The role of fosfomycin for multidrug-resistant gram-negative infections

Pharmacokinetic/pharmacodynamic analysis of meropenem and fosfomycin combinations in in vitro time-kill and hollow-fibre infection models against multidrug-resistant and carbapenemase-producing Klebsiella pneumoniae

BACKGROUND

MDR Gram-negative bacteria, such as ESBL-producing and carbapenemase-producing Klebsiella pneumoniae, represent major global health threats. Treatment options are limited due to increasing resistance and slowed development of novel antimicrobials, making it necessary to apply effective combination therapies based on approved antibiotics.

OBJECTIVES

To quantitatively evaluate the synergistic potential of meropenem and fosfomycin against carbapenem-resistant K. pneumoniae strains isolated from clinics.

METHODS

We evaluated four MDR K. pneumoniae strains, each expressing KPC-2 or KPC-3, using static time-kill assays that accounted for measured meropenem degradation. This was followed by pharmacokinetic/pharmacodynamic (PK/PD) interaction modelling, which estimated meropenem degradation rate constants and identified perpetrator-victim relationships in PD interactions. Dynamic hollow-fibre infection model (HFIM) experiments were used to confirm synergy.

RESULTS

Static time-kill assays demonstrated high killing effects and suppressed regrowth for the combination of meropenem and fosfomycin, compared with the failure of monotherapy. Meropenem degradation was significantly higher in the presence of bacteria, attributable to carbapenemase activity. Pharmacometric models indicated a synergistic interaction primarily driven by meropenem as the perpetrator, enhancing the potency of fosfomycin. HFIM experiments confirmed in vitro synergy, demonstrating continuous bacterial suppression of the combination therapy.

CONCLUSIONS

Meropenem and fosfomycin exhibited additive or synergistic potential against carbapenemase-expressing single- or double-resistant K. pneumoniae at clinically achievable concentrations. This combination therapy may offer a strategy against MDR infections, possibly improving clinical treatment outcomes. Further in vivo research is needed to translate these findings into clinical practice, emphasizing the importance of PK/PD modelling in rationalizing antibiotic use.

Evaluation of pharmacokinetic pharmacodynamic target attainment of meropenem in pediatric patients

BACKGROUND

Meropenem is a widely used carbapenem for treating severe pediatric infections. However, few studies have assessed its pharmacokinetics/pharmacodynamics (PK/PD) in pediatric patients. This study aimed to evaluate the proportion of Saudi pediatric patients achieving the PK/PD target of meropenem.

METHODS

A prospective observational study was conducted at King Saud University Medical City from July to September 2022. Pediatric patients receiving meropenem for suspected or proven infections were included in the study. The primary outcome was the percentage of patients achieving the recommended PK/PD target for critically ill or non-critically ill pediatric patients.

RESULTS

The study included 30 patients (nine neonates and 21 older pediatric patients). All neonates were critically ill. Among them, 55 % achieved the PK/PD target of 100 % free time above the MIC. In older ICU pediatric patients, only 11 % attained this target, whereas 58 % of older pediatrics in the general wards achieved the PK/PD target of 50 % free time above the MIC. Augmented renal clearance (ARC) was identified in 57 % of our pediatric patient population, none of whom achieved the recommended PK/PD targets. The median trough concentrations in patients with and without ARC were 0.75 and 1.3 μg/mL, respectively (P < 0.05).

CONCLUSIONS

The majority of patients in our cohort did not achieve the PK/PD target for meropenem. ARC emerged as a major risk factor for target attainment failure in both critically ill and non-critically ill pediatric patients.

fosA11, a novel chromosomal-encoded fosfomycin resistance gene identified in Providencia rettgeri

