Fosfomycin for the treatment of infections caused by Gram-positive cocci with advanced antimicrobial drug resistance: a review of microbiological, animal and clinical studies

Antibiotic Resistance to Molecules Commonly Prescribed for the Treatment of Antibiotic-Resistant Gram-Positive Pathogens: What Is Relevant for the Clinician?

Antibiotic resistance in Gram-positive pathogens is a relevant concern, particularly in the hospital setting. Several antibiotics are now available to treat these drug-resistant pathogens, such as daptomycin, dalbavancin, linezolid, tedizolid, ceftaroline, ceftobiprole, and fosfomycin. However, antibiotic resistance can also affect these newer molecules. Overall, this is not a frequent phenomenon, but it is a growing concern in some settings and can compromise the effectiveness of these molecules, leaving few therapeutic options. We reviewed the available evidence about the epidemiology of antibiotic resistance to these antibiotics and the main molecular mechanisms of resistance, particularly methicillin-resistant Sthaphylococcus aureus, methicillin-resistant coagulase-negative staphylococci, vancomycin-resistant Enterococcus faecium, and penicillin-resistant Streptococcus pneumoniae. We discussed the interpretation of susceptibility tests when minimum inhibitory concentrations are not available. We focused on the risk of the emergence of resistance during treatment, particularly for daptomycin and fosfomycin, and we discussed the strategies that can be implemented to reduce this phenomenon, which can lead to clinical failure despite appropriate antibiotic treatment. The judicious use of antibiotics, epidemiological surveillance, and infection control measures is essential to preserving the efficacy of these drugs.

Airborne antibiotic resistome and human health risk in railway stations during COVID-19 pandemic

Antimicrobial resistance is recognized as one of the greatest public health concerns. It is becoming an increasingly threat during the COVID-19 pandemic due to increasing usage of antimicrobials, such as antibiotics and disinfectants, in healthcare facilities or public spaces. To explore the characteristics of airborne antibiotic resistome in public transport systems, we assessed distribution and health risks of airborne antibiotic resistome and microbiome in railway stations before and after the pandemic outbreak by culture-independent and culture-dependent metagenomic analysis. Results showed that the diversity of airborne antibiotic resistance genes (ARGs) decreased following the pandemic, while the relative abundance of core ARGs increased. A total of 159 horizontally acquired ARGs, predominantly confering resistance to macrolides and aminoglycosides, were identified in the airborne bacteria and dust samples. Meanwhile, the abundance of horizontally acquired ARGs hosted by pathogens increased during the pandemic. A bloom of clinically important antibiotic (tigecycline and meropenem) resistant bacteria was found following the pandemic outbreak. 251 high-quality metagenome-assembled genomes (MAGs) were recovered from 27 metagenomes, and 86 genera and 125 species were classified. Relative abundance of ARG-carrying MAGs, taxonomically assigned to genus of Bacillus, Pseudomonas, Acinetobacter, and Staphylococcus, was found increased during the pandemic. Bayesian source tracking estimated that human skin and anthropogenic activities were presumptive resistome sources for the public transit air. Moreover, risk assessment based on resistome and microbiome data revealed elevated airborne health risks during the pandemic.

First comparative genomic characterization of the MSSA ST398 lineage detected in aquaculture and other reservoirs

Staphylococcus aureus ST398 can cause diseases in several different animals. In this study we analyzed ten S. aureus ST398 previously collected in three different reservoirs in Portugal (humans, gilthead seabream from aquaculture and dolphin from a zoo). Strains tested against sixteen antibiotics, by disk diffusion or minimum inhibitory concentration, showed decreased susceptibility to benzylpenicillin (all strains from gilthead seabream and dolphin) and to erythromycin with an iMLS_B phenotype (nine strains), and susceptibility to cefoxitin (methicillin-susceptible S. aureus, MSSA). All strains from aquaculture belonged to the same spa type, t2383, whereas strains from the dolphin and humans belonged to spa type t571. A more detailed analysis using single nucleotide polymorphisms (SNPs)-based tree and a heat map, showed that all strains from aquaculture origin were highly related with each other and the strains from dolphin and humans were more distinct, although they were very similar in ARG, VF and MGE content. Mutations F3I and A100V in glpT gene and D278E and E291D in murA gene were identified in nine fosfomycin susceptible strains. The blaZ gene was also detected in six of the seven animal strains. The study of the genetic environment of erm(T)-type (found in nine S. aureus strains) allowed the identification of MGE (rep13-type plasmids and IS431R-type), presumably involved in the mobilization of this gene. All strains showed genes encoding efflux pumps from major facilitator superfamily (e.g., arlR, lmrS-type and norA/B-type), ATP-binding cassettes (ABC; mgrA) and multidrug and toxic compound extrusion (MATE; mepA/R-type) families, all associated to decreased susceptibility to antibiotics/disinfectants. Moreover, genes related with tolerance to heavy metals (cadD), and several VF (e.g., scn, aur, hlgA/B/C and hlb) were also identified. Insertion sequences, prophages, and plasmids made up the mobilome, some of them associated with ARG, VF and genes related with tolerance to heavy metals. This study highlights that S. aureus ST398 can be a reservoir of several ARG, heavy metals resistance genes and VF, which are essential in the adaption and survival of the bacterium in the different environments and an active agent in its dissemination. It makes an important contribution to understanding the extent of the spread of antimicrobial resistance, as well as the virulome, mobilome and resistome of this dangerous lineage.

