Plasmid-determined resistance to fosfomycin in Serratia marcescens

In vitro susceptibility to fosfomycin in clinical and environmental extended-spectrum beta-lactamase producing and/or ciprofloxacin-non-susceptible Escherichia coli isolates

Extended-spectrum beta-lactamase producing and ciprofloxacin-non-susceptible Escherichia coli are clinical and environmental issues. We evaluated the susceptibility profile of fosfomycin in non-susceptible E. coli isolated from urine and the environment. We measured the activity of fosfomycin against 319 and 36 E. coli strains from urine and environmental isolates, respectively, collected from rivers. Fosfomycin resistance profiles were investigated using the minimal inhibitory concentration (MIC), according to the Clinical and Laboratory Standards Institute (CLSI) and the European Committee for Antimicrobial Susceptibility Testing (EUCAST) guidelines. Antibiotic susceptibility testing revealed that 5% and 6.6% of urine samples were non-susceptible to fosfomycin according to CLSI and EUCAST guidelines, respectively. The fosfomycin MIC50/90 was 0.5/4 mg/L. Of the 36 E. coli isolates from river water, 11.1% and 13,8% were non-susceptible to fosfomycin according to CLSI and EUCAST, respectively (range ≤0.25 ≥512 mg/L). All the isolates with MIC ≥512 mg/L for fosfomycin showed the fosA3 gene. Fosfomycin resistance was more frequent in the environment than in clinical samples.

Fosfomycin resistance mechanisms in Enterobacterales: an increasing threat

Antimicrobial resistance is well-known to be a global health and development threat. Due to the decrease of effective antimicrobials, re-evaluation in clinical practice of old antibiotics, as fosfomycin (FOS), have been necessary. FOS is a phosphonic acid derivate that regained interest in clinical practice for the treatment of complicated infection by multi-drug resistant (MDR) bacteria. Globally, FOS resistant Gram-negative pathogens are raising, affecting the public health, and compromising the use of the antibiotic. In particular, the increased prevalence of FOS resistance (FOS^R) profiles among Enterobacterales family is concerning. Decrease in FOS effectiveness can be caused by i) alteration of FOS influx inside bacterial cell or ii) acquiring antimicrobial resistance genes. In this review, we investigate the main components implicated in FOS flow and report specific mutations that affect FOS influx inside bacterial cell and, thus, its effectiveness. FosA enzymes were identified in 1980 from Serratia marcescens but only in recent years the scientific community has started studying their spread. We summarize the global epidemiology of FosA/C2/L1-2 enzymes among Enterobacterales family. To date, 11 different variants of FosA have been reported globally. Among acquired mechanisms, FosA3 is the most spread variant in Enterobacterales, followed by FosA7 and FosA5. Based on recently published studies, we clarify and represent the molecular and genetic composition of fosA/C2 genes enviroment, analyzing the mechanisms by which such genes are slowly transmitting in emerging and high-risk clones, such as E. coli ST69 and ST131, and K. pneumoniae ST11. FOS is indicated as first line option against uncomplicated urinary tract infections and shows remarkable qualities in combination with other antibiotics. A rapid and accurate identification of FOS^R type in Enterobacterales is difficult to achieve due to the lack of commercial phenotypic susceptibility tests and of rapid systems for MIC detection.

Spread and Molecular Characteristics of Enterobacteriaceae Carrying fosA -Like Genes from Farms in China

Here, we report the widespread and complex genetic environments of fosA -like genes in animal-derived strains in China. The fosA7.5 gene was identified in this study and was found to confer resistance to fosfomycin.

High Prevalence and Overexpression of Fosfomycin-Resistant Gene fosX in Enterococcus faecium From China

Enterococci are one of the main causes of gastrointestinal tract infections in the healthcare system and can develop resistance to fosfomycin through plasmid or chromosomally encoded fosfomycin resistance genes. To investigate the mechanisms of fosfomycin resistance, a total of 4,414 clinical isolates of non-replicated clinical enterococci collected from 62 hospitals in 26 provinces or cities in China were tested. Antibiotic susceptibility testing, detection of fosfomycin resistance genes, and cloning of the fosX gene were done. The PFGE, MLST, qRT-PCR, and next genome sequencing were carried out. The results revealed that the fosfomycin-resistant rate of enterococci was 3.5% (153/4,414), and the major resistance mechanism was fosX (101/153) and fosB (52/153) genes. The fosX gene could increase 4- fold fosfomycin MIC in Enterococcus faecium BM4105RF transformants, and the results of PFGE showed the 101 E. faecium carrying fosX were grouped into 48 pulse types. The multilocus sequence typing identified ST555 as the vast majority of STs, mostly distributed in Shanghai, China. Furthermore, the fosX gene expression was strongly related to the fosfomycin-resistant levels of enterococci. The present study was the first to describe the high prevalence presence of the fosX gene in E. faecium from China.

