Antimicrobial susceptibility of multidrug-resistant Gram negative bacteria to fosfomycin

Intravenous Fosfomycin for Systemic Multidrug-Resistant Pseudomonas aeruginosa Infections

Human Pseudomonas infections have high morbidity and mortality rates. Pseudomonas bacteria can cause sepsis or septic shock; they produce biofilm and commonly exhibit a multidrug-resistant phenotype. The choice of antimicrobial therapy in many cases is challenging, and deep knowledge of clinical, microbiological, and pharmacological issues is required. Intravenous fosfomycin is being repurposed in a combination given its favorable pharmacokinetic/pharmacodynamic properties (a small molecule with favorable kinetic both in bloodstream infection and in deep-seated infections), antibiofilm activity, and its interesting synergistic effects with other antimicrobials. Recent literature on epidemiological, microbiological, pharmacological, and clinical data on intravenous fosfomycin therapy against Pseudomonas is herein reviewed and discussed.

Kinetics, Thermodynamics, and Structural Effects of Quinoline-2-Carboxylates, Zinc-Binding Inhibitors of New Delhi Metallo-β-lactamase-1 Re-sensitizing Multidrug-Resistant Bacteria for Carbapenems

Carbapenem resistance mediated by metallo-β-lactamases (MBL) such as New Delhi metallo-β-lactamase-1 (NDM-1) has become a major factor threatening the efficacy of essential β-lactam antibiotics. Starting from hit fragment dipicolinic acid (DPA), 8-hydroxy- and 8-sulfonamido-quinoline-2-carboxylic acids were developed as inhibitors of NDM-1 with highly improved inhibitory activity and binding affinity. The most active compounds formed reversibly inactive ternary protein-inhibitor complexes with two zinc ions as proven by native protein mass spectrometry and bio-layer interferometry. Modification of the NDM-1 structure with remarkable entropic gain was shown by isothermal titration calorimetry and NMR spectroscopy of isotopically labeled protein. The best compounds were potent inhibitors of NDM-1 and other representative MBL with no or little inhibition of human zinc-binding enzymes. These inhibitors significantly reduced the minimum inhibitory concentrations (MIC) of meropenem for multidrug-resistant bacteria recombinantly expressing blaNDM-1 as well as for several multidrug-resistant clinical strains at concentrations non-toxic to human cells.

Intravenous fosfomycin for the treatment of patients with bone and joint infections: a review

INTRODUCTION

Fosfomycin is a wide spectrum bactericidal antibiotic with a unique mode of action, low toxicity, and good penetration in tissues with deep-seated infections, including bone and joint infections.

AREAS COVERED

Data were extracted from 19 published articles. Three hundred and sixty-five patients, with broad age range, received intravenous fosfomycin for the treatment of bone and joint infections (including arthritis, acute and chronic osteomyelitis, discitis, periprosthetic joint infection). Fosfomycin was given as part of a combination antimicrobial therapy in the majority of patients (93.7%). The dosage of fosfomycin ranged from 4 g/day (in one case) to 24 g/day. The dosage of fosfomycin, in some cases, mostly pediatric, was calculated based on body weight, ranging from 50 mg/kg/day to 250 mg/kg/day. The duration of fosfomycin treatment ranged from a couple of days up to 3 months. The most common isolated pathogen was Staphylococcus aureus (38.9%). Three hundred patients (82.2%) were successfully treated. Fosfomycin was well tolerated, as few patients developed mild adverse events, mostly gastrointestinal discomfort, hypernatremia, skin rash, and neutropenia.

EXPERT OPINION

The available data suggests that intravenous fosfomycin may be beneficial for the treatment of patients with bone and joint infections, especially when used as part of a combination antibiotic regimen.

In vitro synergistic activity of fosfomycin in combination with meropenem, amikacin and colistin against OXA-48 and/or NDM-producing Klebsiella pneumoniae

Objective: Carbapenemase-producing Klebsiella pneumoniae (CPKp) infections are increasing worldwide. We investigated the in vitro synergistic activity of fosfomycin (FOS) with meropenem (MRP), amikacin (AMK) and colistin (COL) against OXA-48 and/or New Delhi metallo-beta-lactamase (NDM)-producing Kp blood isolates. Materials and Methods: Seventeen CPKp blood isolates were studied. The broth microdilution method was used for COL, MRP and AMK susceptibilities, while agar dilution for FOS. Synergy was tested by agar dilution chequerboard technique and also was confirmed by a time-kill assay for FOS/MRP and FOS/COL using three representative isolates that were found to be synergistic. Results: FOS in combination with MRP was found to be the most synergistic (15/17 strains, 88%), while 29% and 41% with AMK and COL, respectively. Antagonism was only determined in 2 isolates with the COL/FOS. Conclusions: The MRP/FOS combination demonstrated synergistic activity against CRKp, especially against the two common enzyme-producing isolates in Turkey (OXA-48 and NDM).

Oral fosfomycin for the treatment of lower urinary tract infections among kidney transplant recipients-Results of a Spanish multicenter cohort

Oral fosfomycin may constitute an alternative for the treatment of lower urinary tract infections (UTIs) in kidney transplant recipients (KTRs), particularly in view of recent safety concerns with fluroquinolones. Specific data on the efficacy and safety of fosfomycin in KTR are scarce. We performed a retrospective study in 14 Spanish hospitals including KTRs treated with oral fosfomycin (calcium and trometamol salts) for posttransplant cystitis between January 2005 and December 2017. A total of 133 KTRs developed 143 episodes of cystitis. Most episodes (131 [91.6%]) were produced by gram-negative bacilli (GNB), and 78 (54.5%) were categorized as multidrug resistant (including extended-spectrum β-lactamase-producing Enterobacteriaceae [14%] or carbapenem-resistant GNB [3.5%]). A median daily dose of 1.5 g of fosfomycin (interquartile range [IQR]: 1.5-2) was administered for a median of 7 days (IQR: 3-10). Clinical cure (remission of UTI-attributable symptoms at the end of therapy) was achieved in 83.9% (120/143) episodes. Among those episodes with follow-up urine culture, microbiological cure at month 1 was achieved in 70.2% (59/84) episodes. Percutaneous nephrostomy was associated with a lower probability of clinical cure (adjusted odds ratio: 10.50; 95% confidence interval: 0.98-112.29; P = 0.052). In conclusion, fosfomycin is an effective orally available alternative for treating cystitis among KTRs.