Assessment of the Susceptibility of Clinical Gram-Negative and Gram-Positive Bacterial Strains to Fosfomycin and Significance of This Antibiotic in Infection Treatment

Multidrug resistance of bacteria has prompted intensive development work on new medicines, but also the search for effective options among the oldest antibiotics. Although intravenous fosfomycin (IVFOS) seems to be an interesting proposal, the recommended agar dilution method for susceptibility determination poses a major problem in routine diagnostic testing. As a consequence, there is a lack of comprehensive data on the frequency of isolation of susceptible or resistant strains. This fact triggered the disposition of EUCAST concerning the revision of IVFOS breakpoints (BPs), including withdrawal of BPs for Enterobacterales (excluding E. coli) and coagulase-negative staphylococci. Therefore, the aim of this study was to assess the activity of fosfomycin against numerous clinical strains using recommended methods. Materials and methods: A total of 997 bacterial strains were tested from the following genera: Enterobacterales, Pseudomonas spp., Staphylococcus spp., Acinetobacter spp., and Enterococcus spp., for which there are currently no BPs. The strains were isolated from various clinical materials from patients hospitalized in five hospitals. During the investigation, the recommended agar dilution method was used. Susceptibility to other antibiotics and resistance mechanisms were determined using an automatic method (Phoenix) the disk diffusion method, and E-tests. MIC values of fosfomycin were estimated for all strains and for susceptible and multidrug-resistant (MDR) strains individually. Results: Except for Acinetobacter and Enterococcus, 83% of the strains were susceptible to IVFOS, including the largest percentage of S. aureus and E. coli. Klebsiella spp. turned out to be the least susceptible strains (66%). The highest proportion of susceptibility to fosfomycin was found among strains that were sensitive to other antibiotics (80.9%), and the lowest was found among Gram-negative carbapenemase-producing bacteria (55.6%) and ESBL+ bacteria (61.6%). The MIC evaluation revealed the lowest MIC₅₀ and MIC₉₀ values for S. aureus (0.5 mg/L and 1 mg/L, respectively) and E. coli (4 mg/L and 32 mg/L, respectively). The highest values of MIC₅₀ were found for Acinetobacter spp. (256 mg/L), while the highest values of MIC₉₀ were found for Acinetobacter spp. and Klebsiella spp. (256 mg/L and 512 mg/L, respectively). Conclusions: IVFOS appears to be suitable for the treatment of many infections, including the empirical treatment of polymicrobial infections and those caused by MDR strains, since the sensitivity of the studied strains to this antibiotic in different groups ranged from 66% to as much as 99%. Sensitivity to fosfomycin was also demonstrated by 60% of carbapenem-resistant strains; therefore, IVFOS is one of the few therapeutic options that can be effective against the most resistant Gram-negative rods. In light of the general consultation posted by EUCAST, obtaining data such as IVFOS MIC value distributions may be vital for the decision of implementing fosfomycin into breakpoint tables.

Antibacterial and anti-biofilm activities of fosfomycin combined with rifampin against Carbapenem-resistant Pseudomonas aeruginosa

Increasing prevalence of carbapenem-resistant Pseudomonas aeruginosa(CRPA)strains in the hospital setting represents an emerging challenge to clinical treatment for Pseudomonas aeruginosa (PA) infections, as the range of therapeutic agents active against these pathogens becomes increasingly constrained. This study demonstrated for the first time that fosfomycin (FOS) combined with rifampin (RIF) showed strong synergistic effects against CRPA and carbapenem-susceptible PA, with 100% synergistic rates. Additionally, time-killing curve further proves dynamic antibacterial activity of FOS+RIF against CRPA. Further experiments determined that antibacterial mechanisms of FOS+RIF might be inhibition of biofilm formation and eradication of pre-formed biofilm. The results of inhibition biofilm formation assay demonstrated that RIF and FOS at 1/8MIC, 1/16MIC and 1/32MIC have better inhibitory effects on CRPA biofilm formation VS FOS alone (96%, 90% and 78% VS 29%, 24% and 22%) (p<0.0001) or RIF alone (96%, 90% and 78% VS 86%, 67% and 29%) (p<0.01). The rates of eradicating pre-formed biofilm with combination therapy at 1/2MIC, 1/4MIC and 1/8MIC of both antibiotics, increased 46%, 61% and 55% compared with FOS alone (p<0.001) and 37%, 33% and 46% compared with RIF alone (p<0.01). This finding will provide new insights for the treatment of bacterial infection caused by CRPA, which can be further explored in the clinical practice.

Fosfomycin susceptibility of uropathogens at Charlotte Maxeke Johannesburg Academic Hospital

Background

Multidrug-resistant uropathogens are becoming widespread both in community and hospital setting. Safe yet effective treatments are a priority. Fosfomycin is an antibacterial that displays good activity against most bacteria causing urinary tract infections (UTIs), including multidrug-resistant bacteria. The aim of this study was to evaluate fosfomycin susceptibility for uropathogens isolated from a microbiology laboratory at a tertiary academic hospital. In addition, this was compared to the susceptibility of other oral antimicrobials.

Methods

We conducted a retrospective analysis of laboratory reports for uropathogens isolated at Charlotte Maxeke Johannesburg Academic Hospital from September 2015 to August 2017. Antimicrobial susceptibility testing of the isolates was performed using the Kirby–Bauer disk diffusion method or the Vitek^® 2 system according to the Clinical and Laboratory Standards Institute.

Results

Overall susceptibility of fosfomycin for the 4700 Enterobacteriaceae isolates was 95.7%; 95% confidence interval (CI) 95.1–96.2. The overall susceptibility for fosfomycin against the gram-positives was 98.6%. There were 37.9% multidrug-resistant Enterobacteriaceae (MDRE) isolated during the study period. Fosfomycin displayed activity against 94.4% of extended-spectrum β-lactamase (ESBL) producers and 90.7% for carbapenem-resistant Enterobacteriaceae (CRE). None of the methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus isolates tested was fosfomycin resistant. The overall in vitro susceptibility was significantly higher for fosfomycin (p = 0.0001) compared to amoxicillin/clavulanic acid, cephalexin, cefuroxime, ciprofloxacin, trimethoprim/sulfamethoxazole and nitrofurantoin.

