituted system together with the high value of K_i and the results of complementation study well explain the usual high dosage of this drug for the treatment of the urinary tract infections.

General significance

This is the first report regarding functional analysis of interaction between fosfomycin and GlpT.