Performance of disk diffusion and broth microdilution for fosfomycin susceptibility testing of multidrug-resistant clinical isolates of Enterobacterales and Pseudomonas aeruginosa

OBJECTIVES

This study aimed to evaluate the susceptibility of clinical isolates of Enterobacterales and Pseudomonas aeruginosa to fosfomycin and to determine the concordance of disk diffusion (DD) and broth microdilution (BMD) with agar dilution (AD) for fosfomycin susceptibility testing.

METHODS

The activity of fosfomycin against 225 clinical isolates of Escherichia coli (n = 64), Klebsiella pneumoniae (n = 68), Enterobacter spp. (n = 28) and P. aeruginosa (n = 65) was tested by AD, DD and BMD. For DD, results were recorded considering and not considering colonies growing within the inhibition halo as recommended by the CLSI and EUCAST, respectively. Escherichia coli breakpoints were used for all Enterobacterales. Results were reported as categorical agreement (CA), major error (ME; false-resistant), very major error (VME; false-susceptible) and minor error (any other discrepancies).

RESULTS

Fosfomycin susceptibility of all tested species was >90% by AD. Following CLSI guidelines, DD was the only method reaching ≥90% CA with AD for E. coli and K. pneumoniae, albeit yielding 6% ME. Neither DD nor BMD achieved acceptable CA percentages for Enterobacter spp. Following EUCAST guidelines, none of the methods had CA ≥ 90%. For Enterobacterales, the best performance of DD is achieved when read as indicated by EUCAST but interpreted according the CLSI breakpoints (>97% CA; 0% VME; ≤2% ME). For P. aeruginosa, BMD yielded the best results (89% CA; 0% VME; 11% ME).

CONCLUSION

Neither DD or BMD provide accurate results owing to unacceptable ME and VME percentages even when performed as intended by the guidelines.

Synergistic Activities of Colistin Combinations with Meropenem, Sulbactam, Minocycline, Disodium Fosfomycin, or Vancomycin Against Different Clones of Carbapenem-Resistant Acinetobacter baumannii Strains

Aims: Colistin became the primary treatment option for Acinetobacters that had developed a high rate of resistance to carbapenems which were the first-line therapy in the past, and now Acinetobacters become resistant to nearly all antibiotics. Because of the resistance potential to colistin and the concerns about toxicity, especially for high doses, colistin combination therapies are preferred nowadays. In this study, we aimed to investigate whether combinations of colistin with meropenem, sulbactam, fosfomycin, vancomycin, and minocycline are synergic or not and to determine minocycline susceptibility rate, which is not in use in our country. Results: For the studied 23 Acinetobacter strains, the highest synergy was between colistin and vancomycin, which was shown in 4 (17.4%) strains. The synergy of colistin with meropenem and fosfomycin was detected for 1 (4.3%) strain, the synergy of colistin with minocycline was detected for 2 (8.6%) strains, and no synergy was detected for colistin-sulbactam combination. All the strains were susceptible to minocycline. Conclusion: None of the antibiotic combinations was antagonistic. They had synergistic and additive interactions. Thus, these combinations can be used in clinical practices. The remarkable synergistic interaction of colistin-vancomycin combination and high susceptibility to minocycline highlight the need for more researches on these subjects.

Peptidoglycan recycling contributes to intrinsic resistance to fosfomycin in Acinetobacter baumannii

Background

Acinetobacter baumannii is intrinsically resistant to fosfomycin; however, the mechanisms underlying this resistance are poorly understood.

Objectives

To identify and characterize genes that contribute to intrinsic fosfomycin resistance in A. baumannii.

Methods

More than 9000 individual transposon mutants of the A. baumannii ATCC 17978 strain (fosfomycin MIC ≥1024 mg/L) were screened to identify mutations conferring increased susceptibility to fosfomycin. In-frame deletion mutants were constructed for the identified genes and their susceptibility to fosfomycin was characterized by MIC determination and growth in the presence of fosfomycin. The effects of these mutations on membrane permeability and peptidoglycan integrity were characterized. Susceptibilities to 21 antibiotics were determined for the mutant strains.

Results

Screening of the transposon library identified mutants in the ampD and anmK genes, both encoding enzymes of the peptidoglycan recycling pathway, that demonstrated increased susceptibility to fosfomycin. MIC values for in-frame deletion mutants were ≥42-fold (ampD) and ≥8-fold (anmK) lower than those for the parental strain, and growth of the mutant strains in the presence of 32 mg/L fosfomycin was significantly reduced. Neither mutation resulted in increased cell permeability; however, the ampD mutant demonstrated decreased peptidoglycan integrity. Susceptibility to 21 antibiotics was minimally affected by mutations in ampD and anmK.

Conclusions

This study demonstrates that AmpD and AnmK of the peptidoglycan recycling pathway contribute to intrinsic fosfomycin resistance in A. baumannii, indicating that inhibitors of these enzymes could be used in combination with fosfomycin as a novel treatment approach for MDR A. baumannii.