Conclusion

This study confirmed the high susceptibility of fosfomycin against UTI pathogens isolated at our institution. In an era of increasing antimicrobial resistance, fosfomycin represents a potential option for the treatment of UTIs at Charlotte Maxeke Johannesburg Academic Hospital.

In vitro and in vivo Antibacterial Activity of Linezolid Plus Fosfomycin Against Staphylococcus aureus with Resistance to One Drug

Objective

The purpose of this study is to assess the in vitro/vivo activities of linezolid plus fosfomycin against Staphylococcus aureus (S. aureus) isolates with varying susceptibility to the study drugs.

Methods

The increasing concentration stepwise method was used to induce S. aureus resistant strains. The in vitro antibacterial activity of linezolid combined with fosfomycin against S. aureus in vitro was studied by time-kill curve and PAE. The transmission electron microscopy (TEM) was employed to observe the cell morphology of bacteria treated with drug, and the changes of cell wall thickness were recorded. The Galleria mellonella infection model was established to demonstrate the in vivo efficacy of linezolid and fosfomycin against S. aureus with varying susceptibility.

Results

The antibiotic combination showed excellent synergistic or additive effects on the original and the linezolid-resistant strain, but showed indifferent effect for fosfomycin-resistant strain. TEM images showed that fosfomycin alone and in combined could reduce the cell wall thickness of the strains resistant to linezolid and cell lysis, while linezolid increases the cell wall thickness of the strains resistant to fosfomycin. In the Galleria mellonella infection model, the survival rate of the antibiotic combined was improved compared with that of the single drug. There was a good correlation between in vivo efficacy and in vitro susceptibility.

Conclusion

The type of interaction expressed in the test combination was highly dependent on fosfomycin resistance.

Use of fosfomycin combination therapy to treat multidrug-resistant urinary tract infection among paediatric surgical patients - a tertiary care centre experience

With increasing resistance to currently used antibiotics, antibiotic combinations are being resorted to. The present study deals with five children with complicated urinary tract infection (UTI) whose urine cultures grew multidrug-resistant (MDR) organisms. In all of these five cases, MDR organisms were the causative agents for UTI and the currently available antibiotics, including colistin, were ineffective, although the organisms were sensitive in vitro. In all of these cases, the isolates reverted to being susceptible to the quinolones and cephalosporins tested, namely ceftriaxone and ceftazidime. All were treated using a combination of fosfomycin with other antibiotics, since it has no interference with other classes of antibiotics. Our observations suggest that the use of a combination of fosfomycin with either a carbapenem or an aminoglycoside in a clinical setting would be a reasonable choice to treat UTIs caused by MDR organisms, especially in complicated cases that require chronic therapy.

Cartography of opportunistic pathogens and antibiotic resistance genes in a tertiary hospital environment

Although disinfection is key to infection control, the colonization patterns and resistomes of hospital-environment microbes remain underexplored. We report the first extensive genomic characterization of microbiomes, pathogens and antibiotic resistance cassettes in a tertiary-care hospital, from repeated sampling (up to 1.5 years apart) of 179 sites associated with 45 beds. Deep shotgun metagenomics unveiled distinct ecological niches of microbes and antibiotic resistance genes characterized by biofilm-forming and human-microbiome-influenced environments with corresponding patterns of spatiotemporal divergence. Quasi-metagenomics with nanopore sequencing provided thousands of high-contiguity genomes, phage and plasmid sequences (>60% novel), enabling characterization of resistome and mobilome diversity and dynamic architectures in hospital environments. Phylogenetics identified multidrug-resistant strains as being widely distributed and stably colonizing across sites. Comparisons with clinical isolates indicated that such microbes can persist in hospitals for extended periods (>8 years), to opportunistically infect patients. These findings highlight the importance of characterizing antibiotic resistance reservoirs in hospitals and establish the feasibility of systematic surveys to target resources for preventing infections.

Spatiotemporal characterization of microbial diversity and antibiotic resistance in a tertiary-care hospital reveals broad distribution and persistence of antibiotic-resistant organisms that could cause opportunistic infections in a healthcare setting.

Oral Fosfomycin Treatment for Enterococcal Urinary Tract Infections in a Dynamic In Vitro Model

There are limited treatment options for enterococcal urinary tract infections, especially vancomycin-resistant Enterococcus (VRE). Oral fosfomycin is a potential option, although limited data are available guiding dosing and susceptibility. We undertook pharmacodynamic profiling of fosfomycin against E. faecalis and E. faecium isolates using a dynamic in vitro bladder infection model. Eighty-four isolates underwent fosfomycin agar dilution susceptibility testing (E. faecalis MIC_50/90 32/64 μg/ml; E. faecium MIC_50/90 64/128 μg/ml). Sixteen isolates (including E. faecalis ATCC 29212 and E. faecium ATCC 35667) were chosen to reflect the MIC range and tested in the bladder infection model with synthetic human urine (SHU). Under drug-free conditions, E. faecium demonstrated greater growth restriction in SHU compared to E. faecalis (E. faecium maximal growth 5.8 ± 0.6 log₁₀ CFU/ml; E. faecalis 8.0 ± 1.0 log₁₀ CFU/ml). Isolates were exposed to high and low fosfomycin urinary concentrations after a single dose, and after two doses given over two days with low urinary concentration exposure. Simulated concentrations closely matched the target (bias 2.3%). E. faecalis isolates required greater fosfomycin exposure for 3 log₁₀ kill from the starting inoculum compared with E. faecium The ƒAUC_0-72/MIC and ƒ%T > MIC_0-72 for E. faecalis were 672 and 70%, compared to 216 and 51% for E. faecium, respectively. There was no rise in fosfomycin MIC postexposure. Two doses of fosfomycin with low urinary concentrations resulted in equivalent growth inhibition to a single dose with high urinary concentrations. With this urinary exposure, fosfomycin was effective in promoting suppression of regrowth (>3 log₁₀ kill) in the majority of isolates.