This study investigated resistance genes corresponding to the fosfomycin resistance phenotype in clinical isolate Providencia rettgeri W986, as well as characterizing the enzymatic activity of FosA11 and the genetic environment. Antimicrobial susceptibility testing was performed using the agar microdilution method based on the Clinical and Laboratory Standards Institute guidelines. The whole genomic sequence of Providencia rettgeri W986 was obtained using Illumina sequencing and the PacBio platform. The fosA-11 gene was amplified by PCR and cloned into the pUCP20 vector. The recombinant strain pCold1-fosA11-BL21 was expressed to extract the target protein, and absorbance photometry was applied for enzymatic parameter determination. Minimal inhibitory concentration (MIC) tests showed that W986 conferred fosfomycin resistance and was inhibited by phosphonoformate, thereby indicating the presence of a FosA protein. A novel resistance gene designated as fosA11 was identified by whole-genome sequencing and bioinformatics analysis, and it shared 54.41%-64.23% amino acid identity with known FosA proteins. Cloning fosA11 into Escherichia coli obtained a significant increase (32-fold) in the MIC with fosfomycin. Determination of the enzyme kinetics showed that FosA11 had a high catalytic effect on fosfomycin, with Km = 18 ± 4 and Kcat = 56.1 ± 3.2. We also found that fosA11 was located on the chromosome, but the difference in the GC content between the chromosome and fosA11 was dubious, and thus further investigation is required. In this study, we identified and characterized a novel fosfomycin inactivation enzyme called FosA11. The origin and prevalence of the fosA11 gene in other bacteria require further investigation.IMPORTANCEFosfomycin is an effective antimicrobial agent against Enterobacterales strains. However, the resistance rate of fosfomycin is increasing year by year. Therefore, it is necessary to study the deep molecular mechanism of bacterial resistance to fosfomycin. We identified a novel chromosomal fosfomycin glutathione S-transferase, FosA11 from Providencia rettgeri, which shares a very low identity (54.41%-64.23%) with the previously known FosA and exhibits highly efficient catalytic ability against fosfomycin. Analysis of the genetic context and origin of fosA11 displays that the gene and its surrounding environments are widely conserved in Providencia and no mobile elements are discovered, implying that FosA11 may be broadly important in the natural resistance to fosfomycin of Providencia species.

Evaluation of in vitro pharmacodynamic drug interactions of ceftazidime/avibactam and fosfomycin in Escherichia coli

BACKGROUND

Combination therapy can increase efficacy of antibiotics and prevent emergence of resistance. Ceftazidime/avibactam and fosfomycin may be empirically combined for this purpose, but a systematic and quantitative evaluation of this combination is needed.

OBJECTIVES

In this study, a systematic analysis of the pharmacodynamic interactions of ceftazidime/avibactam and fosfomycin in clinical and isogenic Escherichia coli strains carrying genes coding for several carbapenemases or ESBLs was performed and pharmacodynamic interactions were quantified by modelling and simulations.

METHODS

Pharmacodynamic interactions were evaluated in 'dynamic' chequerboard experiments with quantification of viable bacteria in eight isogenic and six clinical E. coli strains. Additionally, supplemental time-kill experiments were performed and genomic analyses were conducted on representative fosfomycin-resistant subpopulations. Models were fitted to all data using R and NONMEM®.

RESULTS

Synergistic drug interactions were identified for 67% of the clinical and 75% of the isogenic isolates with a mean EC50 reduction of >50%. Time-kill experiments confirmed the interactions and modelling quantified EC50 reductions up to 97% in combination and synergy prevented regrowth of bacteria by enhanced killing effects. In 9 out of 12 fosfomycin-resistant mutants, genomic analyses identified previously reported mutations.

CONCLUSIONS

The broad synergistic in vitro activity of ceftazidime/avibactam and fosfomycin confirms the potential of the application of this drug combination in clinics. The substantial reduction of the EC50 in combination may allow use of lower doses or treatment of organisms with higher MIC values and encourage further research translating these findings into the clinical setting.