Chromosomally Located fosA7 in Salmonella Isolates From China

This study aimed to investigate the prevalence of fosfomycin fosA7 in Salmonella enterica isolates from food animals and retail meat products in China and the impact of fosA7 on bacterial fitness. A total of 360 Salmonella isolates collected from 11 provinces and cities in China were detected for fosA7. All fosA7-positive Salmonella isolates were determined minimum inhibitory concentrations (MICs) and sequenced by Illumina Hiseq. The fosA7 gene of S. Derby isolate HA2-WA5 was knocked out. The full length of fosA7 was cloned into vector pBR322 and then transformed into various hosts. MICs of fosfomycin, growth curves, stability, and fitness of fosA7 were evaluated. The fosA7 gene was identified in S. Derby (ST40, n = 30) and S. Reading (ST1628, n = 5). MICs to fosfomycin of 35 fosA7-positive isolates were 1 to 32 mg/L. All fosA7 were located on chromosomes of Salmonella. The deletion of fosA7 in HA2-WA5 decreased fosfomycin MIC by 16-fold and slightly affected its fitness. The acquisition of plasmid-borne fosA7 enhanced MICs of fosfomycin in Salmonella (1,024-fold) and Escherichia coli (16-fold). The recombinant plasmid pBR322-fosA7 was stable in Salmonella Typhimurium, S. Pullorum, S. Derby, and E. coli, except for Salmonella Enteritidis, and barely affected on the growth of them but significantly increased biological fitness in Salmonella. The spread of specific Salmonella serovars such as S. Derby ST40 will facilitate the dissemination of fosA7. fosA7 can confer high-level fosfomycin resistance and enhance bacterial fitness in Salmonella if transferred on plasmids; thus, it has the potential to be a reservoir of the mobilized fosfomycin resistance gene.

Mobile fosfomycin resistance genes in Enterobacteriaceae-An increasing threat

Antimicrobial resistance is one of the major threats to the health and welfare of both humans and animals. The shortage of new antimicrobial agents has led to the re-evaluation of old antibiotics such as fosfomycin as a potential regimen for treating multidrug-resistant bacteria especially extended-spectrum-beta-lactamase- and carbapenemase-producing Enterobacteriaceae. Fosfomycin is a broad-spectrum bactericidal antibiotic that inhibits the initial step of the cell wall biosynthesis. Fosfomycin resistance can occur due to mutation in the drug uptake system or by the acquisition of fosfomycin-modifying enzymes. In this review, we focus on mobile fosfomycin-resistant genes encoding glutathione-S-transferase which are mainly responsible for fosfomycin resistance in Enterobacteriaceae, that is, fosA and its subtypes, fosC2, and the recently described fosL1-L2. We summarized the proposed origins of the different resistance determinants and highlighted the different plasmid types which are attributed to the dissemination of fosfomycin-modifying enzymes. Thereby, IncF and IncN plasmids play a predominant role. The detection of mobile fosfomycin-resistant genes in Enterobacteriaceae has increased in recent years. Similar to the situation in (East) Asia, the most frequently detected fosfomycin-resistant gene in Europe is fosA3. Mobile fosfomycin-resistant genes have been detected in isolates of human, animal, food, and environmental origin which leads to a growing concern regarding the risk of spread of such bacteria, especially Escherichia coli and Salmonella, at the human-animal-environment interface.

Coexistence of Fosfomycin Resistance Determinant fosA and fosA3 in Enterobacter cloacae Isolated from Pets with Urinary Tract Infection in Taiwan

To analyze the characteristics of fosA and fosA3 in Enterobacter cloacae isolated from aspirated and catheterized urine culture specimens of companion pets in Taiwan. A total of 19 E. cloacae isolates from pets with urinary tract infection were screened for the presence of fosA, fosA3, and fosC2 and for the genetic context of them by PCR amplification and sequencing. The transferability, resistance phenotypes, plasmid replicon typing properties and genetic environments of fosA- and/or fosA3-positive strains were characterized. Five E. cloacae isolates were positive for fosA and three coharbored fosA and fosA3. No fosC determinant was detected. Transconjugants of fosA3 were successfully acquired, while the acquisition of fosA transconjugants was failed. The minimum inhibitory concentrations (MICs) of the three fosA3-positive isolates and their transconjugants were ≥256 mg/L, whereas the MICs of the five fosA-positive isolates ranged from 64 mg/L to 256 mg/L. Three plasmid replicons (InCFrepB, InCL/M, and InCHI2) were identified in fosA- and fosA3-positive E. cloacae isolates. Different genetic contexts lay in the downstream region of fosA and fosA3, respectively. Eight distinct patterns based on the similarity value of more than 80% were typed for all the 8 fosA-positive isolates. In conclusion, the fosA concomitant with fosA3 were found in E. cloacae isolates. The fosA3 not only exhibits stronger activity of inactivating fosfomycin than fosA but also possesses stronger potential to spread than fosA. Different genetic backgrounds exist in these fosA- and fosA3-positive isolates, and different mobile elements may confer the dissemination of fosA and fosA3.

