Fosfomycin kills bacteria by permanently disabling an enzyme they need to build their cell walls. Specifically, it locks onto MurA, the enzyme responsible for kicking off peptidoglycan production, and shuts it down irreversibly. This mechanism is unlike that of any other antibiotic in clinical use, which has kept fosfomycin effective against many drug-resistant organisms even decades after its discovery. But the story of how the drug reaches its target, why some bacteria shrug it off, and where it fits in modern medicine is more layered than a single enzyme interaction suggests.
How Fosfomycin Disables Cell Wall Construction
Bacteria depend on a mesh-like structure called peptidoglycan to keep their cell walls intact. Without it, the internal pressure of the cell ruptures the membrane, and the bacterium dies. Building peptidoglycan starts inside the cell’s cytoplasm with a series of enzymatic steps, and the very first committed step is carried out by an enzyme called MurA (formally, UDP-N-acetylglucosamine enolpyruvyl transferase). MurA’s job is to attach a small molecule called phosphoenolpyruvate, or PEP, to a sugar substrate. That reaction produces the first building block in the peptidoglycan assembly line.
Fosfomycin is a small, simple molecule that looks enough like PEP to fool MurA into accepting it. Once fosfomycin enters MurA’s active site, it forms a permanent covalent bond with a cysteine residue there, locking the enzyme in an inactive state.1PubMed Central. Structure of MurA (UDP-N-acetylglucosamine enolpyruvyl transferase) from Vibrio fischeri in complex with substrate UDP-N-acetylglucosamine and the drug fosfomycin In the well-studied case of E. coli, that cysteine sits at position 115. Once fosfomycin is covalently attached to it, the enzyme cannot release product or process substrate, and the entire downstream chain of peptidoglycan synthesis stalls.2Journal of Biological Chemistry. Evidence That the Fosfomycin Target Cys115 in UDP-N-acetylglucosamine Enolpyruvyl Transferase (MurA) Is Essential for Product Release Because the bond is covalent and irreversible, the enzyme molecule that fosfomycin latches onto is permanently disabled. The bacterium would need to synthesize entirely new MurA molecules to recover, and fosfomycin in the surrounding environment will disable those too.3PubMed. Molecular Pharmacology of the Antibiotic Fosfomycin, an Inhibitor of Peptidoglycan Biosynthesis
This target is shared by both Gram-positive and Gram-negative bacteria, which gives fosfomycin its unusually broad reach. The enzyme is essential for survival in virtually all bacteria that rely on peptidoglycan, and no human enzyme performs the same reaction, so the drug has a favorable safety profile rooted in its basic chemistry.4PubMed Central. Molecular Mechanisms and Clinical Impact of Acquired and Intrinsic Fosfomycin Resistance
How Fosfomycin Gets Inside Bacteria
A drug that targets an intracellular enzyme is only useful if it can get through the bacterial membrane. Fosfomycin does not diffuse passively. Instead, it hijacks transport systems that bacteria normally use to import nutrients. In E. coli, two main transporters do the job: GlpT, which normally ferries in glycerol-3-phosphate, and UhpT, which imports hexose phosphates like glucose-6-phosphate. Fosfomycin is small and phosphonate-containing, so it mimics these natural substrates well enough to ride in on the same channels.5PubMed Central. The glycerol-3-phosphate permease GlpT is the only fosfomycin transporter in Pseudomonas aeruginosa
The relative importance of these two transporters varies by species. In Pseudomonas aeruginosa, GlpT is the sole fosfomycin transporter; UhpT is absent.5PubMed Central. The glycerol-3-phosphate permease GlpT is the only fosfomycin transporter in Pseudomonas aeruginosa That single-route dependence has consequences for both susceptibility and resistance, since knocking out one transporter is easier than knocking out two. An interesting wrinkle is that conditions inside the body can change how much transporter the bacterium produces. Under low-oxygen conditions, E. coli ramps up production of both GlpT and UhpT, pulling more fosfomycin into the cell and making the bacterium more susceptible.6PubMed Central. Elevated Expression of GlpT and UhpT via FNR Activation Contributes to Increased Fosfomycin Susceptibility in Escherichia coli under Anaerobic Conditions Since many sites of infection in the body are low in oxygen, this is a practical advantage.
How Bacteria Resist Fosfomycin
Bacteria have evolved several distinct strategies to survive fosfomycin exposure. These fall into three broad categories: keeping the drug out, destroying the drug, and changing the target.