Efficacy and safety of intravenous fosfomycin for the treatment of carbapenem-resistant Klebsiella pneumoniae

The aim of the study was to evaluate clinical and microbiological efficacy and safety of intravenous fosfomycin for the treatment of carbapenem-resistant K. pneumoniae infections. All adult inpatients receiving 48 h of intravenous fosfomycin, alone or combined with other antibiotics were included in the study. Overall favorable clinical response rate was 75.3% among 94 patients. Clinical response rates were 92.3%, 72.2% and 56.0% for urinary tract infections, bacteremia and pneumonia, respectively. Microbiological eradication was achieved in 55 of 86 patients. 30-day mortality was 33.0%. Adverse events were generally mild. Common adverse events were hypokalemia (37.2%) and hypernatremia (22.3%). Intravenous fosfomycin is an effective antibiotic option with a good safety profile for the treatment of carbapenem-resistant K. pneumoniae infections. The most favorable clinical and microbiological responses are obtained in urinary tract infections. The efficacy of the drug in more severe infections, such as pneumonia and bacteremia, is comparable to the literature.

Performance Evaluation of BD Phoenix and MicroScan WalkAway Plus for Determination of Fosfomycin Susceptibility in Enterobacterales

BACKGROUND

Fosfomycin is an old bactericidal drug that has gained increasing interest in the last decade for its potential use in multi-drug resistant gram-negative infections. However, evidence on fosfomycin susceptibility testing reports a poor correlation between commercial methods vs. reference agar dilution (AD) for Enterobacterales (EB). The study aimed at assessing the performance of two automated systems for the determination of fosfomycin susceptibility in EB clinical isolates.

METHODS

Fosfomycin susceptibility testing results of two collections of 100 non-duplicate clinical EB strains obtained using two different platforms (BD Phoenix and MicroScan WalkAway Plus) were compared with those obtained by AD. Categorical agreement (CA), major error (ME) and very major error (VME) rates were calculated.

RESULTS

BD Phoenix exhibited a 6.9% rate of false-resistant results and achieved a CA of 69%, whereas MicroScan WalkAway Plus achieved 3.7% of false-resistant results and 72% of CA. Both automated systems showed poor detection of resistant isolates, with 49.1% and 56.2% of false-susceptible results for BD Phoenix and Microscan WalkAway Plus, respectively.

CONCLUSIONS

Overall, agar dilution remains the most suitable method for routine laboratory antimicrobial susceptibility testing of fosfomycin on Enterobacterales strains, given the poor performance of automated systems. The application of both automated systems, in the clinical laboratories reporting of fosfomycin, should be reviewed in light of the accuracy results falling below the acceptable threshold.

The Use of Intravenous Fosfomycin in Clinical Practice: A 5-Year Retrospective Study in a Tertiary Hospital in Italy

Fosfomycin in intravenous (IV) formulation has re-emerged as a valuable tool in the treatment of multi-drug resistant (MDR) and extensively drug-resistant (XDR) infections because of its broad spectrum of antibacterial action and pharmacokinetic characteristics. This retrospective study aimed to evaluate how fosfomycin was used in patients admitted to the Polyclinic of Palermo between January 2017 and July 2022. Clinical indications, therapeutic associations, clinical outcomes, and any side effects were analyzed. Intravenous fosfomycin was used in 343 patients, 63% male, with a mean age of 68 years (range 15-95). Urinary tract infections (UTIs) and hospital-acquired pneumonia (HAP) were the main indications for treatment (19% and 18% of the total cases, respectively), followed by skin and soft tissue infections and sepsis. IV fosfomycin was administered in combination with other antibacterial agents, the most common of which were ceftazidime/avibactam (35%), meropenem (17%), and colistin (14%). Nineteen patients received it as monotherapy for UTIs. About 66% had resolution of the infectious process with clinical remission (cure or discharge). Electrolyte disturbances occurred in 2.6% and gastrointestinal symptoms occurred in 2.9%. The data showed that IV fosfomycin is a safe and effective therapeutic option in the treatment of infections with multidrug-resistant microorganisms.