Update on fosfomycin-modified genes in Enterobacteriaceae

The long-used antibiotic fosfomycin has recently been re-evaluated as a potential regimen for treating extended-spectrum β-lactamases (ESBLs) and carbapenem-resistant Enterobacteriaceae (CRE). Fosfomycin is known for its robust bactericidal effect against ESBL-producing Enterobacteriaceae and CRE. However, fosfomycin-modified genes have been reported in transposon elements and conjugative plasmids, resulting in fosfomycin resistance in parts of East Asia. Here we review reports of fosfomycin-modified (fos) genes in Enterobacteriaceae and assess the efficacy of fosfomycin against multidrug-resistant Enterobacteriaceae infections. At least 10 kinds of fos genes have been identified in the past decade; of these, fosA (and fosA subtypes) and fosC2 are primarily found in Enterobacteriaceae. All fosA subtypes except fosA2 are found in plasmids and transposons, nearby insertion sequence elements, or integrons, indicating that mobilizing elements also play an important role in plasmid-mediated fos genes in Enterobacteriaceae. fosA3, which is prevalent in East Asia, has been transmitted (mostly by animals) within and across continents via IS26 mobile elements. The acquisition of multiple antibiotic resistance genes via plasmids and mobile elements has resulted in a need for combined treatments for Enterobacteriaceae cases. The combination of fosfomycin and carbapenem has been the focus of many in vitro studies, but there is a clear need for additional in vivo investigations involving pharmacokinetics.

Widespread Fosfomycin Resistance in Gram-Negative Bacteria Attributable to the Chromosomal fosA Gene

Fosfomycin is a decades-old antibiotic which is being revisited because of its perceived activity against many extensively drug-resistant Gram-negative pathogens. FosA proteins are Mn²⁺ and K⁺-dependent glutathione S-transferases which confer fosfomycin resistance in Gram-negative bacteria by conjugation of glutathione to the antibiotic. Plasmid-borne fosA variants have been reported in fosfomycin-resistant Escherichia coli strains. However, the prevalence and distribution of fosA in other Gram-negative bacteria are not known. We systematically surveyed the presence of fosA in Gram-negative bacteria in over 18,000 published genomes from 18 Gram-negative species and investigated their contribution to fosfomycin resistance. We show that FosA homologues are present in the majority of genomes in some species (e.g., Klebsiella spp., Enterobacter spp., Serratia marcescens, and Pseudomonas aeruginosa), whereas they are largely absent in others (e.g., E. coli, Acinetobacter baumannii, and Burkholderia cepacia). FosA proteins in different bacterial pathogens are highly divergent, but key amino acid residues in the active site are conserved. Chromosomal fosA genes conferred high-level fosfomycin resistance when expressed in E. coli, and deletion of chromosomal fosA in S. marcescens eliminated fosfomycin resistance. Our results indicate that FosA is encoded by clinically relevant Gram-negative species and contributes to intrinsic fosfomycin resistance.IMPORTANCE There is a critical need to identify alternate approaches to treat infections caused by extensively drug-resistant (XDR) Gram-negative bacteria. Fosfomycin is an old antibiotic which is routinely used for the treatment of urinary tract infections, although there is substantial interest in expanding its use to systemic infections caused by XDR Gram-negative bacteria. In this study, we show that fosA genes, which encode dimeric Mn²⁺- and K⁺-dependent glutathione S-transferase, are widely distributed in the genomes of Gram-negative bacteria-particularly those belonging to the family Enterobacteriaceae-and confer fosfomycin resistance. This finding suggests that chromosomally located fosA genes represent a vast reservoir of fosfomycin resistance determinants that may be transferred to E. coli Furthermore, they suggest that inhibition of FosA activity may provide a viable strategy to potentiate the activity of fosfomycin against XDR Gram-negative bacteria.

First Detection of a Fosfomycin Resistance Gene, fosA7, in Salmonella enterica Serovar Heidelberg Isolated from Broiler Chickens

We previously described Salmonella enterica serovar Heidelberg isolates harboring a chromosomal gene cluster similar to the glutathione S-transferase gene, a putative fosA gene conferring resistance to fosfomycin. Here, we show that this new gene, named fosA7, confers resistance to fosfomycin. The introduction of fosA7 into the fosfomycin-susceptible Salmonella enterica serovar Enteritidis resulted in a substantial increase in the fosfomycin MIC. This finding increases the awareness of antibiotic resistance in Salmonella Heidelberg from broilers as related to the food safety and public health.

Antibacterial activity of fosfomycin against uropathogens

BACKGROUND

In recent years, the rising rates of resistance to antimicrobial drugs among pathogens have caused great difficulty for clinicians treating infectious diseases. The aim of the study was to assess the curative effect of fosfomycin in treating urinary tract infections (UTIs) in China.

METHODS

We collected clinical isolates of UTIs to determine their susceptibility to fosfomycin by the agar dilution method and to analyze extended-spectrum β-lactamase (ESBL)-producing isolates by the double-disc method on Mueller-Hinton agar. Fosfomycin-modifying enzyme analysis was conducted by PCR. Differences between the different groups were determined by the χ(2) test.

RESULTS

We collected 433 UTI isolates, and the result showed that the susceptibility rates of clinical isolates were all above 80%. Only Klebsiella pneumoniae was fosA positive, with a positive rate of 26.7%. No correlation was found for the resistance between the antibiotic drugs tested and fosfomycin in the other bacteria, except for cefepime, levofloxacin and ciprofloxacin in Enterobacter cloacae and imipenem in K. pneumoniae.