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).

Calculated initial parenteral treatment of bacterial infections: Safety and tolerabilty

This is the fourth chapter of the guideline "Calculated initial parenteral treatment of bacterial infections in adults - update 2018" in the 2nd updated version. The German guideline by the Paul-Ehrlich-Gesellschaft für Chemotherapie e.V. (PEG) has been translated to address an international audience. Safety and tolerability of antimicrobial agents will be discussed in this chapter. Toxic, allergic and biological effects can be differentiated on the basis of their pathogenesis. The question of differences in the tolerability of specific antibiotics is of particular importance. However, due to limitations of the available data, it cannot be answered for most agents with the desired accuracy. For an assessment of rare side effects, results from the postmarketing surveillance have to be used.

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.

Elucidation of the pharmacokinetic/pharmacodynamic determinants of fosfomycin activity against Pseudomonas aeruginosa using a dynamic in vitro model

Objectives

To identify the fosfomycin pharmacokinetic (PK)/pharmacodynamic (PD) index (fT>MIC, fAUC/MIC or fCmax/MIC) most closely correlated with activity against Pseudomonas aeruginosa and determine the PK/PD target associated with various extents of bacterial killing and the prevention of emergence of resistance.

Methods

Dose fractionation was conducted over 24 h in a dynamic one-compartment in vitro PK/PD model utilizing P. aeruginosa ATCC 27853 and two MDR clinical isolates (CR 1005 and CW 7). In total, 35 different dosing regimens were examined across the three strains. Microbiological response was examined by log changes and population analysis profiles. A Hill-type Emax model was fitted to the killing effect data (expressed as the log10 ratio of the area under the cfu/mL curve for treated regimens versus controls).

Results

Bacterial killing of no more than ∼3 log10 cfu/mL was achieved irrespective of regimen. The fAUC/MIC was the PK/PD index most closely correlated with efficacy (R2 = 0.80). The fAUC/MIC targets required to achieve 1 and 2 log10 reductions in the area under the cfu/mL curve relative to growth control were 489 and 1024, respectively. No regimen was able to suppress the emergence of resistance, and near-complete replacement of susceptible with resistant subpopulations occurred with virtually all regimens.

Conclusions

Bacterial killing for fosfomycin against P. aeruginosa was most closely associated with the fAUC/MIC. Suppression of fosfomycin-resistant subpopulations could not be achieved even with fosfomycin exposures well above those that can be safely achieved clinically.

Vancomycin-resistant enterococcal infection in a Thai university hospital: clinical characteristics, treatment outcomes, and synergistic effect

Purpose

The incidence of infections with vancomycin-resistant enterococci (VRE) is increasing, with associated high mortality rates and limited therapeutic choices. We investigated the clinical characteristics and treatment outcomes of VRE infection and also determined the in vitro effect of monotherapy and combined antimicrobials against clinical VRE isolates.

Patients and methods

Clinical data and bacterial isolates obtained from patients with VRE infections between January 2014 and April 2018 at Phramongkutklao Hospital were reviewed. The clinical outcomes included in-hospital mortality, 30-day mortality, and microbiological eradication. Clonal relationships were assessed by random amplified polymorphic DNA analysis. In vitro activity of linezolid, tigecycline, fosfomycin, gentamicin, chloramphenicol, and ampicillin were determined by minimum inhibitory concentration (MIC) values. Tests of synergy of fosfomycin- or gentamicin-based combinations by the checkerboard method were reported with the fractional inhibitory concentration index or MIC reduction, respectively.

Results

Among 26 cases of VRE infection, nosocomial and gastrointestinal infections were the most common. There were various treatment regimens, but linezolid-containing regimens were generally used. In-hospital and 30-day mortality were 73.1% and 57.7%, respectively. Higher mortality was significantly associated with illness severity. The VRE isolates tested were universally susceptible to linezolid and tigecycline. A synergistic or additive effect was observed for fosfomycin combined with linezolid (100%) and with tigecycline (83.3%). Fourfold or greater MIC reduction was observed for linezolid or fosfomycin plus gentamicin at concentrations 1 (58.3%, 62.5%), 2 (83.3%, 62.5%), and 4 μg/mL (91.6%, 62.5%).

Conclusion

In-hospital mortality among patients with VRE infection was high. Linezolid remains a treatment of choice. However, combination therapy such as linezolid plus fosfomycin and linezolid plus gentamicin should be considered in cases of serious infection.

Current clinical use of intravenous fosfomycin in ICU patients in two European countries

PURPOSE

In Europe, intravenous fosfomycin (IV) is used particularly in difficult-to-treat or complex infections, caused by both Gram-positive and Gram-negative pathogens including multidrug-resistant strains. Here, we investigated the efficacy and safety of intravenous fosfomycin under real-life conditions.

METHODS

Prospective, multi-center, and non-interventional study in patients with bacterial infections from 20 intensive care units (ICU) in Germany and Austria (NCT01173575).