Fosfomycin Resistance in Bacteria Isolated from Companion Animals (Dogs and Cats)

Fosfomycin is an old antibacterial agent, which is currently used mainly in human medicine, in uncomplicated Urinary Tract Infections (UTIs). The purpose of this review is to investigate the presence and the characteristics of Fosfomycin resistance in bacteria isolated from canine or feline samples, estimate the possible causes of the dissemination of associated strains in pets, and underline the requirements of prospective relevant studies. Preferred Reporting Items for Systematic Reviews (PRISMA) guidelines were used for the search of current literature in two databases. A total of 33 articles were finally included in the review. Relevant data were tracked down, assembled, and compared. Referring to the geographical distribution, Northeast Asia was the main area of origin of the studies. E. coli was the predominant species detected, followed by other Enterobacteriaceae, Staphylococci, and Pseudomonas spp. FosA and fosA3 were the more frequently encountered Antimicrobial Resistance Genes (ARGs) in the related Gram-negative isolates, while fosB was regularly encountered in Gram-positive ones. The majority of the strains were multidrug-resistant (MDR) and co-carried resistance genes against several classes of antibiotics and especially β-Lactams, such as bla_CTX-M and mecA. These results demonstrate the fact that the cause of the spreading of Fosfomycin-resistant bacteria among pets could be the extended use of other antibacterial agents, that promote the prevalence of MDR, epidemic strains among an animal population. Through the circulation of these strains into a community, a public health issue could arise. Further research is essential though, for the comprehensive consideration of the issue, as the current data are limited.

Extended-spectrum β-lactamase-producing and carbapenem-resistant Enterobacterales bloodstream infection after solid organ transplantation: Recent trends in epidemiology and therapeutic approaches

Background

Infections caused by multidrug‐resistant gram‐negative bacilli (MDR GNB), in particular extended‐spectrum β‐lactamase‐producing (ESBL‐E) and carbapenem‐resistant Enterobacterales (CRE), pose a major threat in solid organ transplantation (SOT). Outcome prediction and therapy are challenging due to the scarcity of randomized clinical trials (RCTs) or well‐designed observational studies focused on this population.

Methods

Narrative review with a focus on the contributions provided by the ongoing multinational INCREMENT‐SOT consortium (ClinicalTrials identifier NCT02852902) in the fields of epidemiology and clinical management.

Results

The Spanish Society of Transplantation (SET), the Group for Study of Infection in Transplantation of the Spanish Society of Infectious Diseases and Clinical Microbiology (GESITRA‐SEIMC), and the Spanish Network for Research in Infectious Diseases (REIPI) recently published their recommendations for the management of MDR GNB infections in SOT recipients. We revisit the SET/GESITRA‐SEIMC/REIPI document taking into consideration new evidence that emerged on the molecular epidemiology, prognostic stratification, and treatment of post‐transplant ESBL‐E and CRE infections. Results derived from the INCREMENT‐SOT consortium may support the therapeutic approach to post‐transplant bloodstream infection (BSI). The initiatives devoted to sparing the use of carbapenems in low‐risk ESBL‐E BSI or to repurposing existing non‐β‐lactam antibiotics for CRE in both non‐transplant and transplant patients are reviewed, as well as the eventual positioning in the specific SOT setting of recently approved antibiotics.

Conclusion

Due to the clinical complexity and relative rarity of ESBL‐E and CRE infections in SOT recipients, multinational cooperative efforts such as the INCREMENT‐SOT Project should be encouraged. In addition, RCTs focused on post‐transplant serious infection remain urgently needed.

Permeation of Fosfomycin through the Phosphate-Specific Channels OprP and OprO of Pseudomonas aeruginosa