CONCLUSION

Our data suggest that fosfomycin may be a suitable antimicrobial agent for UTI isolates and ESBL-producing bacteria in our hospital.

Structure and function of the genomically encoded fosfomycin resistance enzyme, FosB, from Staphylococcus aureus

The Gram-positive pathogen Staphylococcus aureus is a leading cause of global morbidity and mortality. Like many multi-drug-resistant organisms, S. aureus contains antibiotic-modifying enzymes that facilitate resistance to a multitude of antimicrobial compounds. FosB is a Mn²⁺-dependent fosfomycin-inactivating enzyme found in S. aureus that catalyzes nucleophilic addition of either l-cysteine (l-Cys) or bacillithiol (BSH) to the antibiotic, resulting in a modified compound with no bactericidal properties. The three-dimensional X-ray crystal structure of FosB from S. aureus (FosB^Sa) has been determined to a resolution of 1.15 Å. Cocrystallization of FosB^Sa with either l-Cys or BSH results in a disulfide bond between the exogenous thiol and the active site Cys9 of the enzyme. An analysis of the structures suggests that a highly conserved loop region of the FosB enzymes must change conformation to bind fosfomycin. While two crystals of FosB^Sa contain Zn²⁺ in the active site, kinetic analyses of FosB^Sa indicated that the enzyme is inhibited by Zn²⁺ for l-Cys transferase activity and only marginally active for BSH transferase activity. Fosfomycin-treated disk diffusion assays involving S. aureus Newman and the USA300 JE2 methicillin-resistant S. aureus demonstrate a marked increase in the sensitivity of the organism to the antibiotic in either the BSH or FosB null strains, indicating that both are required for survival of the organism in the presence of the antibiotic. This work identifies FosB as a primary fosfomycin-modifying pathway of S. aureus and establishes the enzyme as a potential therapeutic target for increased efficacy of fosfomycin against the pathogen.

Fosfomycin resistance among vancomycin-resistant enterococci owing to transfer of a plasmid harbouring the fosB gene

The presence and characterisation of plasmid-mediated fosfomycin resistance determinants were investigated among 45 clinical vancomycin-resistant enterococci (VRE) isolated in Zhejiang Province, China. In total, 19 VRE were resistant to fosfomycin, of which 18 isolates had conjugative fosfomycin resistance and were positive for fosB. No reported fos genes were detected in the remaining isolate. Among the 18 fosB-carrying isolates, the fosB gene was always flanked by tnpA, suggesting the same novel fosB transposon. In 10 of the 18 fosB-carrying isolates, the fosB and tnpA genes were found reversely inserted in the vanA transposon Tn1546. In the remaining eight isolates the fosB and vanA genes were located on different plasmids. These findings indicate that acquisition of the conjugative plasmid harbouring the novel fosB transposon (ISL3-like transposon) and the Tn1546-like transposon (containing vanA and fosB) may explain, at least in part, the recent increase in fosfomycin-resistant Enterococcus faecium in China.

Prevalence and molecular epidemiology of plasmid-mediated fosfomycin resistance genes among blood and urinary Escherichia coli isolates

A total of 1878 non-duplicate clinical Escherichia coli isolates (comprising 1711 urinary isolates and 167 blood-culture isolates), which were collected from multiple centres in Hong Kong during 1996-2008, were used to investigate the prevalence and molecular epidemiology of plasmid-mediated fosfomycin (fos) resistance genes. Eighteen of the 1878 clinical E. coli isolates were fosfomycin resistant, of which six were fosA3 positive and two were positive for another fosA variant (designated fosKP96). No isolates had the fosC2 gene. The clones of the eight isolates were diverse: sequence type (ST) 95 (n = 2), ST118 (n = 1), ST131 (n = 1), ST617 (n = 1), ST648 (n = 1), ST1488 (n = 1) and ST2847 (n = 1). In the isolates, fosA3 and blaCTX-M genes were co-harboured on conjugative plasmids with F2:A-:B- (n = 2), N (n = 1), F-:A-:B1 and N (n = 1) and untypable (n = 2) replicons. Both fosKP96-carrying plasmids belonged to replicon N. RFLP analysis showed that the two F2:A-:B- plasmids carrying fosA3 and blaCTX-M-3 genes shared the same pattern. Complete sequencing of one of the two F2:A-:B- plasmids, pFOS-HK151325 (69 768 bp) demonstrated it to be >99 % identical to the previously sequenced plasmid pHK23a originating from a pig E. coli isolate in the same region. This study demonstrated the dissemination of fosA3 genes in diverse E. coli clones on multiple blaCTX-M-carrying plasmid types, of which F2:A-:B- plasmids closely related to pHK23a were shared by isolates from human and animal sources.