In vitro and in vivo activity of ciprofloxacin/fosfomycin combination therapy against ciprofloxacin-resistant Shigella flexneri isolates

Objective: Ciprofloxacin resistance (CIP^R) for Shigella isolates is becoming more prevalent. This study systematically investigated the antibacterial activity of ciprofloxacin (CIP)/fosfomycin (FOS) combination in vitro and in vivo against CIP^R S. flexneri isolates. Method: Eighty CIP^R S. flexneri isolates were selected for synergy studies by the microtiter plate checkerboard assay. Two S. flexneri isolates (GN120471, CIP^RFOS^R; GN120454, CIP^RFOS^S) were used to investigate the efficacy of the CIP/FOS combination by the time-kill methodology. Clinically relevant concentrations (CIP, 0.5, 1, or 2.5 μg/mL; FOS, 30, 150, or 300 μg/mL) were combined, and the colony counts were conducted at 3, 5, 8, and 24 hours. The in vivo activity of the CIP/FOS combination was assessed using a Galleria mellonella larvae model. Results: In checkerboard assays, 31 strains (38.75%) showed synergy for the CIP/FOS combination. For the isolate GN120471, monotherapy with CIP or FOS at all concentrations produced little or no bacterial killing, while the CIP/FOS combination produced enhanced bacterial killing with FOS concentrations of 150 and 300 μg/mL, especially when combined with CIP at 2.5 μg/mL. For the isolate GN120454, the CIP/FOS combination at all concentrations produced more rapid and extensive killing (up to 5log₁₀ colony forming units (CFU)/mL with many combinations) than with either antibiotic alone. Mortality at 96 hours was around 80% at approximately 10⁴ CFU/larva for GN120471 and GN120454. When CIP at 2.5 μg/mL was combined with FOS at 150 μg/mL for the bactericidal activity in vivo, the survival rates for CIP/FOS combination against GN120471-infected and GN120454-infected larvae were significantly higher than that of CIP (68.75% vs 25%, P=0.013; 81.25% vs 37.5%, P=0.012, respectively). Conclusion: Against CIP^R S. flexneri isolates, the CIP/FOS combination induced synergy, and increased bacterial killing in vitro and in a simple invertebrate model of infection.

Antimicrobial treatment challenges in the era of carbapenem resistance

Infections due to carbapenem-resistant Gram-negative bacteria are burdened by high mortality and represent an urgent threat to address. Clinicians are currently at a dawn of a new era in which antibiotic resistance in Gram-negative bacilli is being dealt with by the availability of the first new antibiotics in this field for many years. Although new antibiotics have shown promising results in clinical trials, there is still uncertainty over whether their use will improve clinical outcomes in real world practice. Some observational studies have reported a survival benefit in carbapenem-resistant Enterobacteriaceae bloodstream infections using combination therapy, often including "old" antibiotics such as colistin, aminoglycosides, tigecycline, and carbapenems. These regimens, however, are linked to increased risk of antimicrobial resistance, and their efficacy has yet to be compared to new antimicrobial options. While awaiting more definitive evidence, antibiotic stewards need clear direction on how to optimize the use of old and novel antibiotic options. Furthermore, carbapenem-sparing regimens should be carefully considered as a potential tool to reduce selective antimicrobial pressure.

In Vitro Activity of Newer and Conventional Antimicrobial Agents, Including Fosfomycin and Colistin, against Selected Gram-Negative Bacilli in Kuwait

Limited data are available on susceptibilities of these organisms to some of the recently made accessible antimicrobial agents. The in vitro activities of newer antibiotics, such as, ceftolozane/tazobactam (C/T) and ceftazidime/avibactam (CZA) along with some “older” antibiotics, for example fosfomycin (FOS) and colistin (CL) were determined against selected strains (resistant to ≥3 antimicrobial agents) of Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa. Minimum inhibitory concentrations (MIC) were determined by Clinical and Laboratory Standards Institute microbroth dilution. 133 isolates: 46 E. coli, 39 K. pneumoniae, and 48 P. aeruginosa were tested. Results showed that E. coli isolates with MIC_50/90, 0.5/1 μg/mL for CL; 4/32 μg/mL for FOS; 0.25/32 μg/mL for C/T; 0.25/8 μg/mL for CZA, exhibited susceptibility rates of 95.7%, 97.8%, 76.1%, and 89.1%, respectively. On the other hand, K. pneumoniae strains with MIC_50/90, 0.5/1 μg/mL for CL; 256/512 μg/mL for FOS; 2/128 μg/mL for C/T; 0.5/128 μg/mL for CZA showed susceptibility rates of 92.3%, 7.7%, 51.3%, and 64.1%, respectively. P. aeruginosa isolates with MIC_50/90, 1/1 μg/mL for CL; 128/128 μg/mL for C/T; 32/64 μg/mL for CZA presented susceptibility rates of 97.9%, 33.3%, and 39.6%, respectively. Higher MICs were demonstrated against most of the antibiotics. However, CL retained efficacy at low MICs against most of the isolates tested.

Molecular Epidemiology of Plasmid-Mediated Fosfomycin Resistance Gene Determinants in Klebsiella pneumoniae Carbapenemase-Producing Klebsiella pneumoniae Isolates in China

The Klebsiella pneumoniae carbapenemase (KPC)-producing K. pneumoniae has become a serious problem because the species is wide ranging and there are few treatment options. Fosfomycin has attracted renewed interest in combination therapy for infections caused by KPC-producing K. pneumoniae isolates. Because of the increasing use of fosfomycin, resistant isolates have been continually reported in carbapenem-resistant K. pneumoniae (CRKP). At present, multiple mechanisms can result in fosfomycin resistance. However, there is limited knowledge with respect to plasmid-mediated fosfomycin resistance gene (fosA3) determinants in KPC-producing K. pneumoniae isolates. In this study, a total of 101 CRKP strains were collected from four hospitals in Zhejiang province from January 2013 to August 2014; 28.7% (29/101) of CRKP isolates were resistant to fosfomycin. Gene fosA3 was detected in 29 fosfomycin-resistant KPC-producing K. pneumoniae isolates, whereas genes fosA, fosB, fosB2, fosC, fosC2, and fosX were all negative among the resistant isolates. In addition, among 29 fosfomycin-resistant KPC-producing K. pneumoniae isolates, pulsed-field gel electrophoresis (PFGE) analysis revealed five pulsotypes. S1-PFGE and Southern blot showed that the fosA3 gene was located on an approximately 140-kb plasmid in all isolates. Eight of the 29 isolates (27.6%) tested could successfully transfer their fosfomycin-resistant phenotype to Escherichia coli strain J53. All fosA3-positive isolates were determined to have an identical genetic background, IS26-tetR-cadC-orf1-fosA3-IS26, which is the same as that of the fosA3-positive plasmid pFOS18 in China. The primary resistance mechanism to fosfomycin was caused by a plasmid-mediated fosA3. Furthermore, it is noteworthy that the plasmid genetically carrying a combination of the fosA3 and bla_KPC-2 genes could accelerate the spread of antibiotic resistance. Effective and persistent monitoring and surveillance will be vital to prevent further dissemination of these resistance genes.