RESULTS

Overall, 209 patients were included (77 females, 132 males, mean age: 59 ± 16 years), 194 of which were treated in intensive care (APACHE II score at the beginning of fosfomycin therapy: 23 ± 8). Main indications (± bacteremia or sepsis) were infections of the CNS (21.5%), community- (CAP) and hospital-acquired pneumonia (HAP)/ventilator-associated pneumonia (VAP, 15.3%), bone and joint infections (BJI, 11%), abdominal infections (11%), and bacteremia (10.5%). Most frequently identified pathogens were S. aureus (22.3%), S. epidermidis (14.2%), Enterococcus spp. (10.8%), E. coli (12.3%) and Klebsiella spp. (7.7%). At least one multidrug-resistant (MDR) pathogen was isolated from 51 patients (24.4%). Fosfomycin was administered with an average daily dose of 13.7 ± 3.5 g over 12.4 ± 8.6 days, almost exclusively (99%) in combination with other antibiotics. The overall clinical success was favorable in 81.3% (148/182) of cases, and in 84.8% (39/46) of patients with ≥ 1 MDR pathogen. Noteworthy, 16.3% (34/209) of patients developed at least one, in the majority of cases non-serious, adverse drug reaction during fosfomycin therapy.

CONCLUSION

Our data suggest that IV fosfomycin is an effective and safe combination partner for the treatment of a broad spectrum of severe bacterial infections in critically ill patients.

A study on combination of daptomycin with selected antimicrobial agents: in vitro synergistic effect of MIC value of 1 mg/L against MRSA strains

Background

Daptomycin is an important drug used in the treatment of methicillin-resistant Staphylococcus aureus (MRSA) infection. A high dose of daptomycin is indicated for an MRSA infection with a minimum inhibitory concentration (MIC) of 1 mg/L for daptomycin. Combination therapies with daptomycin and other antimicrobial agents, including fosfomycin, display in vitro synergism potentially. This study was conducted to investigate the in vitro synergistic effect of daptomycin-based combination therapy against MRSA strains with high daptomycin MIC.

Method

The synergistic effects of daptomycin in combination with fosfomycin, gentamicin, linezolid, oxacillin, or rifampicin against MRSA with an MIC of 1 mg/L for daptomycin were measured using the microbroth checkerboard assay in vitro.

Result

A total of 100 MRSA isolates was tested. The synergistic interactions of the drugs were evaluated using the fractional inhibitory concentration index. The MIC values revealed that all isolates (100%) were found to be susceptible to linezolid, 85% to fosfomycin, 8% to gentamicin, 69% to rifampicin, and no isolate was susceptible to oxacillin. The in vitro synergism rates of daptomycin in combination with fosfomycin, oxacillin, gentamicin, linezolid, and rifampicin were 37, 11, 5, 3, and 1%, respectively.

Conclusion

The combination of daptomycin plus fosfomycin may be an effective therapeutic option for MRSA infection.

Electronic supplementary material

The online version of this article (10.1186/s40360-019-0305-y) contains supplementary material, which is available to authorized users.

Fosfomycin in the pediatric setting: Evidence and potential indications

To date, there has been little experience in using fosfomycin in children. However, its broad spectrum of action and excellent safety profile have renewed interest in this antibiotic, especially for treating infections by multidrug-resistant bacteria. The main indication for fosfomycin in pediatrics is currently community-acquired lower urinary tract infection. Given its good activity against bacteria, fosfomycin can also be useful in urinary infections caused by extended-spectrum beta-lactamase-producing enterobacteria. Fosfomycin presents very good dissemination to tissues including bone and is therefore an option in the combined therapy of osteomyelitis, especially in cases produced by methicillin-resistant Staphylococcus aureus (MRSA) or in cases with beta-lactam allergies. Fosfomycin can also be employed in combination for multidrug-resistant Gram-negative bacteremia (especially carbapenemase-producing enterobacteria), S. aureus (if there is a high suspicion of MRSA or complicated infections) and vancomycin-resistant Enterococcus spp. Other infections in which fosfomycin could be part of a combined therapy include staphylococcal endocarditis (in case of beta-lactam allergy or MRSA), central nervous system infections (mainly by MRSA, S. epidermidis, Listeria and resistant pneumococcus), nosocomial pneumonia and infections associated with mechanical ventilation.

New evidence on the use of fosfomycin for bacteremia and infectious endocarditis

There is growing concern regarding the increased resistance rates of numerous pathogens and the limited availability of new antibiotics against these pathogens. In this context, fosfomycin is of considerable interest due to its activity against a wide spectrum of these microorganisms. We will review the encouraging data on this issue regarding the use of fosfomycin in treating Gram-negative bacterial infections. We will also cover fosfomycin's role against 2 of the main causal agents of bacteremia and endocarditis worldwide (nosocomial and community-acquired): enterococci, whose growing resistance to glycopeptides and aminoglycosides represents a serious threat, and methicillin-resistant Staphylococcus aureus, whose infection, despite efforts, continues to be associated with high morbidity and mortality and a high risk of complications. Thanks also to its considerable synergistic capacity with various antibiotics, fosfomycin is a tool for extending the therapeutic arsenal against these types of infections.

Antibiotic Resistance and the MRSA Problem

Staphylococcus aureus is capable of becoming resistant to all classes of antibiotics clinically available and resistance can develop through de novo mutations in chromosomal genes or through acquisition of horizontally transferred resistance determinants. This review covers the most important antibiotics available for treatment of S. aureus infections and a special emphasis is dedicated to the current knowledge of the wide variety of resistance mechanisms that S. aureus employ to withstand antibiotics. Since resistance development has been inevitable for all currently available antibiotics, new therapies are continuously under development. Besides development of new small molecules affecting cell viability, alternative approaches including anti-virulence and bacteriophage therapeutics are being investigated and may become important tools to combat staphylococcal infections in the future.