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen responsible for many nosocomial infections. It is quite resistant to various antibiotics, caused by the absence of general diffusion pores in the outer membrane. Instead, it contains many substrate-specific channels. Among them are the two phosphate- and pyrophosphate-specific porins OprP and OprO. Phosphonic acid antibiotics such as fosfomycin and fosmidomycin seem to be good candidates for using these channels to enter P. aeruginosa bacteria. Here, we investigated the permeation of fosfomycin through OprP and OprO using electrophysiology and molecular dynamics (MD) simulations. The results were compared to those of the fosmidomycin translocation, for which additional MD simulations were performed. In the electrophysiological approach, we noticed a higher binding affinity of fosfomycin than of fosmidomycin to OprP and OprO. In MD simulations, the ladder of arginine residues and the cluster of lysine residues play an important role in the permeation of fosfomycin through the OprP and OprO channels. Molecular details on the permeation of fosfomycin through OprP and OprO channels were derived from MD simulations and compared to those of fosmidomycin translocation. In summary, this study demonstrates that the selectivity of membrane channels can be employed to improve the permeation of antibiotics into Gram-negative bacteria and especially into resistant P. aeruginosa strains.

The Genomic Characterization of KPC-Producing Klebsiella pneumoniae from the ICU of a Teaching Hospital in Shanghai, China

Purpose

This study retrospectively analyzed the genome characteristics of blaKPC-2 in multidrug-resistant Klebsiella pneumoniae collected from the ICU of a teaching hospital in Shanghai, China.

Methods

From February 2018 to December 2019, 36 strains of multidrug-resistant Klebsiella pneumoniae were collected from the bronchoalveolar lavage fluid of critically ill patients. The genome of all isolates was obtained through the Illumina sequence, and single nucleotide polymorphisms of the blaKPC-2 gene were analyzed to explore blaKPC-2’s evolutionary characteristics. Different strains’ genetic relationships and homology were studied by constructing an evolutionary tree on a single copy orthologue. Pacbio combined Illumina sequence was conducted to evaluate the structure and potential mobility of drug-resistant plasmids of the strain KP-s26.

Results

The distribution of resistance and virulence genes had little difference, but most strains had significant differences in the plasmid-encoded region. Most strains (31/36) carried the carbapenemase gene blaKPC-2, with no single nucleotide polymorphism in different strains. Extended-spectrum β-lactamase resistance genes, such as blaCTX-M and blaSHV, were found in the isolates, but no metallo-β-lactamases were detected. All strains with blaKPC-2 coexisted with chromosomal-associated fosfomycin resistance genes fosA6, and the coexistence of blaKPC-2 and blaCTX variants (blaCTX-M-15, blaCTX-M-65, and blaCTX-M-27) was also detected in 29/31 strains. The isolate KP-s26 carried five circular plasmids. pA and pB were conjugate plasmids, as they carried drug resistance genes and contained a complete IV secretion system.

Conclusion

The blaKPC-2 carbapenemase gene is relatively conservative in the process of evolution; drug-resistant plasmids containing conjugated transfer elements contribute to the spreading of drug resistance. The coexistence of blaKPC-2 with fosA6 or blaCTX-M variants was associated with increased fosfomycin resistance and broad-spectrum β-lactam resistance, respectively.

Clinical Trials Registration

Clinical Trials.gov Identifier: NCT03950544

Continuous versus intermittent infusion of antibiotics in Gram-negative multidrug-resistant infections

PURPOSE OF REVIEW

The aim of this review was to perform a critical reappraisal of the real-world evidence supporting administration by prolonged infusion of novel beta-lactams for the management of multidrug-resistant Gram-negative infections.

RECENT FINDINGS

Real-world evidence support the use of novel beta-lactams by prolonged infusion over intermittent infusion in terms of achieving aggressive pharmacokinetic/pharmacodynamic (PK/PD) target for either maximizing efficacy and clinical outcome or suppressing the emergence of resistance development. Continuous infusion of ceftolozane-tazobactam showed a marked superiority toward both intermittent and extended infusion (EI) in achieving a PK/PD target of 100%fT> 4 X MIC in infections caused by less-susceptible Pseudomonas aeruginosa isolates. No resistance development was found in critically ill or immunocompromised patients treated with EI ceftolozane-tazobactam compared to intermittent infusion. Prolonged infusion of ceftazidime-avibactam was negatively associated with mortality in patients affected by Klebsiella pneumoniae carbapenemase-producing K. pneumoniae infections. Different challenging scenarios (patients showing augmented renal clearance of affected by deep-seated infections) could benefit from prolonged infusion to optimize the efficacy of novel agents.