Fosfomycin as a potential therapy for the treatment of systemic infections: a population pharmacokinetic model to simulate multiple dosing regimens

Fosfomycin has emerged as a potential therapy for multidrug-resistant bacterial infections. In most European countries, the oral formulation is only approved as a 3 g single dose for treatment of uncomplicated cystitis. However, for the treatment of complicated systemic infections, this dose regimen is unlikely to reach efficacious serum and tissue concentrations. This study aims to investigate different fosfomycin-dosing regimens to evaluate its rationale for treatment of systemic infections. Serum concentration-time profiles of fosfomycin were simulated using a population pharmacokinetic model based on published pharmacokinetic parameter values, their uncertainty, inter-individual variability and covariates. The model was validated on published data and used to simulate a wide range of dosing regimens for oral and intravenous administration of fosfomycin. Finally, based on the minimum inhibitory concentration for E. coli, surrogate pharmacodynamic indices were calculated for each dosing regimen. This is the first population pharmacokinetic model to describe the oral pharmacokinetics of fosfomycin using data from different literature sources. The model and surrogate pharmacodynamic indices provide quantitative evidence that a dosing regimen of 6-12 g per day divided in 3 doses is required to obtain efficacious exposure and may serve as a first step in the treatment of systemic multi-drug-resistant bacterial infections.

Fosfomycin: Pharmacological, Clinical and Future Perspectives

Fosfomycin is a bactericidal, low-molecular weight, broad-spectrum antibiotic, with putative activity against several bacteria, including multidrug-resistant Gram-negative bacteria, by irreversibly inhibiting an early stage in cell wall synthesis. Evidence suggests that fosfomycin has a synergistic effect when used in combination with other antimicrobial agents that act via a different mechanism of action, thereby allowing for reduced dosages and lower toxicity. Fosfomycin does not bind to plasma proteins and is cleared via the kidneys. Due to its extensive tissue penetration, fosfomycin may be indicated for infections of the CNS, soft tissues, bone, lungs, and abscesses. The oral bioavailability of fosfomycin tromethamine is <50%; therefore, oral administration of fosfomycin tromethamine is approved only as a 3-gram one-time dose for treating urinary tract infections. However, based on published PK parameters, PK/PD simulations have been performed for several multiple-dose regimens, which might lead to the future use of fosfomycin for treating complicated infections with multidrug-resistant bacteria. Because essential pharmacological information and knowledge regarding mechanisms of resistance are currently limited and/or controversial, further studies are urgently needed, and fosfomycin monotherapy should be avoided.

Compatibility of fosfomycin with different commercial peritoneal dialysis solutions

For treatment of peritoneal dialysis-related peritonitis, intraperitoneal administration of antibiotics remains the preferable route. For home-based therapy, patients are commonly supplied with peritoneal dialysis fluids already containing antimicrobial agents. The present study set out to investigate the compatibility of fosfomycin with different peritoneal dialysis fluids, namely, Extraneal^®, Nutrineal^®, Physioneal^® 1.36% and Physioneal^® 2.27%, under varying storage conditions. The peritoneal dialysis fluid bags including 4 g fosfomycin were stored over 14 days at refrigeration temperature (6°C) and room temperature (25°C) and over 24 h at body temperature (37°C). Drug concentrations over time were determined by using high-performance liquid chromatography coupled to a mass spectrometer. In addition, drug activity was assessed by a disk diffusion method, diluent stability by visual inspection and drug adsorption by comparison of the measured and calculated concentrations. Blank peritoneal dialysis fluids and deionized water were used as comparator solutions. Fosfomycin was stable in all peritoneal dialysis fluids and at each storage condition investigated over the whole study period. The remaining drug concentrations ranged between 94% and 104% of the respective initial concentrations. No significant drug adsorption was observed for any peritoneal dialysis fluid at any storage condition. No relevant reduction of antimicrobial activity was observed. Fosfomycin is compatible with Extraneal^®, Nutrineal^® and Physioneal^® for up to two weeks at refrigeration or room temperature and may be used for home-based therapy. No dose adjustment is needed due to adsorption or degradation.

Exploring the hidden potential of fosfomycin for the fight against severe Gram-negative infections

Gram-negative resistance is a serious global crisis putting the world on the cusp of 'pre-antibiotic era'. This serious crisis has been catalysed by the rapid increase in carbapenem-resistant Enterobacteriaceae (CRE). Spurge in colistin usage to combat CRE infections leads to the reports of (colistin and carbapenem resistant enterobacteriaceae) CCRE (resistance to colistin in isolates of CRE) infections further jeopardising our last defence. The antibacterial apocalypse imposed by global resistance crisis requires urgent alternative therapeutic options. Interest in the use of fosfomycin renewed recently for serious systemic infections caused by multidrug-resistant Enterobacteriaceae. This review aimed at analysing the recent evidence on intravenous fosfomycin to explore its hidden potential, especially when fosfomycin disodium is going to be available in India. Although a number of promising evidence are coming up for fosfomycin, there are still areas where more work is required to establish intravenous fosfomycin as the last resort antibacterial for severe Gram-negative infections.