ZTI-01 (fosfomycin for injection) in the treatment of hospitalized patients with complicated urinary tract infections

Fosfomycin is a bactericidal antibiotic available since the 1970s whose intravenous formulation has been available in many countries outside the USA. Given the rise in drug-resistant bacteria, its introduction into the US market has become a necessity for addressing these organisms. This review provides an overview of the microbiology, clinical pharmacology and initial clinical experiences of the intravenous fosfomycin product (ZTI-01) that is undergoing clinical development in the USA for the treatment of complicated urinary tract infections and acute pyelonephritis.

Fosfomycin: mechanisms and the increasing prevalence of resistance

There are challenges regarding increased global rates of microbial resistance and the emergence of new mechanisms that result in microorganisms becoming resistant to antimicrobial drugs. Fosfomycin is a broad-spectrum bactericidal antibiotic effective against Gram-negative and certain Gram-positive bacteria, such as Staphylococci, that interfere with cell wall synthesis. During the last 40 years, fosfomycin has been evaluated in a wide range of applications and fields. Although numerous studies have been done in this area, there remains limited information regarding the prevalence of resistance. Therefore, in this review, we focus on the available data concerning the mechanisms and increasing resistance regarding fosfomycin.

Management of complicated skin and soft tissue infections with a special focus on the role of newer antibiotics

Complicated skin and soft tissue infections (cSSTIs) represent the severe form of infectious disease that involves deeper soft tissues. Involvement of methicillin-resistant Staphylococcus aureus (MRSA) further complicates cSSTI with increased hospitalization, health care costs, and overall mortality. Various international guidelines provide recommendations on the management of cSSTIs, with the inclusion of newer antibiotics. This literature-based review discusses the overall management of cSSTI, including appropriate use of antibiotics in clinical practice. Successful treatment of cSSTIs starts with early and precise diagnosis, including identification of causative pathogen and its load, determination of infection severity, associated complications, and risk factors. The current standard-of-care for cSSTIs involves incision, drainage, surgical debridement, broad-spectrum antibiotic therapy, and supportive care. In recent years, the emergence of newer antibiotics (eg, ceftaroline, tigecycline, daptomycin, linezolid, etc) has provided clinicians wider options of antimicrobial therapy. Selection of antibiotics should be based on the drug characteristics, effectiveness, safety, and treatment costs, alongside other aspects such as host factors and local multidrug resistance rates. However, larger studies on newer antibiotics are warranted to refine the decision making on the appropriate antimicrobial therapy. Local Antimicrobial Stewardship Program strategies in health care settings could guide clinicians for early initiation of specific treatments to combat region-specific antimicrobial resistance, minimize adverse effects, and to improve outcomes such as reduction in Clostridium difficile infections. These strategies involving iv-to-oral switch, de-escalation to narrow-spectrum antibiotics, and dose optimization have an impact on the overall improvement of cSSTI therapy outcomes, especially in countries like Singapore that has a high disease burden.

Resistance to fosfomycin: Mechanisms, Frequency and Clinical Consequences

Fosfomycin has been used for the treatment of infections due to susceptible and multidrug-resistant (MDR) bacteria. It inhibits bacterial cell wall synthesis through a unique mechanism of action at a step prior to that inhibited by β-lactams. Fosfomycin enters the bacterium through membrane channels/transporters and inhibits MurA, which initiates peptidoglycan (PG) biosynthesis of the bacterial cell wall. Several bacteria display inherent resistance to fosfomycin mainly through MurA mutations. Acquired resistance involves, in order of decreasing frequency, modifications of membrane transporters that prevent fosfomycin from entering the bacterial cell, acquisition of plasmid-encoded genes that inactivate fosfomycin, and MurA mutations. Fosfomycin resistance develops readily in vitro but less so in vivo. Mutation frequency is higher among Pseudomonas aeruginosa and Klebsiella spp. compared with Escherichia coli and is associated with fosfomycin concentration. Mutations in cAMP regulators, fosfomycin transporters and MurA seem to be associated with higher biological cost in Enterobacteriaceae but not in Pseudomonas spp. The contribution of fosfomycin inactivating enzymes in emergence and spread of fosfomycin resistance currently seems low-to-moderate, but their presence in transferable plasmids may potentially provide the best means for the spread of fosfomycin resistance in the future. Their co-existence with genes conferring resistance to other antibiotic classes may increase the emergence of MDR strains. Although susceptibility rates vary, rates seem to increase in settings with higher fosfomycin use and among multidrug-resistant pathogens.

Clinical efficacy of fosfomycin combinations against a variety of gram-positive cocci

INTRODUCTION

Over recent years we have witnessed an increase in the resistance of microorganisms to the available antimicrobials and a decrease in the number of new antimicrobials. Fosfomycin is a safe and cheap broad-spectrum antibiotic which has shown very promising results in combination therapy, mainly against gram-negative microorganisms. Little is known, however, about its clinical efficacy against gram-positive microorganisms.

METHODS

We performed a retrospective review of all patients with severe gram-positive infections who received fosfomycin as part of their treatment from 2011 to 2017. We also performed in vitro time-kill assays to study the behaviour of fosfomycin with different antimicrobials against two strains of methicillin-resistant Staphylococcus aureus (MRSA) and two strains of methicillin-susceptible S. aureus (MSSA).