Mechanistic studies of FosB: a divalent-metal-dependent bacillithiol-S-transferase that mediates fosfomycin resistance in Staphylococcus aureus

FosB is a divalent-metal-dependent thiol-S-transferase implicated in fosfomycin resistance among many pathogenic Gram-positive bacteria. In the present paper, we describe detailed kinetic studies of FosB from Staphylococcus aureus (SaFosB) that confirm that bacillithiol (BSH) is its preferred physiological thiol substrate. SaFosB is the first to be characterized among a new class of enzyme (bacillithiol-S-transferases), which, unlike glutathione transferases, are distributed among many low-G+C Gram-positive bacteria that use BSH instead of glutathione as their major low-molecular-mass thiol. The K(m) values for BSH and fosfomycin are 4.2 and 17.8 mM respectively. Substrate specificity assays revealed that the thiol and amino groups of BSH are essential for activity, whereas malate is important for SaFosB recognition and catalytic efficiency. Metal activity assays indicated that Mn(2+) and Mg(2+) are likely to be the relevant cofactors under physiological conditions. The serine analogue of BSH (BOH) is an effective competitive inhibitor of SaFosB with respect to BSH, but uncompetitive with respect to fosfomycin. Coupled with NMR characterization of the reaction product (BS-fosfomycin), this demonstrates that the SaFosB-catalysed reaction pathway involves a compulsory ordered binding mechanism with fosfomycin binding first followed by BSH which then attacks the more sterically hindered C-1 carbon of the fosfomycin epoxide. Disruption of BSH biosynthesis in S. aureus increases sensitivity to fosfomycin. Together, these results indicate that SaFosB is a divalent-metal-dependent bacillithiol-S-transferase that confers fosfomycin resistance on S. aureus.

Molecular Mechanisms and Clinical Impact of Acquired and Intrinsic Fosfomycin Resistance

Bacterial infections caused by antibiotic-resistant isolates have become a major health problem in recent years, since they are very difficult to treat, leading to an increase in morbidity and mortality. Fosfomycin is a broad-spectrum bactericidal antibiotic that inhibits cell wall biosynthesis in both Gram-negative and Gram-positive bacteria. This antibiotic has a unique mechanism of action and inhibits the initial step in peptidoglycan biosynthesis by blocking the enzyme, MurA. Fosfomycin has been used successfully for the treatment of urinary tract infections for a long time, but the increased emergence of antibiotic resistance has made fosfomycin a suitable candidate for the treatment of infections caused by multidrug-resistant pathogens, especially in combination with other therapeutic partners. The acquisition of fosfomycin resistance could threaten the reintroduction of this antibiotic for the treatment of bacterial infection. Here, we analyse the mechanism of action and molecular mechanisms for the development of fosfomycin resistance, including the modification of the antibiotic target, reduced antibiotic uptake and antibiotic inactivation. In addition, we describe the role of each pathway in clinical isolates.

Genomewide overexpression screen for fosfomycin resistance in Escherichia coli: MurA confers clinical resistance at low fitness cost

To determine whether the overexpression of chromosomal genes can confer fosfomycin resistance, genomewide screening of a complete set of 5,272 plasmid-expressed open reading frames of Escherichia coli (ASKA collection) was performed. Major results are that (i) no clinical level of resistance is achieved by overexpressing chromosomal genes, except murA; (ii) this level is reached at a low fitness cost; and (iii) this cost is much lower than that imposed by other mutations conferring fosfomycin resistance.

Challenges of antibacterial discovery

The discovery of novel small-molecule antibacterial drugs has been stalled for many years. The purpose of this review is to underscore and illustrate those scientific problems unique to the discovery and optimization of novel antibacterial agents that have adversely affected the output of the effort. The major challenges fall into two areas: (i) proper target selection, particularly the necessity of pursuing molecular targets that are not prone to rapid resistance development, and (ii) improvement of chemical libraries to overcome limitations of diversity, especially that which is necessary to overcome barriers to bacterial entry and proclivity to be effluxed, especially in Gram-negative organisms. Failure to address these problems has led to a great deal of misdirected effort.

Prevalence of fosfomycin resistance among CTX-M-producing Escherichia coli clinical isolates in Japan and identification of novel plasmid-mediated fosfomycin-modifying enzymes

We evaluated the in vitro activity of fosfomycin against a total of 192 CTX-M beta-lactamase-producing Escherichia coli strains isolated in 70 Japanese clinical settings. Most of the isolates (96.4%) were found to be susceptible to fosfomycin. On the other hand, some of the resistant isolates were confirmed to harbor the novel transferable fosfomycin resistance determinants named FosA3 and FosC2, which efficaciously inactivate fosfomycin through glutathione S-transferase activity.

Fosfomycin: an old, new friend?

Fosfomycin (FOM) is an antibiotic which has varying application indications across the globe. European, Japanese, South African and Brazilian usage practices are much broader, involving multiple formulations of FOM than the currently limited application of FOM in the United States, where uncomplicated urinary tract infection represents the only indication for FOM-tromethamine. Based on early difficulty in determining FOMs genuine in vitro activity, there was initial skepticism about its efficacy and application range. However, in the mid 1970s, correctly executed experiments coupled with an improved understanding of microbiological concepts opened the door for broader use of FOM. During the following 40 years FOM was evaluated in pre-clinical and clinical trials in a wide range of applications and in a multitude of settings. The gathering of pharmacokinetic and pharmacodynamic data was incorporated into large scale studies in which FOM efficacy was further explored and proven. Among European nations, intravenous FOM-disodium for patients presenting with soft tissue infections, sepsis or deep seated infectious processes has become well accepted over the last two decades. The recent emergence of bacterial strains, which impede and encumber pharmacotherapy, namely, MRSA, ESBL and MSSA, lends itself to the idea of reviving long-standing, sensibly used antimicrobial agents like FOM. This review provides a comprehensive conspectus on FOM's history, mode of action, tissue penetration characteristics, resistance, antibacterial activity, combination partners and clinical uses among other facets of interest.