Characterisation of fosfomycin resistance mechanisms and molecular epidemiology in extended-spectrum β-lactamase-producing Klebsiella pneumoniae isolates

Although fosfomycin is a treatment option for infections caused by extended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae, fosfomycin resistance has been documented. To our knowledge, fosfomycin resistance mechanisms in Klebsiella pneumoniae have not been systematically investigated. A total of 108 ESBL-producing K. pneumoniae isolates collected from Kaohsiung Medical University Hospital, Taiwan, from August 2012 to May 2013 were analysed in this study. Pulsed-field gel electrophoresis (PFGE) revealed 64 pulsotypes and six non-typeable isolates, indicating high genetic diversity. Moreover, pulsotypes V (n = 6), VII (n = 11) and LI (n = 4) belonging to ST11 were major types. Among 30 (27.8%) fosfomycin-non-susceptible isolates, 21 (70%) had a MurA amino acid substitution, and seven new variations increased the fosfomycin minimum inhibitory concentration (MIC) by 8- to 16-fold compared with wild-type MurA in Escherichia coli DH5α.strain. Functionless transporters (GlpT and UhpT) with various mutations were found in 29 isolates (97%). No known fosfomycin-modifying enzymes were detected in this study. The major resistance mechanisms to fosfomycin in K. pneumoniae were amino acid variations in the drug target and transporters.

Clinically relevant concentrations of fosfomycin combined with polymyxin B, tobramycin or ciprofloxacin enhance bacterial killing of Pseudomonas aeruginosa, but do not suppress the emergence of fosfomycin resistance

OBJECTIVES

Fosfomycin resistance occurs rapidly with monotherapy. This study systematically investigated bacterial killing and emergence of fosfomycin resistance with fosfomycin combinations against Pseudomonas aeruginosa.

METHODS

Four clinical isolates and a reference strain of P. aeruginosa were employed. Combinations of fosfomycin plus polymyxin B, tobramycin or ciprofloxacin were examined over 24 h using time-kill studies (inocula ∼10(6) cfu/mL) incorporating clinically relevant concentrations (fosfomycin, 30, 150 or 300 mg/L; polymyxin B, 0.5, 1 or 2 mg/L; tobramycin, 0.5, 1.5 or 4 mg/L; ciprofloxacin, 0.5, 1 or 2.5 mg/L). Microbiological response was examined by log changes and population analysis profiles.

RESULTS

Against susceptible isolates, monotherapy produced varying degrees of initial killing followed by rapid regrowth. Fosfomycin plus polymyxin B or tobramycin produced greater initial killing (up to ∼4 log10 cfu/mL) with many concentrations compared with monotherapy against fosfomycin-susceptible (FOF(S)) isolates. With these combinations, synergy or additivity was observed in 54 (67%) and 49 (60%) of 81 cases (nine combinations across three isolates at three timepoints) for polymyxin B and tobramycin, respectively. Substantial improvements in killing were absent against fosfomycin-resistant (FOF(R)) isolates. For fosfomycin/ciprofloxacin combinations, synergy or additivity was observed against FOF(R) isolates in 33 of 54 (61%) cases (nine combinations across two isolates at three timepoints), while improvements in killing were largely absent against FOF(S) isolates. No combination prevented emergence of fosfomycin resistance.

CONCLUSIONS

Against P. aeruginosa, fosfomycin in combination with polymyxin B or tobramycin (FOF(S) isolates) or ciprofloxacin (FOF(R) isolates) increased bacterial killing, but did not suppress emergence of fosfomycin resistance.

Treatment options for carbapenem-resistant and extensively drug-resistant Acinetobacter baumannii infections

Acinetobacter baumannii is a leading cause of healthcare-associated infections worldwide. Because of various intrinsic and acquired mechanisms of resistance, most β-lactam agents are not effective against many strains, and carbapenems have played an important role in therapy. Recent trends show many infections are caused by carbapenem-resistant or even extensively drug-resistant (XDR) strains, for which effective therapy is not well established. Evidence to date suggests that colistin constitutes the backbone of therapy, but the unique pharmacokinetic properties of colistin have led many to suggest the use of combination antimicrobial therapy. However, the combination of agents and dosing regimens that delivers the best clinical efficacy while minimizing toxicity is yet to be defined. Carbapenems, sulbactam, rifampin and tigecycline have been the most studied in the context of combination therapy. Most data regarding therapy for invasive, resistant A. baumannii infections come from uncontrolled case series and retrospective analyses, though some clinical trials have been completed and others are underway. Early institution of appropriate antimicrobial therapy is shown to consistently improve survival of patients with carbapenem-resistant and XDR A. baumannii infection, but the choice of empiric therapy in these infections remains an open question. This review summarizes the most current knowledge regarding the epidemiology, mechanisms of resistance, and treatment considerations of carbapenem-resistant and XDR A. baumannii.

Acinetobacter baumannii: evolution of antimicrobial resistance-treatment options

The first decade of the 20th century witnessed a surge in the incidence of infections due to several highly antimicrobial-resistant bacteria in hospitals worldwide. Acinetobacter baumannii is one such organism that turned from an occasional respiratory pathogen into a major nosocomial pathogen. An increasing number of A. baumannii genome sequences have broadened our understanding of the genetic makeup of these bacteria and highlighted the extent of horizontal transfer of DNA. Animal models of disease combined with bacterial mutagenesis have provided some valuable insights into mechanisms of A. baumannii pathogenesis. Bacterial factors known to be important for disease include outer membrane porins, surface structures including capsule and lipopolysaccharide, enzymes such as phospholipase D, iron acquisition systems, and regulatory proteins. A. baumannii has a propensity to accumulate resistance to various groups of antimicrobial agents. In particular, carbapenem resistance has become commonplace, accounting for the majority of A. baumannii strains in many hospitals today. Carbapenem-resistant strains are often resistant to all other routinely tested agents. Treatment of carbapenem-resistant A. baumannii infection therefore involves the use of combinations of last resort agents such as colistin and tigecycline, but the efficacy and safety of these approaches are yet to be defined. Antimicrobial-resistant A. baumannii has high potential to spread among ill patients in intensive care units. Early recognition and timely implementation of appropriate infection control measures is crucial in preventing outbreaks.

Carbapenem-resistant Enterobacteriaceae and Acinetobacter baumannii: assessing their impact on organ transplantation

PURPOSE OF REVIEW

This review highlights the impact of carbapenem-resistant Enterobacteriaceae and carbapenem-resistant Acinetobacter baumannii on patients who have undergone organ transplantation and explores both available and potential agents to treat infections caused by these multidrug-resistant (MDR) pathogens.