SUMMARY

Although available data are still limited, real-world evidence regarding mainly ceftolozane-tazobactam and ceftazidime-avibactam could support the administration of novel beta-lactams by prolonged infusion in some specific scenarios in which achievement of aggressive PK/PD target is quite challenging.

Epidemiology and Pattern of Resistance of Gram-Negative Bacteria Isolated from Blood Samples in Hospitalized Patients: A Single Center Retrospective Analysis from Southern Italy

BACKGROUND

Blood culturing remains the mainstream tool to inform an appropriate treatment in hospital-acquired bloodstream infections and to diagnose any bacteremia.

METHODS

A retrospective investigation on the prevalence of Gram-negative bacteria (GNB) and their resistance in hospitalized patients by age, sex, and units from blood cultures (BCs) was conducted from January 2018 to April 2020 at Sant'Elia hospital, Caltanissetta, southern Italy. We divided the patient age range into four equal intervals.

RESULTS

Multivariate demographic and microbiological variables did not show an association between bacteria distributions and gender and age. The distribution by units showed a higher prevalence of Klebsiella pneumoniae and Acinetobacter baumannii in the intensive care unit (ICU) and Escherichia coli in the non-intensive care units (non-ICUs). The analysis of antibiotic resistance showed that E. coli was susceptible to a large class of antibiotics such as carbapenem and trimethoprim-sulfamethoxazole. K. pneumoniae showed a significant susceptibility to colistin, tigecycline, and trimethoprim-sulfamethoxazole. From the survival analysis, patients with E. coli had a higher survival rate.

CONCLUSIONS

The authors stress the importance of the implementation of large community-level programs to prevent E. coli bacteremia. K. pneumoniae and E. coli susceptibility patterns to antibiotics, including in the prescription patterns of general practitioners, suggest that the local surveillance and implementation of educational programs remain essential measures to slow down the spread of resistance and, consequently, increase the antibiotic lifespan.

Fosfomycin therapeutic drug monitoring in real-life: development and validation of a LC-MS/MS method on plasma samples

Individualization of fosfomycin dosing based on therapeutic drug monitoring (TDM) of plasma concentrations could reduce drug-related adverse events and improve clinical outcome in complex clinical conditions. Quantification of fosfomycin in plasma samples was performed by a rapid ultraperformance liquid chromatography mass spectrometry method. Sample preparation involved protein precipitation with [¹³C₃]-fosfomycin benzylamine salt as internal standard. The calibration curve ranged from 2 to 800 mg/L. Within- and between-day precision and accuracy, sensitivity, selectivity, dilution integrity, recovery were investigated and the results met the acceptance criteria. In patients, multiple drug dosing (every 6 or 8 hours) or in continuous administration were adopted, resulting in a large interpatient variability in drug concentrations (from 7.4 mg/L and 644.6 mg/L; CV: 91.1%). In critical care patient setting TDM can represent an important tool to identify the best fosfomycin dosing in single patients, taking into consideration clinical characteristics, infection sites and susceptibility of the treated pathogens.

Intravenous fosfomycin as salvage therapy for osteomyelitis caused by multidrug-resistant Pseudomonas aeruginosa

PURPOSE

A case of osteomyelitis caused by multidrug-resistant (MDR) Pseudomonas aeruginosa is reported.

