Emergence of fosfomycin-resistant isolates of Shiga-like toxin-producing Escherichia coli O26

Functional analysis of active site residues of the fosfomycin resistance enzyme FosA from Pseudomonas aeruginosa

The metalloglutathione transferase FosA catalyzes the conjugation of glutathione to carbon-1 of the antibiotic fosfomycin, rendering it ineffective as an antibacterial drug. Codon randomization and selection for the ability of resulting clones to confer fosfomycin resistance to Escherichia coli were used to identify residues critical for FosA function. Of the 24 codons chosen for randomization, 16 were found to be essential because only the wild type amino acid was selected. These included ligands to the Mn(2+) and the K(+), residues that furnish hydrogen bonds to fosfomycin, and residues located in a putative glutathione/fosfomycin-binding site. The remaining eight positions randomized were tolerant to substitutions. Site-directed mutagenesis of some of the essential and tolerant amino acids to alanine was performed, and the activity of the purified proteins was determined. Mutation of the residues that are within hydrogen bonding distance to the oxirane or phosphonate oxygens of fosfomycin resulted in variants with very low or no activity. Mutation of Ser(94), which bridges one of the phosphonate oxygens with a potassium ion, resulted in insoluble protein. The Y39A mutation in the putative glutathione-binding site resulted in a 4-fold increase in the apparent K(m) for glutathione. Only two of the amino acids in the substrate-binding site are conserved in the related fosfomycin resistance proteins FosB and FosX, whereas no amino acids in the putative glutathione-binding site are conserved.

Glycerol-3-phosphate transporter of Escherichia coli: structure, function and regulation

Glycerol-3-phosphate (G3P) plays a major role in glycolysis and phospholipid biosynthesis in the cell. Escherichia coli uses a secondary membrane transporter protein, GlpT, to uptake G3P into the cytoplasm. The crystal structure of the protein was recently determined to 3.3 A resolution. The protein consists of an N- and a C-terminal domain, each formed by a compact bundle of six transmembrane alpha-helices. The substrate-translocation pore is found at the domain interface and faces the cytoplasm. At the closed end of the pore is the substrate binding site, which is formed by two arginine residues. In combination with biochemical data, the crystal structure suggests a single binding site, alternating access mechanism for substrate translocation, namely, the substrate bound at the N- and C-terminal domain interface is transported across the membrane via a rocker-switch type of movement of the domains. Furthermore, GlpT may serve as a structural and mechanistic paradigm for other secondary active membrane transporters.

Characterization of Pseudomonas aeruginosa isolates from patients with urinary tract infections during antibiotic therapy

We characterized susceptibilities and genotypes in a series of Pseudomonas aeruginosa isolates from five cases of urinary tract infections (UTIs) to evaluate clonal shifts of carbapenem resistance. In one case, a series of isolates showed different susceptibility patterns for carbapenems but an identical genotype. In another case, genotypes varied among 4 P. aeruginosa isolates from recurrent UTIs over 9 months. Although the patient had been treated with no antibiotic immediately before isolation, the susceptibility patterns for carbapenems and ceftazidime varied. Further analysis in these two cases of outer membrane protein profiles showed that loss of OprD production resulted in reduced susceptibilities to carbapenems in all of the carbapenem-resistant isolates. Loss of OprD production was likely due to oprD gene inactivation in both of cases, since the carbapenem-resistant isolates showed no cross resistance to levofloxacin and chloramphenicol compared with the carbapenem-susceptible isolates. There was another case in which all isolates showed similar susceptibility patterns for carbapenems and ceftazidime, and an identical genotype during the intermittent use of antibiotics over 5 months. In two cases, a single course of antibiotic therapy resulted in eradication of P. aeruginosa. Our results suggest that clonal shifts of carbapenem resistance in P. aeruginosa may result from loss of OprD during antibiotic treatment. Therefore, it is important for clinicians to monitor susceptibilities to antibiotics, especially carbapenems, in P. aeruginosa isolated during therapy.