Transporter Mutations
The most straightforward route to resistance is losing or crippling the transporters that bring fosfomycin into the cell. Research in Staphylococcus aureus illustrates this clearly. When the gene for UhpT alone was deleted, resistance climbed substantially. When GlpT alone was knocked out, resistance rose modestly. But when both transporters were removed, the bacterium became extremely resistant, with its minimum inhibitory concentration jumping from 0.5 to over 1,024 micrograms per milliliter.7PubMed Central. Mutations of the Transporter Proteins GlpT and UhpT Confer Fosfomycin Resistance in Staphylococcus aureus UhpT mutations appear to play the larger role in S. aureus resistance, though both transporters matter.
Drug-Modifying Enzymes
Perhaps the most clinically worrying resistance mechanism involves enzymes that chemically alter fosfomycin, rendering it unable to bind MurA. Several families of these enzymes exist. The Fos enzymes, including FosA, FosB, FosC, and FosX, each use a slightly different chemical strategy to neutralize the drug. FosA and FosC are glutathione-S-transferases that open fosfomycin’s epoxide ring by attaching glutathione. FosB does something similar using the thiol group of bacillithiol or cysteine. FosX uses water to accomplish the ring-opening.4PubMed Central. Molecular Mechanisms and Clinical Impact of Acquired and Intrinsic Fosfomycin Resistance
A separate resistance kinase called FomA, found in the fosfomycin-producing organism Streptomyces wedmorensis, takes a different approach: it phosphorylates fosfomycin’s phosphonate group using ATP, which effectively neutralizes the drug before it can reach MurA.8PubMed Central. Crystal structure of fosfomycin resistance kinase FomA from Streptomyces wedmorensis
Plasmid-Borne Spread of Resistance
The chromosomal mutations described above typically arise in individual bacterial lineages and spread only as that lineage multiplies. Far more alarming is the spread of resistance genes on plasmids, mobile pieces of DNA that can hop between bacteria, even between different species. In the Enterobacteriaceae family, FosA variants carried on plasmids are a growing concern. A study of E. coli and Klebsiella isolates from food and environmental samples found high-level fosfomycin resistance driven by plasmids carrying fosA3 or fosA8. These fosA genes were frequently embedded in mobile genetic elements flanked by insertion sequences, and in most cases were located near genes for other resistance determinants, including extended-spectrum beta-lactamase genes.9PubMed. Characteristics of fosA-carrying plasmids in E. coli and Klebsiella spp. isolates originating from food and environmental samples That physical proximity on plasmids means that using one class of antibiotics can co-select for fosfomycin resistance as well, accelerating its spread even in settings where fosfomycin itself is not being used.10PubMed Central. Mobile fosfomycin resistance genes in Enterobacteriaceae-An increasing threat
Target Modification
Mutations in the MurA gene itself can also confer resistance, though this route appears less common clinically. Among MRSA isolates, researchers identified several distinct MurA mutations. One created a premature stop codon, truncating the protein, and was found in a fosfomycin-resistant isolate. Others caused amino acid substitutions; some of these were found in sensitive isolates, suggesting that not every MurA change eliminates drug binding.11PubMed Central. Prevalence of Fosfomycin Resistance and Mutations in murA, glpT, and uhpT in Methicillin-Resistant Staphylococcus aureus Strains Isolated from Blood and Cerebrospinal Fluid Samples The critical cysteine that fosfomycin bonds to is highly conserved across bacterial species, which is part of why target-based resistance is relatively rare. Mutating that residue tends to cripple the enzyme’s normal function too, imposing a steep fitness cost on the bacterium.
What Fosfomycin Works Against
Fosfomycin’s broad-spectrum activity is one of its key selling points, particularly against bacteria that have become resistant to other drug classes. Among ESBL-producing E. coli, susceptibility to fosfomycin has consistently remained high. A Canadian study of 160 ESBL-producing Enterobacteriaceae found that fewer than 4% were resistant to fosfomycin.12PubMed Central. Determination of susceptibility to fosfomycin and tigecycline of Enterobacteriaceae, particularly Escherichia coli isolates, producing extended-spectrum β-lactamases from multiple regional Canadian hospitals Another study found fosfomycin active against about 89% of ESBL-producing E. coli.13Journal of Global Antimicrobial Resistance. In vitro fosfomycin study on concordance of susceptibility testing methods against ESBL and carbapenem-resistant Enterobacteriaceae
The picture is less rosy for Klebsiella pneumoniae, especially carbapenem-resistant strains. Resistance rates among K. pneumoniae ranged from about a quarter in KPC-producing isolates to 100% in strains co-producing NDM and OXA-48 carbapenemases.13Journal of Global Antimicrobial Resistance. In vitro fosfomycin study on concordance of susceptibility testing methods against ESBL and carbapenem-resistant Enterobacteriaceae This makes clinical sense: Klebsiella species frequently carry chromosomal fosA genes that confer some baseline resistance, and the accumulation of plasmid-borne resistance elements on top of that can push them past the threshold for clinical usefulness.