RESULTS

Seventy-five patients were treated with different fosfomycin combinations. Among them, 61 (81%) were successfully treated. Daptomycin plus fosfomycin was the most effective combination. Overall, the treatment with fosfomycin was safe, and side effects were minor. There was only one major side effect that resolved after discontinuation of therapy. Time-kill studies demonstrated increased activity of fosfomycin combinations, with daptomycin-fosfomycin being the most active combination against both MRSA and MSSA strains.

CONCLUSIONS

Our results suggest that antimicrobial combinations including fosfomycin are an alternative and effective approach for gram-positive infections.

High-Level Fosfomycin Resistance in Vancomycin-Resistant Enterococcus faecium

Of 890 vancomycin-resistant Enterococcus faecium isolates obtained by rectal screening from patients in Pittsburgh, Pennsylvania, USA, 4 had MICs >1,024 μg/mL for fosfomycin. These isolates had a Cys119Asp substitution in the active site of UDP-N-acetylglucosamine enolpyruvyl transferase. This substitution increased the fosfomycin MIC >4-fold and rendered this drug inactive in biochemical assays.

Prevalence of Fosfomycin Resistance in Methicillin-Resistant Staphylococcus aureus Isolated from Patients in a University Hospital in China from 2013 to 2015

In this study, we investigated the fosfomycin susceptibility rates among different methicillin-resistant Staphylococcus aureus (MRSA) clones. A total of 293 MRSA isolates obtained from Sir Run Run Shaw hospital during 2013-2015 were tested for fosfomycin susceptibility. The overall fosfomycin resistance rate among these MRSA isolates was 53.2%. Although 91.9% of the ST5 MRSA isolates (MIC50>1,024 mg/L) were resistant to fosfomycin, no fosfomycin-resistant isolate was found among the 69 ST59 MRSA isolates (MIC50/90, 0.5/4 mg/L). The fosfomycin resistance rate among the MRSA isolates recovered from skin and soft tissue infections was 19.1%, which was lower than the rates detected among MRSA isolates from other types of invasive infections. The fosfomycin resistance rate in community-onset MRSA was 30.2%, which was lower than that detected in healthcare-associated MRSA of 70.7%. One MRSA isolate had the fosB7 gene, whereas most (127/156) of the fosfomycin-resistant MRSA isolates had deletions in glpT genes. These findings highlight the importance of monitoring the fosfomycin susceptibility in MRSA isolates for epidemiological purposes.

Evaluation of perioperative prophylaxis with fosfomycin tromethamine in ureteroscopic stone removal: an investigator-driven prospective, multicenter, randomized, controlled study

PURPOSE

To compare efficacy, safety, and cost-effectiveness of fosfomycin tromethamine with other standard-of-care antibiotics in patients undergoing ureteroscopic lithotripsy.

METHODS

This study was a prospective, multicenter, randomized, controlled trial. Eligible patients scheduled for ureteroscopic lithotripsy were randomly assigned to receive either fosfomycin (fosfomycin group, N = 101 patients) or standard-of-care antibiotic therapy as prophylaxis (control group, N = 115 patients). The incidence of infectious complications and adverse events was analyzed between the two groups, as well as the cost-benefit analysis.

RESULTS

The incidence of infections following lithotripsy was 3.0% in the fosfomycin group and 6.1% in the control group (p > 0.05). Only asymptomatic bacteriuria was reported in fosfomycin group. In the control group was reported asymptomatic bacteriuria (3.5%), fever (0.9%), bacteremia (0.9%), and genitourinary infection (0.9%). The rate of adverse events was very low, with no adverse event reported in the fosfomycin group and only one in the control group (forearm phlebitis). The average cost per patient of antibiotic therapy with fosfomycin was 151.45 ± 8.62 yuan (22.7 ± 1.3 USD), significantly lower compared to the average cost per patient of antibiotics used in the control group 305.10 ± 245.95 yuan (45.7 ± 36.9 USD; p < 0.001).

CONCLUSIONS

Two oral doses of 3 g fosfomycin tromethamine showed good efficacy and safety and low cost in perioperative prophylaxis of infections following ureteroscopic stone removal.

In vitro and in vivo activity of fosfomycin alone and in combination with rifampin and tigecycline against Gram-positive cocci isolated from surgical wound infections

Complicated skin and soft tissue infections constitute a heterogeneous group of severe disorders, with surgical site infections being the most common hospital-acquired ones. The aim of our study was to investigate the synergistic and bactericidal activities of antimicrobial combinations of fosfomycin with rifampicin and tigecycline against Enterococcus faecalis, Enterococcus faecium and methicillin-resistant Staphylococcus aureus (MRSA) clinical isolates, and also to evaluate their in vivo effects in a mouse wound infection model. In in vitro studies, the combinations of fosfomycin with rifampicin and tigecycline were both synergistic. These synergies were confirmed in in vivo studies: the drug combinations showed the highest antimicrobial effects compared to monotherapy. In conclusion, the efficacy of fosfomycin combinations, also confirmed in our in vivo model, may suggest new directions in the treatment of infected skin and a possible alternative way to control bacterial skin infection.

Vigilance on use of drugs, herbal products, and food supplements during pregnancy: focus on fosfomycin

PURPOSE

Urinary tract infection (UTI) is defined as a common bacterial infection that can lead to significant morbidity such as stricture, fistula, abscess formation, bacteremia, sepsis, pyelonephritis, and kidney dysfunction with a mortality rates reported of 1% in men and 3% in women because of development of pyelonephritis. UTIs are more common in women and the 33% of them require antimicrobials treatment for at least one episode by the age of 24 years. UTIs are the most common infections observed during pregnancy and up to 30% of mothers with not treated asymptomatic bacteriuria may develop acute pyelonephritis which consequently can be associated to adverse maternal and fetal outcomes. All bacteriuria in pregnancy should be treated with antimicrobial treatments being safe for both the mother and the fetus. Approximately one every four women receives prescription of antibiotic treatment during pregnancy, nearly 80% of all the prescription medications during gestation. The use of fosfomycin to treat cystitis in pregnancy generally considered safe and effective. Even though use on antibiotics for urinary tract infections is considered generally safe for the fetus and mothers, this opinion is not based on specific studies monitoring the relationship of among urinary infections, consumption of antibiotics, and pregnancy outcomes.