RECENT FINDINGS

Few antimicrobials exist to treat carbapenem-resistant Gram-negative infections, and resistance to salvage therapies is escalating. Organ transplantation appears to be a risk factor for infections with Klebsiella pneumoniae carbapenemase-producing Enterobacteriaceae. Isolation of these MDR bacteria is increasing and may be associated with allograft failure and mortality. In the majority of cases, aminoglycosides, polymyxins, and tigecycline have been employed to treat these infections. Anecdotal successes exist but these antibiotics may be unreliable. Few novel agents are in development.

SUMMARY

Bacterial infections remain a leading cause of posttransplantation morbidity and mortality. Carbapenem resistance is a significant threat to allograft and patient survival. With few antimicrobials being developed, transplant centers may be forced to make decisions regarding surveillance, empiric antimicrobial regimens, and transplant candidacy in the setting of carriage of MDR pathogens. There is an urgent need for collaborative studies to address the clinical impact of these infections on transplantation.

Blocking peptidoglycan recycling in Pseudomonas aeruginosa attenuates intrinsic resistance to fosfomycin

Gram-negative bacteria recycle as much as half of their cell wall per generation. Here we show that interference with cell wall recycling in Pseudomonas aeruginosa strains results in four- to eight-fold increased susceptibility to the antibiotic fosfomycin, pushing the minimal inhibitory concentration for strains PA14 and PA01 to therapeutically appropriate values of 2-4 and 8-16 mg/L, respectively. A newly discovered metabolic pathway that connects cell wall recycling with peptidoglycan de novo biosynthesis is responsible for the high intrinsic resistance of P. aeruginosa to fosfomycin. The pathway comprises an anomeric cell wall amino sugar kinase (AmgK) and an uridylyl transferase (MurU), which together convert N-acetylmuramic acid (MurNAc) through MurNAc α-1-phosphate to uridine diphosphate (UDP)-MurNAc, thereby bypassing the fosfomycin-sensitive de novo synthesis of UDP-MurNAc. Thus, inhibition of peptidoglycan recycling can be applied as a new strategy for the combinatory therapy against multidrug-resistant P. aeruginosa strains.

Fosfomycin for the treatment of resistant gram-negative bacterial infections. Insights from the Society of Infectious Diseases Pharmacists

The antimicrobial agent fosfomycin was discovered in 1969, at a time when bacteria had not yet developed extended-spectrum β-lactamases or carbapenemases. Decades later, it is not uncommon for gram-negative organisms to be multidrug-resistant and even pan-resistant to available antibiotic regimens, leaving clinicians with few therapeutic alternatives. Because fosfomycin has been shown to retain activity against these virulent pathogens, there is renewed interest in its use as a therapeutic agent. Fosfomycin formulations including fosfomycin disodium and the newer tromethamine salt are less toxic than other alternatives and are attractive options for resistant gram-negative and gram-positive infections. Oral fosfomycin tromethamine is approved for urinary tract infections in the United States, and an intravenous formulation is also available outside of the United States for systemic disease. The bactericidal action of fosfomycin occurs at an earlier step in cell wall synthesis than that of β-lactam antibiotics. From an in vitro standpoint, fosfomycin generally has high activity against ESBL- and carbapenemase-producing Enterobacteriaceae; multidrug-resistant Pseudomonas aeruginosa susceptibility appears to be more dependent on the local antibiogram. Fosfomycin formulations have a large volume of distribution, penetrate biofilms, and concentrate in the urine. Both oral and intravenous fosfomycin formulations are effective for a wide range of gram-negative infections and disease severities; however, clinical studies are limited. Fosfomycin formulations are well-tolerated, and mild gastrointestinal distress is the most common adverse effect. The primary limitations of fosfomycin are the lack of established regimens for complicated infections and the lack of availability of the intravenous formulation in the United States. Further study of this promising agent seems warranted in the current climate of antibiotic resistance.

Potentiation effects of amikacin and fosfomycin against selected amikacin-nonsusceptible Gram-negative respiratory tract pathogens

The amikacin-fosfomycin inhalation system (AFIS) is a combination of 2 antibiotics and an in-line nebulizer delivery system that is being developed for adjunctive treatment of pneumonia caused by Gram-negative organisms in patients on mechanical ventilation. AFIS consists of a combination of amikacin and fosfomycin solutions at a 5:2 ratio (amikacin, 3 ml at 100 mg/ml; fosfomycin, 3 ml at 40 mg/ml) and the PARI Investigational eFlow Inline System. In this antibiotic potentiation study, the antimicrobial activities of amikacin and fosfomycin, alone and in a 5:2 combination, were assessed against 62 Gram-negative pathogens from a worldwide antimicrobial surveillance collection (SENTRY). The amikacin MICs for 62 isolates of Acinetobacter baumannii, Pseudomonas aeruginosa, and Klebsiella pneumoniae were ≥32 μg/ml (intermediate or resistant according to the Clinical and Laboratory Standards Institute [CLSI]; resistant according to the European Committee on Antimicrobial Susceptibility Testing [EUCAST]). Each isolate was tested against amikacin (0.25 to 1,024 μg/ml), fosfomycin (0.1 to 409.6 μg/ml), and amikacin-fosfomycin (at a 5:2 ratio) using CLSI reference agar dilution methods. The median MIC values for amikacin and fosfomycin against the 62 isolates each decreased 2-fold with the amikacin-fosfomycin (5:2) combination from that with either antibiotic alone. Interactions between amikacin and fosfomycin differed by isolate and ranged from no detectable interaction to high potentiation. The amikacin-fosfomycin (5:2) combination reduced the amikacin concentration required to inhibit all 62 isolates from >1,024 to ≤ 256 μg/ml and reduced the required fosfomycin concentration from 204.8 to 102.4 μg/ml. These results support continued development of the amikacin-fosfomycin combination for aerosolized administration, where high drug levels can be achieved.