SUMMARY

An 84-year-old Caucasian male with an underlying history of type 2 diabetes, peripheral vascular disease, and coronary artery disease had chronic nonhealing wounds on his right foot. Wound care and a course of intravenous (IV) ertapenem with oral ciprofloxacin were ineffective. His initial wound culture grew Staphylococcus aureus, group G streptococcus and P. aeruginosa; the Pseudomonas was susceptible to multiple agents. The patient eventually required midtarsal amputation and angioplasties to his right leg. Twenty days after the operation, 2 openings were discovered at the surgical site, 1 of which was probed to the bone. He was readmitted 5 weeks after the operation. A repeat wound swab grew MDR P. aeruginosa and Finegoldia magna. The Pseudomonas was susceptible to gentamicin and colistin. The patient had revision of the infected amputation site with the goal of salvaging his right lower limb. The patient developed acute renal failure after 26 days of IV gentamicin, IV ceftriaxone, and oral metronidazole. Additional susceptibility testing was performed to identify alternatives. The bacteria were considered susceptible to IV fosfomycin, the last resort, by our microbiology laboratory. This was combined with ceftolozane/tazobactam followed by meropenem to treat the residual infection. After 2 weeks of IV fosfomycin, the patient's wound improved and further amputation was avoided.

CONCLUSION

Our case demonstrates that IV fosfomycin may provide an effective salvage therapy when combined with β-lactams for the treatment of severe diabetic foot infection or osteomyelitis caused by MDR P. aeruginosa.

Clonal spread of Escherichia coli O101: H9-ST10 and O101: H9-ST167 strains carrying fosA3 and bla CTX-M-14 among diarrheal calves in a Chinese farm, with Australian Chroicocephalus as the possible origin of E. coli O101: H9-ST10

During a 2018 antimicrobial resistance surveillance of Escherichia coli isolates from diarrheal calves in Xinjiang Province, China, an unexpectedly high prevalence (48.5%) of fosfomycin resistance was observed. This study aimed to reveal the determinants of fosfomycin resistance and the underlying transmission mechanism. Polymerase chain reaction (PCR) screening showed that all fosfomycin-resistant E. coli carried the fosA3 gene. Pulsed-field gel electrophoresis (PFGE) and southern blot hybridization revealed that the 16 fosA3-positive isolates belonged to four different PFGE patterns (i.e., A, B, C, D). The fosA3 genes of 11 clonally related strains (pattern D) were located on the chromosome, while others were carried by plasmids. Whole-genome and long-read sequencing indicated that the pattern D strains were E. coli O101:H9-ST10, and the pattern C, B, and A strains were O101:H9-ST167, O8:H30-ST1431, and O101:H9 with unknown ST, respectively. Among the pattern C strains, the bla_CTX-M-14 gene was co-localized with the fosA3 gene on the F18:A-:B1 plasmids. Interestingly, phylogenetic analysis based on core genome single nucleotide polymorphisms (cgSNPs) showed that the O101:H9-ST10 strains were closely related to a Australian-isolated Chroicocephalus-origin E. coli O101:H9-ST10 strain producing CTX-M-14 and FosA3, with a difference of only 11 SNPs. These results indicate possible international dissemination of the high-risk E. coli clone O101:H9-ST10 by migratory birds.

Biosynthesis of fosfomycin in pseudomonads reveals an unexpected enzymatic activity in the metallohydrolase superfamily

The epoxide-containing phosphonate natural product fosfomycin is a broad-spectrum antibiotic used in the treatment of cystitis. Fosfomycin is produced by both the plant pathogen Pseudomonas syringae and soil-dwelling streptomycetes. While the streptomycete pathway has recently been fully elucidated, the pseudomonad pathway is still mostly elusive. Through a systematic evaluation of heterologous expression of putative biosynthetic enzymes, we identified the central enzyme responsible for completing the biosynthetic pathway in pseudomonads. The missing transformation involves the oxidative decarboxylation of the intermediate 2-phosphonomethylmalate to a new intermediate, 3-oxo-4-phosphonobutanoate, by PsfC. Crystallographic studies reveal that PsfC unexpectedly belongs to a new class of diiron metalloenzymes that are part of the polymerase and histidinol phosphatase superfamily.