Glutathione transferases in bacteria

Bacterial glutathione transferases (GSTs) are part of a superfamily of enzymes that play a key role in cellular detoxification. GSTs are widely distributed in prokaryotes and are grouped into several classes. Bacterial GSTs are implicated in a variety of distinct processes such as the biodegradation of xenobiotics, protection against chemical and oxidative stresses and antimicrobial drug resistance. In addition to their role in detoxification, bacterial GSTs are also involved in a variety of distinct metabolic processes such as the biotransformation of dichloromethane, the degradation of lignin and atrazine, and the reductive dechlorination of pentachlorophenol. This review article summarizes the current status of knowledge regarding the functional and structural properties of bacterial GSTs.

Fosfomycin resistance proteins: a nexus of glutathione transferases and epoxide hydrolases in a metalloenzyme superfamily

Three similar but mechanistically distinct fosfomycin resistance proteins that catalyze the opening of the oxirane ring of the antibiotic are known. FosA is a Mn(II) and K(+)-dependent glutathione transferase. FosB is a Mg(2+)-dependent L-cysteine thiol transferase. FosX is a Mn(II)-dependent fosfomycin-specific epoxide hydrolase. The expression, purification, kinetic, and physical characteristics of six fosfomycin resistance proteins including the FosA proteins from transposon TN2921 and Pseudomonas aeruginosa, the FosB proteins from Bacillus subtilis and Staphylococcus aureus, and the FosX proteins from Mesorhizobium loti and Listeria monocytogenes are reported.

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.

Outbreaks and sporadic cases of Salmonella serovar panama studied by DNA fingerprinting and antimicrobial resistance

In the Principality of Asturias (PA), Spain. three Salmonella serovar Panama outbreaks were registered in August 1998. In order to achieve an accurate identification of the strains implicated in the outbreaks and to study the molecular epidemiology of this serovar in the PA, the isolates collected over 1990-1999 were examined by DNA fingerprinting and antimicrobial resistance analysis. The origin of the isolates was: human (65, of which 20 were associated with the three outbreaks), octopus (2), beef (2), eggs (7), poultry faeces (2), sea water (5), sewage (2) and unknown (1). Sixteen lineages were defined by ribotyping, enterobacterial repetitive intergenic consensus sequences analysis, and randomly amplified polymorphic DNA segment analysis. One lineage was endemic in the PA and was also represented by isolates from other Spanish regions. The organisms of this lineage can be differentiated (by resistance-, plasmid- and integron-profiles) into 19 types. The three outbreaks were caused by organisms falling into a single type (nalidixic acid-resistant, plasmid- and integron-free) belonging to the endemic lineage, which was associated with poultry as the reservoir. Isolates showing drug-resistance (71%) fell into six lineages and 23 types. Ten multidrug-resistant types carried class I integrons with three types of variable regions containing resistance gene cassettes.

Molecular characterisation of emergent multiresistant Salmonella enterica serotype [4,5,12:i:-] organisms causing human salmonellosis

Salmonella multidrug-resistant clinical organisms identified as serotype [4,5,12:i:-] were typed using selected genetic procedures and compared with typhimurium organisms collected in the same Spanish region. Results showed a low genetic heterogeneity among [4,5,12:i:-] organisms, which generated identical ribotypes and similar but not identical XbaI PFGE, RAPD, and plasmid profiles. Multidrug resistance could be eliminated by curing and seems to be mediated by 140-kb (spvC+) and 120-kb (spvC-) non-self-transferable plasmids. The [4,5,12:i:-] organisms fall into a single genetic lineage, which emerged in 1997 and presents a different degree of genetic relationship with typhimurium lineages.

Characterization of the fomA and fomB gene products from Streptomyces wedmorensis, which confer fosfomycin resistance on Escherichia coli

Together, the fomA and fomB genes in the fosfomycin biosynthetic gene cluster of Streptomyces wedmorensis confer high-level fosfomycin resistance on Escherichia coli. To elucidate their functions, the fomA and fomB genes were overexpressed in E. coli and the gene products were characterized. The recombinant FomA protein converted fosfomycin to fosfomycin monophosphate, which was inactive on E. coli, in the presence of a magnesium ion and ATP. On the other hand, the recombinant FomB protein did not inactivate fosfomycin. However, a reaction mixture containing FomA and FomB proteins converted fosfomycin to fosfomycin monophosphate and fosfomycin diphosphate in the presence of ATP and a magnesium ion, indicating that FomA and FomB catalyzed phosphorylations of fosfomycin and fosfomycin monophosphate, respectively. These results suggest that the self-resistance mechanism of the fosfomycin-producing organism S. wedmorensis is mono- and diphosphorylation of the phosphonate function of fosfomycin catalyzed by FomA and FomB.

