Fosfomycin: Use Beyond Urinary Tract and Gastrointestinal Infections

Biosynthesis of phosphonic and phosphinic acid natural products

Natural products containing carbon-phosphorus bonds (phosphonic and phosphinic acids) have found widespread use in medicine and agriculture. Recent years have seen a renewed interest in the biochemistry and biology of these compounds with the cloning of the biosynthetic gene clusters for several family members. This review discusses the commonalities and differences in the molecular logic that lie behind the biosynthesis of these compounds. The current knowledge regarding the metabolic pathways and enzymes involved in the production of a number of natural products, including the approved antibiotic fosfomycin, the widely used herbicide phosphinothricin (PT), and the clinical candidate for treatment of malaria FR-900098, is presented. Many of the enzymes involved in the biosynthesis of these compounds catalyze chemically and biologically unprecedented transformations, and a wealth of new biochemistry has been revealed through their study. These investigations have also suggested new strategies for natural product discovery.

Extended-spectrum beta-lactamase-producing organisms

Extended-spectrum beta-lactamases (ESBLs), which hydrolyse extended-spectrum cephalosporins and are inhibited by clavulanic acid, are spreading among Enterobacteriaceae. The CTX-M enzymes are replacing SHV and TEM enzymes as the prevalent type of ESBLs, principally in community-acquired infections caused by Escherichia coli. Associated infectious syndromes include mainly urinary tract infections, and secondly bloodstream and intra-abdominal infections, and may be serious enough to warrant hospitalisation. Affected patients commonly have various underlying risk factors. This is also observed in hospital-acquired infections. The rates of ESBL-expression among nosocomial Enterobacteriaceae isolates, particularly Klebsiella pneumoniae, have risen substantially in several countries. The hospital epidemiology of these infections is often complex; multiple clonal strains causing focal outbreaks may co-exist with sporadic ones. Relevant infection-control measures should focus on reducing patient-to-patient transmission via the inanimate environment, hospital personnel, and medical equipment. Wise use of antibiotics is also essential. The available therapeutic options for the treatment of ESBL-associated infections are limited by drug resistance conferred by the ESBLs, along with frequently observed co-resistance to various antibiotic classes, including cephamycins, fluoroquinolones, aminoglycosides, tetracyclines, and trimethoprim/sulfamethoxazole. Relevant clinical data regarding the effectiveness of different regimens for ESBL-associated infections are limited. Although certain cephalosporins may appear active in vitro, associated clinical outcomes are often suboptimal. beta-Lactam/beta-lactamase inhibitor combinations may be of value, but the supporting evidence is weak. Carbapenems are regarded as the agents of choice, and may be more effective than fluoroquinolones for serious infections. Tigecycline and polymyxins have substantial antimicrobial activity against ESBL-producing Enterobacteriaceae, and, along with fosfomycin, merit further evaluation.

Identification of a novel UDP-N-acetylglucosamine enolpyruvyl transferase (MurA) from Vibrio fischeri that confers high fosfomycin resistance in Escherichia coli

MurA [UDP-N-acetylglucosamine (UDP-NAG) enolpyruvyl transferase] is a key enzyme involved in bacterial cell wall peptidoglycan synthesis and a target for the antimicrobial agent fosfomycin, a structural analog of the MurA substrate phosphoenol pyruvate. In this study, we identified, cloned and sequenced a novel murA gene from an environmental isolate of Vibrio fischeri that is naturally resistant to fosfomycin. The fosfomycin resistance gene was isolated from a genomic DNA library of V. fischeri. An antimicrobial agent hypersensitive strain of Escherichia coli harboring murA from V. fischeri exhibited a high fosfomycin resistance phenotype, with minimum inhibitory concentration of 3,000 microg/ml. The cloned murA gene was 1,269 bp long encoding a 422 amino acid polypeptide with an estimated pI of 5.0. The deduced amino acid sequence of the putative protein was identified as UDP-NAG enolpyruvyl transferase by homology comparison. The MurA protein with an estimated molecular weight of 44.7 kDa was expressed in E. coli and purified by affinity chromatography. MurA of V. fischeri will be a useful target to identify potential inhibitors of fosfomycin resistance in pharmacological studies.

Temocillin revived

Resistance in Gram-negative pathogens is an increasing concern, with carbapenems often appearing as the only acceptable treatment option in serious infections. Reviving older compounds that have fallen into disuse may help to alleviate this burden. Temocillin (6-alpha-methoxy-ticarcillin) is resistant to most if not all classical and extended-spectrum beta-lactamases and to AmpC enzymes. It is also chemically stable, allowing administration by continuous infusion. Pharmacokinetic/pharmacodynamic analysis, aided by Monte-Carlo simulations, suggests a breakpoint of 8 mg/L for the registered maximum dosage of 4 g daily. Temocillin's weaknesses, explaining its limited previous use, are a lack of activity against Gram-positive organisms, anaerobes and Pseudomonas. In settings where these are unlikely or are covered by other agents, temocillin may be useful, potentially 'sparing' carbapenems and having little apparent potential to select for Clostridium difficile.