In Vitro Susceptibility of Multi-Drug Resistant Klebsiellapneumoniae Strains Causing Nosocomial Infections to Fosfomycin. A Comparison of Determination Methods

INTRODUCTION

Over the past few decades, Klebsiella pneumoniae strains increased their pathogenicity and antibiotic resistance, thereby becoming a major therapeutic challenge. One of the few available therapeutic options seems to be intravenous fosfomycin. Unfortunately, the determination of sensitivity to fosfomycin performed in hospital laboratories can pose a significant problem. Therefore, the aim of the present research was to evaluate the activity of fosfomycin against clinical, multidrug-resistant Klebsiella pneumoniae strains isolated from nosocomial infections between 2011 and 2020, as well as to evaluate the methods routinely used in hospital laboratories to assess bacterial susceptibility to this antibiotic.

MATERIALS AND METHODS

43 multidrug-resistant Klebsiella strains isolates from various infections were tested. All the strains had ESBL enzymes, and 20 also showed the presence of carbapenemases. Susceptibility was determined using the diffusion method (E-test) and the automated system (Phoenix), which were compared with the reference method (agar dilution).

RESULTS

For the reference method and for the E-test, the percentage of strains sensitive to fosfomycin was 65%. For the Phoenix system, the percentage of susceptible strains was slightly higher and stood at 72%. The percentage of fosfomycin-resistant strains in the Klebsiella carbapenemase-producing group was higher (45% for the reference method and E-test and 40% for the Phoenix method) than in carbapenemase-negative strains (25%, 25%, and 20%, respectively). Full (100%) susceptibility categorical agreement was achieved for the E-test and the reference method. Agreement between the automated Phoenix system and the reference method reached 86%.

CONCLUSIONS

Fosfomycin appears to be the antibiotic with a potential for use in the treatment of infections with multidrug-resistant Klebsiella strains. Susceptibility to this drug is exhibited by some strains, which are resistant to colistin and carbapenems. The E-test, unlike the Phoenix method, can be an alternative to the reference method in the routine determination of fosfomycin susceptibility, as it shows agreement in terms of sensitivity categories and only slight differences in MIC values. The Phoenix system, in comparison to the reference method, shows large discrepancies in the MIC values and in the susceptibility category.

Treatment of severe infections due to metallo-β-lactamases-producing Gram-negative bacteria

In the last decades, there was an important paucity of agents for adequately treating infections due to metallo-β-lactamases-producing Gram-negative bacteria (MBL-GNB). Cefiderocol, a novel siderophore cephalosporin showing in vitro activity against MBL-GNB, has been recently marketed, and a combination of aztreonam and ceftazidime/avibactam has shown a possible favorable effect on survival of patients with severe MBL-GNB infections in observational studies. Other agents showing in vitro activity against MBL-GNB are currently in clinical development (e.g., cefepime/taniborbactam, LYS228, cefepime/zidebactam) that could be an important addition to our future armamentarium for severe MBL-GNB infections. Nonetheless, we should not discontinue our efforts to optimize the use of non-β-lactams agents, since they could remain an essential last-resort or alternative option in selected cases.

Synergistic activity of fosfomycin and chloramphenicol against vancomycin-resistant Enterococcus faecium (VREfm) isolates from bloodstream infections

Vancomycin-resistant Enterococcus faecium (VREfm) infections are increasing. Current anti-VREfm options (linezolid and daptomycin) are suboptimal. Fosfomycin maintains good efficacy against VREfm and chloramphenicol is active against ≥ 90% of VREfm. We tested chloramphenicol + fosfomycin (CAF+FOS) against 10 VREfm isolated from blood. MICs were 64 to 512 µg/mL for fosfomycin and 8 to 16 µg/mL for chloramphenicol. The combination decreased both MICs, with a synergic effect in 50% of the isolates and an additive effect in the remaining 50%. Time-kill assays performed on fractional inhibitory concentration index ≤ 0.5 strains confirmed the synergism. The antibiotic combination at ¼ of minimum inhibitory concentrations (MICs) caused a ≥ 2 log₁₀ reduction compared to the two antibiotics alone. Finally, we provided a proof of concept of the in vitro efficacy of CAF+FOS in G. mellonella. The survival of G. mellonella larvae treated with the combination was significantly higher. The activity of fosfomycin and chloramphenicol against VREfm increases when they are used in combination.

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.