Against Gram-positive organisms, fosfomycin performs well. Studies have found all tested MRSA, vancomycin-resistant enterococci, and methicillin-resistant coagulase-negative staphylococci to be susceptible in vitro.14PubMed Central. Increasing antimicrobial resistance among uropathogens: Is fosfomycin the answer? 15Journal of Microbiology and Infectious Diseases. In vitro efficacy of fosfomycin against clinical strains AmpC-producing organisms also showed near-complete susceptibility at 99%.14PubMed Central. Increasing antimicrobial resistance among uropathogens: Is fosfomycin the answer?
Why Fosfomycin Works Best in Combination
For serious systemic infections, fosfomycin is rarely used alone. When given as monotherapy against highly resistant organisms, resistance can emerge quickly during treatment because a single transporter mutation or acquisition of a fos gene is sometimes enough to wipe out efficacy. Combination therapy addresses this problem and often produces synergy, meaning the combined effect is greater than the sum of each drug working individually.
Lab studies against carbapenem-resistant Acinetobacter baumannii demonstrated that pairing fosfomycin with aminoglycosides, glycylcyclines, fluoroquinolones, or colistin reduced the amount of fosfomycin needed by two- to sixteen-fold. Time-kill experiments confirmed that these combinations killed more than 99.9% of bacterial cells.16PubMed Central. Evaluation of the Synergistic Antibacterial Effects of Fosfomycin in Combination with Selected Antibiotics against Carbapenem-Resistant Acinetobacter baumannii Data from dynamic infection models and animal studies reinforce these in-vitro findings, showing that intravenous fosfomycin combined with a beta-lactam, a polymyxin, or an aminoglycoside produces synergistic effects that rival or surpass other commonly used combination regimens.17Journal of Antimicrobial Chemotherapy. Combination therapy with IV fosfomycin for adult patients with serious Gram-negative infections: a review of the literature
Part of why combination therapy suppresses resistance so effectively is probabilistic: the odds of a bacterium simultaneously acquiring resistance to two unrelated drugs are far lower than the odds of surviving either one alone. Fosfomycin’s unique mechanism means it rarely shares cross-resistance with other antibiotic classes, making it a natural partner.
Oral Versus Intravenous Formulations
Fosfomycin comes in two main forms, and they are practically different drugs from a dosing perspective. The oral version, fosfomycin tromethamine, is absorbed at a bioavailability of roughly 33% to 53%, depending on the study and how it is measured.18PubMed Central. Fosfomycin: Mechanisms, Targets, and Resistance in Bacterial Infections 19PubMed Central. Pharmacokinetics, Safety, and Tolerability of Single-Dose Intravenous (ZTI-01) and Oral Fosfomycin in Healthy Volunteers Because so much of the oral dose passes through unabsorbed, it concentrates in the urine rather than the bloodstream. That is actually an advantage for urinary tract infections, and the standard approved use of the oral form is a single 3-gram dose for uncomplicated UTIs.
The intravenous version, fosfomycin disodium, delivers dramatically higher blood levels. After a 3-gram IV dose, peak blood concentrations reach roughly 276 to 370 micrograms per milliliter, compared to only about 22 to 32 micrograms per milliliter after the same oral dose.18PubMed Central. Fosfomycin: Mechanisms, Targets, and Resistance in Bacterial Infections Those higher concentrations make IV fosfomycin suitable for systemic infections beyond the urinary tract. The drug distributes well into tissues, reaching clinically useful levels in the kidneys, lungs, bone, cerebrospinal fluid, prostate, and heart valves.20PubMed. The revival of fosfomycin Both forms are cleared primarily through the kidneys unchanged, with roughly 74% to 80% of an IV dose recovered in urine within 48 hours, compared to about 37% after an oral dose.19PubMed Central. Pharmacokinetics, Safety, and Tolerability of Single-Dose Intravenous (ZTI-01) and Oral Fosfomycin in Healthy Volunteers
Side Effects Worth Knowing About
Fosfomycin has a generally mild side-effect profile. The oral single-dose regimen for UTIs is particularly well tolerated, with the most common complaints being gastrointestinal: nausea, diarrhea, and occasional stomach discomfort. These are usually transient and self-limiting.