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

We evaluated the susceptibilities of 129 Shiga-like toxin-producing Escherichia coli (STEC) isolates to various antibiotics. The numbers of isolates for which MICs were high (> or = 128 micrograms/ml) were as follows: 5 for fosfomycin, 14 for ampicillin, 1 for cefaclor, 6 for kanamycin, 22 for tetracycline, and 2 for doxycycline. For two isolates of STEC O26 MICs of fosfomycin were high (1,024 and 512 micrograms/ml, respectively). Conjugation experiments and glutathione S-transferase assays suggested that the fosfomycin resistance in these isolates was determined not by a plasmid but chromosomally. The amount of active intracellular fosfomycin in these STEC isolates was 100- to 200-fold less than that in E. coli C600 harboring pREFTT47B408 in the presence of either L-alpha-glycerophosphate or glucose-6-phosphate. Cloning, sequencing, and Northern blot analysis demonstrated that the transcriptional level of the murA gene encoding UDP-N-acetylglucosamine enolpyruvoyl transferase in these isolates was greater than that in E. coli C600. Our results suggest that the fosfomycin resistance in these STEC isolates is due to concurrent effects of alteration of the glpT and/or uhp transport systems and of the enhanced transcription of the murA gene.

Replicon typing of 71 multiresistant Serratia marcescens strains

Replicon typing is the identification of plasmids by hybridization with specific DNA probes which contain the genes involved in plasmid maintenance. This new method has been used to classify plasmids into replicon (rep) groups which can often be correlated with incompatibility (Inc) groups. We studied 71 multiresistant Serratia marcescens strains with 19 rep probes constructed from reference plasmid replicons belonging to known Inc groups. These probes are known to react with enteric bacterial plasmids. However, they did not represent the totality of the thirty known Inc groups. For 52% of the studied strains, plasmids were identified and classified into groups FIB, FIC, FIIA, HI2, L/M, N, B/O, P, W, Y and Com9. Most (79%) of the plasmid preparations hybridized with a single rep probe, and 21% hybridized with two different probes. Electrophoretic analysis of DNA suggested that double hybridization could result from the presence of either two different Inc plasmids in the same strain (e.g. S37) or one single plasmid with a multireplicon (e.g. S113).

The comparative activity of fosfomycin trometamol against organisms isolated from infected urines

Five hundred urinary pathogens, collected from patients of general practitioners and hospital in-patients, were identified and tested for susceptibility to fosfomycin, ampicillin, cephalexin, nalidixic acid, nitrofurantoin, trimethoprim and sulfamethoxazole. Overall, 83% of the isolates were sensitive to fosfomycin, comprising 89% of the out-patient strains and 77% of the in-patient isolates. This degree of sensitivity was similar to that of cephalexin, nalidixic acid and trimethoprim, but higher than that observed with ampicillin, nitrofurantoin and sulfamethoxazole. Fosfomycin generally showed a broad spectrum of activity, but was less active than some other compounds against Klebsiella spp. and streptococci. More than 70% of strains resistant to ampicillin, sulfamethoxazole or trimethoprim were sensitive to fosfomycin indicating that cross resistance is not presently a problem.

Nucleotide sequence and intracellular location of the product of the fosfomycin resistance gene from transposon Tn2921

An 863-base-pair DNA fragment containing the fosfomycin resistance gene from transposon Tn2921 was sequenced. Analysis of the sequence revealed the presence of a single open reading frame that encoded the FOS polypeptide of 16 kilodaltons from Tn2921. Minicell fractionation studies showed that the FOS protein was located in the bacterial cytoplasm.

Epidemiological, clinical and microbiological features of Yersinia enterocolitica infections in a community during a four-year period

Epidemiological and clinical features of 17 cases (13 children) of yersiniosis treated in a hospital over a four-year period, as well as factors associated with virulence, antibiotic resistance and plasmids in Yersinia enterocolitica strains from these patients were studied. The proportion of Y. enterocolitica was 3.57% of all fecal cultures positive for enteric pathogens, with 13 sporadic episodes and one outbreak. In 11 cases the infection was unimicrobial. The clinical presentation was: gastroenteritis (14 cases), pseudoappendicitis (1 case) and asymptomatic (2 cases). Eight cases needed hospital care, and 8 required antimicrobial treatment. None of the isolated bacterial strains produced pyrazinamidase, hemolysins, elastase, fibrinolysine, colicines or aerobactin. Eight strains showed calcium dependence, 7 of them exhibited autoagglutination, serum resistance, crystal violet binding and carried a 42 MDa plasmid related with virulence. Two strains carried a 3.3 MDa R-plasmid that encoded streptomycin-sulfadiazine resistance. The restriction analysis showed that the virulence plasmids constitute one variety and R-plasmids another.