Detection of mutations in quinolone resistance-determining regions in levofloxacin- and methicillin-resistant Staphylococcus aureus: effects of the mutations on fluoroquinolone MICs

The minimum inhibitory concentrations (MICs) of 18 antibiotics were determined for 66 clinical isolates of staphylococci. Genotypes, mutations in the quinolone resistance-determining regions (QRDRs), and effect of efflux were determined in the 18 levofloxacin-resistant isolates, for which the MICs of levofloxacin were high (> or =8 microg/ml). The increased levofloxacin resistance mainly resulted from some combinations of mutations in the QRDRs, although NorA-mediated efflux may play a minor role in resistance. A combination of mutations in GrlA (Ser80Phe), GrlB (Pro451Ser), and GyrA (Ser84Leu) was found in 4 methicillin-resistant Staphylococcus aureus (MRSA) isolates that were unrelated genotypically. The mutations in grlA QRDR varied in the isolates classified as being in an identical pulsed-field gel electrophoresis (PFGE) group, although the grlB, gyrA, and gyrB QRDRs were the same. These results suggest that the patterns of amino acid mutations in the QRDRs can provide distinct epidemiologic information from PFGE genotypes in fluoroquinolone-resistant MRSA. A combination of at least three mutations in GrlA, GrlB, and/or GyrA is required to increase the MICs of fluoroquinolones, although all of the levofloxacin-resistant MRSA retained the MICs of sitafloxacin in the range of 1 to 2 microg/ml.

Structure and function of the Mur enzymes: development of novel inhibitors

One of the biggest challenges for recent medical research is the continuous development of new antibiotics interacting with bacterial essential mechanisms. The machinery for peptidoglycan biosynthesis is a rich source of crucial targets for antibacterial chemotherapy. The cytoplasmic steps of the biosynthesis of peptidoglycan precursor, catalysed by a series of Mur enzymes, are excellent candidates for drug development. There has been growing interest in these bacterial enzymes over the last decade. Many studies attempted to understand the detailed mechanisms and structural features of the key enzymes MurA to MurF. Only MurA is inhibited by a known antibiotic, fosfomycin. Several attempts made to develop novel inhibitors of this pathway are discussed in this review. Three novel inhibitors of MurA were identified recently. 4-Thiazolidinone compounds were designed as MurB inhibitors. Many phosphinic acid derivatives and substrate analogues were identified as inhibitors of the MurC to MurF amino acid ligases.

Characterization of a holin (HolNU3-1) in methicillin-resistant Staphylococcus aureus host

The gene encoding holin protein HolNU3-1 from a clinical isolate of methicillin-resistant Staphylococcus aureus (MRSA) NU3-1 was cloned and expressed in S. aureus RN4220. HolNU3-1 encoded by the holNU3-1 gene, which is located upstream of the deleted endolysin gene, was functional. Expression of the holNU3-1 gene induced a decrease in culture turbidity and formation of translucent (empty ghost) cells in S. aureus. We found heterogeneity of the holin genes and diversity of the two-component lysis system, which consists of holin and endolysin, in MRSA hosts. We suggest that this diversity is important in the identification of the evolution of clinical isolates of S. aureus.

Antimicrobial resistance of foodborne pathogens

Emergence of bacterial antimicrobial resistance has become a serious problem worldwide. While much of the resistance observed in human medicine is attributed to inappropriate use in humans, there is increasing evidence that antimicrobial use in animals selects for resistant foodborne pathogens that may be transmitted to humans as food contaminants.

The bacterial cell wall as a source of antibacterial targets

This review focuses on target-based approaches for developing new chemical classes of antibacterial agents aimed at the bacterial cell wall. The clinical success of antibiotics such as beta-lactams and glycopeptides validates this chemotherapeutic strategy and emerging resistance to these agents warrants the development of new antibacterial drugs. Understanding the mechanism of action and resistance to beta-lactams and glycopeptides at a molecular level has supported the development of new agents that prevent transpeptidation and transglycosylation reactions of peptidoglycan polymerisation. The enzymes involved in the synthesis of the peptidoglycan structural unit have also been targeted for antibacterial discovery. The influence of bacterial genetics and genomics, structural biology, assay development and the properties of known inhibitors on these approaches will be discussed in the context of drug discovery.