Combination therapy for carbapenem-resistant Gram-negative bacteria

The emergence of resistant to carbapenems Gram-negative bacteria (CR GNB) has severely challenged antimicrobial therapy. Many CR GNB isolates are only susceptible to polymyxins; however, therapy with polymyxins and other potentially active antibiotics presents some drawbacks, which have discouraged their use in monotherapy. In this context, along with strong pre-clinical evidence of benefit in combining antimicrobials against CR GNB, the clinical use of combination therapy has been raised as an interesting strategy to overcome these potential limitations of a single agent. Polymyxins, tigecycline and even carbapenems are usually the cornerstone agents in combination schemes. Optimization of the probability to attain the pharmacokinetic/pharmacodynamic targets by both cornerstone drug and adjuvant drug is of paramount importance to achieve better clinical and microbiological outcomes. Clinical evidence of the major drugs utilized in combination schemes and how they should be prescribed considering pharmacokinetic/pharmacodynamic characteristics against CR GNB will be reviewed in this article.

"Stormy waters ahead": global emergence of carbapenemases

Carbapenems, once considered the last line of defense against of serious infections with Enterobacteriaceae, are threatened with extinction. The increasing isolation of carbapenem-resistant Gram-negative pathogens is forcing practitioners to rely on uncertain alternatives. As little as 5 years ago, reports of carbapenem resistance in Enterobacteriaceae, common causes of both community and healthcare-associated infections, were sporadic and primarily limited to case reports, tertiary care centers, intensive care units, and outbreak settings. Carbapenem resistance mediated by β-lactamases, or carbapenemases, has become widespread and with the paucity of reliable antimicrobials available or in development, international focus has shifted to early detection and infection control. However, as reports of Klebsiella pneumoniae carbapenemases, New Delhi metallo-β-lactamase-1, and more recently OXA-48 (oxacillinase-48) become more common and with the conveniences of travel, the assumption that infections with highly resistant Gram-negative pathogens are limited to the infirmed and the heavily antibiotic and healthcare exposed are quickly being dispelled. Herein, we provide a status report describing the increasing challenges clinicians are facing and forecast the “stormy waters” ahead.

Experience with fosfomycin for treatment of urinary tract infections due to multidrug-resistant organisms

Fosfomycin has shown promising in vitro activity against multidrug-resistant (MDR) urinary pathogens; however, clinical data are lacking. We conducted a retrospective chart review to describe the microbiological and clinical outcomes of urinary tract infections (UTIs) with MDR pathogens treated with fosfomycin tromethamine. Charts for 41 hospitalized patients with a urine culture for an MDR pathogen who received fosfomycin tromethamine from 2006 to 2010 were reviewed. Forty-one patients had 44 urinary pathogens, including 13 carbapenem-resistant Klebsiella pneumoniae (CR-Kp), 8 Pseudomonas aeruginosa, and 7 vancomycin-resistant Enterococcus faecium (VRE) isolates, 7 extended-spectrum beta-lactamase (ESBL) producers, and 9 others. In vitro fosfomycin susceptibility was 86% (median MIC, 16 μg/ml; range, 0.25 to 1,024 μg/ml). Patients received an average of 2.9 fosfomycin doses per treatment course. The overall microbiological cure was 59%; failure was due to either relapse (24%) or reinfection UTI (17%). Microbiological cure rates by pathogen were 46% for CR-Kp, 38% for P. aeruginosa, 71% for VRE, 57% for ESBL producers, and 100% for others. Microbiological cure (n = 24) was compared to microbiological failure (n = 17). There were significantly more solid organ transplant recipients in the microbiological failure group (59% versus 21%; P = 0.02). None of the patients in the microbiological cure group had a ureteral stent, compared to 24% of patients within the microbiological failure group (P = 0.02). Fosfomycin demonstrated in vitro activity against UTIs due to MDR pathogens. For CR-KP, there was a divergence between in vitro susceptibility (92%) and microbiological cure (46%). Multiple confounding factors may have contributed to microbiological failures, and further data regarding the use of fosfomycin for UTIs due to MDR pathogens are needed.

Fosfomycin: efficacy against infections caused by multidrug-resistant bacteria

OBJECTIVE

To analyze the indications for and the efficacy of parenteral fosfomycin, especially against multidrug-resistant (MDR) and pan-resistant bacterial infections.

PATIENTS AND METHODS

During a unique crisis in fosfomycin production, the supply of this antibiotic had to be carefully monitored in France over a 10-week period. One hundred and sixteen assessable patients were included in a prospective cohort study.

RESULTS

The main indications for use were osteoarthritis, lung infection, urinary tract infection, and bacteraemia. The 2 bacteria most frequently involved were Pseudomonas aeruginosa and methicillin-resistant Staphylococcus. MDR bacteria were seen in 71.5% (83/116) of cases, especially MDR P. aeruginosa (n = 28). Critical situations were common, with 44.0% (51/116) of hospitalizations occurring in an intensive care unit and 22.4% (26/116) of patients with septic shock. The overall outcome was favourable in 76.8% of cases (76/99 assessable patients).

CONCLUSION

This study provided a unique opportunity to describe the use of fosfomycin and assess its efficacy, especially against MDR bacterial infections, even in critical situations.

Tigecycline treatment for urinary tract infections: case report and literature review

Antimicrobial resistance among Gram-negatives is increasing; treatment options are limited. Although tigecycline is used infrequently for urinary tract infection (UTI), greater use is likely as resistance increases. We report successful treatment of an episode of febrile UTI and probable prostatitis with tigecycline, and summarize the relevant literature.