Intravenous fosfomycin carries an additional concern that is less obvious. The disodium salt form delivers a substantial sodium load with each dose. At high IV doses used for serious infections, this can push blood sodium levels above normal, a condition called hypernatremia.21PubMed. Effect of fosfomycin-induced hypernatremia on patients’ hospital stay length and survival A retrospective review found cases of hypernatremia alongside gastrointestinal symptoms in patients receiving IV fosfomycin, and the sodium load can be especially problematic in patients with heart failure or kidney impairment who cannot easily excrete the excess.22PubMed Central. Hypernatremia During Intravenous Treatment With Fosfomycin: A Retrospective Medical Record Review Study and an Analysis of Spontaneous Reports in the EudraVigilance Database Clinicians using IV fosfomycin for extended courses typically monitor electrolytes closely.
Anti-Inflammatory Effects Beyond Killing Bacteria
An increasingly studied property of fosfomycin is its ability to calm the immune system independent of its antibiotic action. In preclinical work, fosfomycin has been shown to suppress major pro-inflammatory signaling molecules, including TNF-α, IL-1β, and IL-6, while leaving anti-inflammatory signals like IL-10 relatively untouched.23PubMed. Anti-inflammatory and immunomodulatory properties of fosfomycin: an overlooked potential in a well-known antibiotic In a human blood model challenged with bacterial endotoxin, fosfomycin reduced IL-6 protein levels by about 56% and TNF-α by about 73%.24Journal of Antimicrobial Chemotherapy. Immunomodulatory effects of fosfomycin in an endotoxin model in human blood
This matters because in serious infections like sepsis, the body’s own inflammatory response often causes as much damage as the bacteria themselves. An antibiotic that simultaneously kills bacteria and dials down excessive inflammation could offer a double benefit. Fosfomycin also appears to limit neutrophil migration and activation and to suppress T-cell proliferation.23PubMed. Anti-inflammatory and immunomodulatory properties of fosfomycin: an overlooked potential in a well-known antibiotic These immunomodulatory properties have not yet been tested rigorously in large clinical trials, so how much they contribute to real-world outcomes remains an open question. Still, they help explain why some clinicians report favorable responses to fosfomycin in scenarios where its antibacterial potency alone would not fully account for the clinical improvement.
Fosfomycin in Children and Newborns
Experience with fosfomycin in pediatric patients has historically been limited, but renewed interest in the drug for multidrug-resistant infections has pushed researchers to study it more carefully in younger populations.25PubMed Central. Fosfomycin in the pediatric setting: Evidence and potential indications A randomized controlled trial evaluated IV fosfomycin added to standard-of-care antibiotics in neonates with sepsis. The study found no evidence that fosfomycin worsened sodium levels or caused gastrointestinal problems in this vulnerable population. Adverse events were actually slightly less frequent in the group receiving fosfomycin than in those on standard care alone. Pharmacokinetic modeling from the trial suggested that neonates need a higher weight-adjusted dose than adults: about 150 milligrams per kilogram twice daily for most infants, reduced to 100 milligrams per kilogram twice daily for the youngest neonates (under seven days old) or the smallest (under 1,500 grams).26Archives of Disease in Childhood. Randomised controlled trial of fosfomycin in neonatal sepsis: pharmacokinetics and safety in relation to sodium overload
The favorable safety data in neonates is particularly encouraging because very few antibiotics have been formally tested in this age group. Neonatal sepsis caused by resistant Gram-negative organisms is a growing problem in neonatal intensive care units worldwide, and having a drug with a different mechanism and good tolerability expands the treatment toolkit meaningfully.
Fosfomycin and Biofilms
Bacteria living in biofilms, the slimy communities that form on surfaces like catheters and implants, are notoriously harder to kill than free-floating bacteria. Fosfomycin’s small size and water solubility help it penetrate biofilm matrices better than many larger antibiotics. In a study of multidrug-resistant and extensively drug-resistant Pseudomonas aeruginosa, fosfomycin exposure inhibited biofilm formation to varying degrees in about 71% of isolates tested. However, roughly 29% of isolates actually showed increased biofilm density after fosfomycin exposure, and all of those were fosfomycin-resistant strains.27PubMed Central. In Vitro Antibiofilm Activity of Fosfomycin Alone and in Combination with Other Antibiotics against Multidrug-Resistant and Extensively Drug-Resistant Pseudomonas aeruginosa The implication is clear: fosfomycin can help with biofilm-related infections, but only if the organism is susceptible. Using it against a resistant biofilm-forming strain risks making the biofilm problem worse, underscoring the importance of susceptibility testing before treatment decisions.
This finding fits a broader pattern in antibiotic therapy where sub-inhibitory drug concentrations sometimes trigger defensive behaviors in bacteria, including thicker biofilm production. Getting the dose right and confirming susceptibility beforehand are essential when targeting biofilm infections with fosfomycin, whether used alone or as part of a combination regimen.