Purification of a glutathione S-transferase that mediates fosfomycin resistance in bacteria

The enzyme that modifies fosfomycin by formation of an adduct with glutathione was purified 12-fold with a 56% activity yield by passage through DEAE Sephacel and high-performance liquid chromatography molecular exclusion columns. Its functional form was a homodimer of two 16,000-dalton polypeptides, which possibly showed an antiparallel alpha tertiary structure and which lacked marked hydrophobic regions. Visualization of the reaction was achieved by precolumn derivatization of glutathione and the adduct, separation by high-performance liquid chromatography, and fluorescence detection of both compounds. Temperature and pH optima were 20 to 30 degrees C and 8.25, respectively; Mn2+, Fe2+, and Co2+ enhanced the rate of modification; and Km values were 9.4 and 11 mM for fosfomycin and glutathione, respectively. Phosphoenolpyruvate did not interfere with fosfomycin modification. The enzyme was stable at 4 degrees C for at least 6 months but progressively lost its activity upon being heated for 60 min at temperatures over 30 degrees C.

Fosfomycin trometamol: activity in vitro against urinary tract pathogens

The spectrum of activity of fosfomycin embraces all the common causes of uncomplicated urinary tract infection. The activity is greatly affected by the conditions of the test. Glucose, phosphates and NaCl all interfere with the activity of the drug, whereas glucose-6-phosphate has a marked potentiating effect against many strains. The activity of fosfomycin is greater at acid than at alkaline pH; inoculum density also has an effect, but this is less marked at acid pH values. Fosfomycin is rapidly bactericidal to susceptible bacteria, causing lysis within 30 min. In contrast, fosmidomycin, which also has a narrower spectrum of activity than fosfomycin, is much more slowly bactericidal. In the form of its trometamol salt, fosfomycin is well absorbed after oral administration, and is excreted in high concentration in the urine. Experiments in an in-vitro model of the treatment of bacterial cystitis suggest that concentrations of fosfomycin achievable in urine after oral administration of high doses of the trometamol salt have a marked suppressive effect on bacterial growth without favouring the emergence of resistant mutants.

Formation of an adduct between fosfomycin and glutathione: a new mechanism of antibiotic resistance in bacteria

Plasmid-borne resistance to fosfomycin in bacteria is due to modification of the antibiotic molecule by a glutathione S-transferase that catalyzes the formation of a covalent bond between the sulfhydryl residue of the cysteine in glutathione and the C-1 of fosfomycin. This reaction results in opening of the epoxide ring of the antibiotic to form an inactive adduct, the structure of which was confirmed by nuclear magnetic resonance. Dialyzed extracts prepared from resistant Escherichia coli strains were unable to modify fosfomycin unless exogenous glutathione was added to the reaction mixtures. Similarly, mutants defective in glutathione biosynthesis were susceptible to fosfomycin, despite harboring a resistance plasmid. Extracts of resistant but not susceptible strains could join glutathione to 1-chloro-2,4-dinitrobenzene, confirming the nature of the enzymatic activity. Adduct formation appeared to be specific for glutathione: none of the other thiols tested (cysteine, N-acetylcysteine, and dithiothreitol) could modify fosfomycin.

Plasmid typing of Shigella sonnei epidemic strains and molecular relationship of their R-plasmids

We conducted a surveillance program on epidemic and/or endemic Shigella strains in Asturias (Spain), their frequency and dispersion in our community, and their R-plasmids. We analyzed initial isolates of Shigella sonnei from two epidemic outbreaks using antibiotic resistance patterns and plasmid profile analysis as epidemiological markers. We found that the 2 outbreaks were caused by different S. sonnei strains, which respectively carried one and two R-plasmids together with other plasmids. The molecular relationship among these and three other R-plasmids from two S. sonnei strains isolated during a previous outbreak, were studied by restriction enzyme analysis and DNA-DNA hybridizations. We were able to establish different levels of relationship among the six R-plasmids.

Cloning and molecular epidemiology of plasmid-determined fosfomycin resistance

The plasmid determinant of resistance to fosfomycin (For) was cloned into pBR322 and located in a 0.7-kilobase segment of DNA by transposon mutagenesis and in vitro deletion analysis. It encodes an 18-kilodalton protein located in the cytoplasm of resistant cells. Its synthesis is constitutive. The For genetic determinant is common to all plasmids isolated since 1975 in an hospital environment as determined by DNA-DNA hybridization. However, plasmids which carry For can be divided into two groups on the basis of size, pattern of antibiotic resistances, incompatibility specificity, and restriction and hybridization properties.

Plasmid-mediated fosfomycin resistance is due to enzymatic modification of the antibiotic

The molecular mechanism of plasmid-mediated resistance to fosfomycin is described. The antibiotic was inactivated intracellularly and remained inside the cells. Modification was also obtained from cell extracts and was not energy dependent. The modifying enzyme seems to have sulfhydryl groups in its active center.

Structural and functional analyses of the fosfomycin resistance transposon Tn2921

The fosfomycin resistance transposon Tn2921 is flanked by directly repeated sequences homologous to the Tn10-related insertion sequence IS10. The nonrepeated DNA sequences of Tn2921 can be deleted without affecting the transposition ability of the element, showing that at least one of the direct repeats is an active insertion sequence. Transposition of Tn2921 seems to occur through direct transposition, since cointegrates have not been observed. The evolutionary relatedness of Tn2921 and IS10 is discussed.