Antibiotic and antimicrobial peptide combinations: synergistic inhibition of Pseudomonas fluorescens and antibiotic-resistant variants

Variants resistant to penicillin G (RvP), streptomycin (RvS), lincomycin (RvL) and rifampicin (RvR) were developed from a colistin-sensitive isolate of Pseudomonas fluorescens LRC-R73 (P. fluorescens). Cell fatty acid composition, K(+) efflux and sensitivity to antimicrobial peptides (nisin Z, pediocin PA-1/AcH and colistin) alone or combined with antibiotics were determined. P. fluorescens was highly sensitive to kanamycin, tetracycline and chloramphenicol at minimal inhibitory concentrations of 0.366, 0.305 and 0.732 μg/ml respectively. P. fluorescens, RvP, RvS, RvL and RvR were resistant to nisin Z and pediocin PA-1/AcH at concentrations ≥100 μg/ml but sensitive to colistin at 0.076, 0.043, 0.344, 0.344 and 0.258 μg/ml respectively. A synergistic inhibitory effect (FICI ≤0.5) was observed when resistant variants were treated with peptide/antibiotic combinations. No significant effect on K(+) efflux from the resistant variants in the presence of antibiotics or peptides alone or combined was observed. The proportion of C16:0 was significantly higher in antibiotic-resistant variants than in the parent strain, accounting for 32.3%, 46.49%, 43.3%, 40.1% and 44.1% of the total fatty acids in P. fluorescens, RvP, RvS, RvL and RvR respectively. Combination of antibiotics with antimicrobial peptides could allow reduced use of antibiotics in medical applications and could help slow the emergence of bacteria resistant to antibiotics.

Fosfomycin: evaluation of the published evidence on the emergence of antimicrobial resistance in Gram-negative pathogens

Fosfomycin has attracted renewed interest for the treatment of lower urinary tract and even systemic infections caused by Gram-negative pathogens with resistance to traditionally used agents. The main concern regarding the clinical utility of fosfomycin refers to the potential for the emergence of resistance during therapy. In this review, we evaluate the available published evidence regarding the mechanisms and the frequency of in vitro mutational resistance to fosfomycin in Gram-negative pathogens. We also review data regarding the emergence of resistance in clinical studies of fosfomycin therapy in various infectious syndromes and data from studies that evaluate the evolution of fosfomycin resistance over time. There appears to be discordance between the high frequency of mutational resistance to fosfomycin in vitro and the lower extent of this phenomenon in clinical studies. This discordance could at least partly be attributed to a biological cost associated with common mutations that confer resistance to fosfomycin, including decreased growth rate and low adherence to epithelial cells for the resistant mutants. The development of resistance appears to be more frequent both in vitro and in clinical studies for Pseudomonas aeruginosa in comparison with Escherichia coli, whereas relevant data for other Enterobacteriaceae are relatively scarce. The urinary tract seems to provide a favourable environment for the use of fosfomycin with a low associated likelihood for the emergence of resistance, owing to high drug concentrations and acidic pH. Additional data are needed to further clarify the optimal use of fosfomycin for different infectious syndromes caused by contemporary multidrug-resistant pathogens.

Therapeutic options for infections with Enterobacteriaceae producing carbapenem-hydrolyzing enzymes

Enterobacteriaceae that produce serine carbapenemases or metallo-β-lactamases, such as KPC, OXA-48, VIM or NDM, respectively, are spreading mostly as nosocomial pathogens worldwide. Such strains are typically resistant to most if not all available antimicrobials. Specific relevant clinical data are scarce to guide the determination of the most appropriate treatment options. Data on antimicrobial susceptibility, resistance development, synergy, pharmacokinetic and pharmacodynamic parameters of the candidate regimens, as well as the experience from the treatment of infections with nonfermenting Gram-negative pathogens, can aid in this regard. Colistin and tigecycline are most likely to be active in vitro against Enterobacteriaceae producing carbapenem-hydrolyzing β-lactamases, but resistance development is of concern. Individual members of the aminoglycoside class can also be active in vitro, while carbapenems or aztreonam (specifically for metallo-β-lactamase producers) can have low minimum inhibitory concentrations. Current data do not reliably support the use of these agents as monotherapy for systemic infections. Several expanded-spectrum cephalosporins, such as ceftazidime, may be active against OXA-48 type producers. Fosfomycin might be useful as a last-resort option as part of combination regimens. Combination antimicrobial therapy with agents exhibiting synergy might also be of benefit, until novel effective agents could become clinically available.

Status report on carbapenemases: challenges and prospects

Antimicrobial resistance in hospital and community-onset bacterial infections is a significant source of patient morbidity and mortality. In the past decade, we have witnessed the increasing recovery of carbapenem-resistant Gram-negative bacteria. For many isolates, carbapenem resistance is due to the production of carbapenemases, β-lactamases that can inactivate carbapenems and frequently other β-lactam antibiotics. Currently, these enzymes are mainly found in three different β-lactamase classes (class A, B and D). Regardless of the molecular classification, there are few antimicrobials available to treat infections with these organisms and data regarding agents in development are limited to in vitro studies. This article focuses on the epidemiology of carbapenemase-producing Gram-negative bacteria. We also review available agents and those in development with potential activity against this evolving threat.

Antimicrobial susceptibilities of commonly encountered bacterial isolates to fosfomycin determined by agar dilution and disk diffusion methods

We studied the antimicrobial activity of fosfomycin against 960 strains of commonly encountered bacteria associated with urinary tract infection using standard agar dilution and disk diffusion methods. Species studied included 3 common species of Enterobacteriaceae, Pseudomonas aeruginosa, Acinetobacter baumannii, and Stenotrophomonas maltophilia; methicillin-susceptible and -resistant Staphylococcus aureus; and vancomycin-susceptible and resistant Enterococcus faecalis and E. faecium. MICs and inhibition zone diameters were interpreted in accordance with both the currently recommended Clinical and Laboratory Standards Institute (CLSI) criteria for urinary tract isolates of Escherichia coli and Enterococcus faecalis and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) criteria for Enterobacteriaceae. Tentative zone diameter interpretive criteria were developed for species not currently published by CLSI or EUCAST. Escherichia coli was uniformly susceptible to fosfomycin, as were most strains of Klebsiella pneumoniae and Enterobacter cloacae. A. baumannii was resistant to fosfomycin, while the prevalence of resistance in P. aeruginosa and S. maltophilia was greatly affected by the choice of MIC breakpoint. New tentative zone diameter criteria for K. pneumoniae, E. cloacae, S. aureus, and E. faecium were able to be set, providing some interim laboratory guidance for disk diffusion until further breakpoint evaluations are undertaken by CLSI and EUCAST.