Identification and characterization of new inhibitors of the Escherichia coli MurA enzyme

The bacterial enzyme MurA catalyzes the transfer of enolpyruvate from phosphoenolpyruvate (PEP) to uridine diphospho-N-acetylglucosamine (UNAG), which is the first committed step of bacterial cell wall biosynthesis. From high-throughput screening of a chemical library, three novel inhibitors of the Escherichia coli MurA enzyme were identified: the cyclic disulfide RWJ-3981, the purine analog RWJ-140998, and the pyrazolopyrimidine RWJ-110192. When MurA was preincubated with inhibitor, followed by addition of UNAG and PEP, the 50% inhibitory concentrations (IC(50)s) were 0.2 to 0.9 microM, compared to 8.8 microM for the known MurA inhibitor, fosfomycin. The three compounds exhibited MICs of 4 to 32 microg/ml against Staphylococcus aureus; however, the inhibition of DNA, RNA, and protein synthesis in addition to peptidoglycan synthesis by all three inhibitors indicated that antibacterial activity was not due specifically to MurA inhibition. The presence of UNAG during the MurA and inhibitor preincubation lowered the IC(50) at least fivefold, suggesting that, like fosfomycin, the three compounds may interact with the enzyme in a specific fashion that is enhanced by UNAG. Ultrafiltration and mass spectrometry experiments suggested that the compounds were tightly, but not covalently, associated with MurA. Molecular modeling studies demonstrated that the compounds could fit into the site occupied by fosfomycin; exposure of MurA to each compound reduced the labeling of MurA by tritiated fosfomycin. Taken together, the evidence indicates that these inhibitors may bind noncovalently to the MurA enzyme, at or near the site where fosfomycin binds.

FosB, a cysteine-dependent fosfomycin resistance protein under the control of sigma(W), an extracytoplasmic-function sigma factor in Bacillus subtilis

We demonstrate that the Bacillus subtilis fosB(yndN) gene encodes a fosfomycin resistance protein. Expression of fosB requires sigma(W), and both fosB and sigW mutants are fosfomycin sensitive. FosB is a metallothiol transferase related to the FosA class of Mn(2+)-dependent glutathione transferases but with a preference for Mg(2+) and L-cysteine as cofactors.

The staphylokinase gene is located in the structural gene encoding N-acetylmuramyl-L-alanine amidase in methicillin-resistant Staphylococcus aureus

The nucleotide sequence of a 15600-bp DNA fragment containing the staphylokinase gene (sakNU3-1) of methicillin-resistant Staphylococcus aureus (MRSA) NU3-1 was determined. The sak gene was found within the ply gene encoding N-acetylmuramyl-L-alanine amidase and thus the ply gene should be inactivated. In the flanking region of the sak gene, the tandem repeat sequences (GAAGTGTT and GAATGGTT) were present as possible junction points between the sak and ply genes. No sequences characteristic of the presence of an IS-like element were found. Upstream from the ply gene, the kdpA, kdpB and kdpC homologues were present. Downstream from the ply gene, the tagA, tagH and tagG homologues were present. The sak gene was inserted into the same position of ply in 5/6 of sak(+) MRSA isolates with different genotypes. In all of these sak(+) isolates, Sak was detected in the culture supernatant.

Medium compositions and culture conditions for the assay of fosfomycin susceptibility by Etest

The Etest (AB Biodisk, Solna, Sweden), a susceptibility testing method, was used for fosfomycin and was evaluated by comparison with the agar dilution method for 73 clinical isolates of Escherichia coli, including two resistant strains, and an optimal Etest method for fosfomycin was determined. Media and culture conditions greatly affected the minimum inhibitory concentrations (MICs) determined by the Etest for fosfomycin, as shown for the agar dilution methods. Our results showed that the most favorable conditions for the Etest for fosfomycin were with Mueller-Hinton agar (Becton-Dickinson Japan, Tokyo, Japan) under aerobic conditions. However, the MICs for the resistant strains were much higher than those determined by agar dilution methods, using Nutrient agar (Becton-Dickinson Japan) under anaerobic conditions. The addition of glucose-6-phosphate did not significantly affect the Etest results.