MATERIALS AND METHODS

On this basis we decided to analyze data from the database of our multicenter study PHYTOVIGGEST, reporting data from 5362 pregnancies, focusing on use of fosfomycin. Principal outcomes of pregnancy in women treated with fosfomycin were taken into consideration.

RESULTS

Women who have been treated with urinary antibiotics during the pregnancy were 183. With respect to the total number of pregnancies of our sample, these women represented the percentage of 3.49% (187/5362). Analysis of different outcomes of pregnancy such as gestational age, neonatal weight, and neonatal Apgar index did not show any significant difference. At the same time, analysis of data of pregnancy complicancies (such as urgent cesarean delivery, use of general anesthesia, need to induce labor) did not show any difference in women taking fosfomycin during pregnancy and those not taking it.

CONCLUSIONS

Our data, based on a large number of pregnancies, confirm the safety use of fosfomycin use in pregnancy.

Linezolid activity against clinical Gram-positive cocci with advanced antimicrobial drug resistance in Iran

OBJECTIVES

The aim of the study was to investigate the linezolid activity against clinical Gram-positive cocci with advanced antimicrobial drug resistance.

METHODS

A collection of methicillin resistant Staphylococcus aureus (MRSA), vancomycin resistant enterococci (VRE), penicillin non-susceptible Streptococcus pneumoniae (PNSP), and group B streptococci (GBS) were isolated from various clinical samples. Antimicrobial susceptibility tests were done using standard methods Subsequently, we investigated linezolid antibacterial activities, the first approved oxazolidinone against isolates by the standard broth microdilution method.

RESULTS

According to our results, MRSA and PNSP isolates were multidrug resistant, and almost half of the VRE isolates were high level gentamicin resistant (HLGR). Furthermore, resistance to linezolid was not seen among the isolates. The MIC₉₀ values for MRSA, VRE, PNSP and GBS isolates were 4μg/ml, 2μg/ml, 1μg/ml, and 0.5μg/ml, respectively. Only 6.25% of vancomycin resistant enterococci showed intermediate susceptibility to this antibiotic.

CONCLUSIONS

These findings indicate that linezolid has an excellent activity against clinical drug resistant Gram-positive isolates in Iran. Constant monitoring and surveillance of linezolid MIC distribution allows the researchers to assess and detect gradual upward MIC drifts.

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.

Mutations of the Transporter Proteins GlpT and UhpT Confer Fosfomycin Resistance in Staphylococcus aureus

With the increasing spread of methicillin-resistant Staphylococcus aureus worldwide, fosfomycin has begun to be used more often, either alone or in combination with other antibiotics, for treating methicillin-resistant S. aureus infections, resulting in the emergence of fosfomycin-resistant strains. Fosfomycin resistance is reported to be mediated by fosfomycin-modifying enzymes (FosA, FosB, FosC, and FosX) and mutations of the target enzyme MurA or the membrane transporter proteins UhpT and GlpT. Our previous studies indicated that the fos genes might not the major fosfomycin resistance mechanism in S. aureus, whereas mutations of glpT and uhpT seemed to be more related to fosfomycin resistance. However, the precise role of these two genes in S. aureus fosfomycin resistance remains unclear. The aim of the present study was to investigate the role of glpT and uhpT in S. aureus fosfomycin resistance. Homologous recombination was used to knockout the uhpT and glpT genes in S. aureus Newman. Gene complementation was generated by the plasmid pRB473 carrying these two genes. The fosfomycin minimal inhibitory concentration (MIC) of the strains was measured by the E-test to observe the influence of gene deletion on antibiotic susceptibility. In addition, growth curves were constructed to determine whether the mutations have a significant influence on bacterial growth. Deletion of uhpT, glpT, and both of them led to increased fosfomycin MIC 0.5 μg/ml to 32 μg/ml, 4 μg/ml, and >1024 μg/ml, respectively. By complementing uhpT and glpT into the deletion mutants, the fosfomycin MIC decreased from 32 to 0.5 μg/ml and from 4 to 0.25 μg/ml, respectively. Moreover, the transporter gene-deleted strains showed no obvious difference in growth curves compared to the parental strain. In summary, our study strongly suggests that mutations of uhpT and glpT lead to fosfomycin resistance in S. aureus, and that uhpT mutation may play a more important role. The high resistance and low biological fitness cost resulting from uhpT and glpT deletion suggest that these strains might have an evolutionary advantage in a fosfomycin-rich clinical situation, which should be closely monitored.

Fosfomycin

The treatment of bacterial infections suffers from two major problems: spread of multidrug-resistant (MDR) or extensively drug-resistant (XDR) pathogens and lack of development of new antibiotics active against such MDR and XDR bacteria. As a result, physicians have turned to older antibiotics, such as polymyxins, tetracyclines, and aminoglycosides. Lately, due to development of resistance to these agents, fosfomycin has gained attention, as it has remained active against both Gram-positive and Gram-negative MDR and XDR bacteria. New data of higher quality have become available, and several issues were clarified further. In this review, we summarize the available fosfomycin data regarding pharmacokinetic and pharmacodynamic properties, the in vitro activity against susceptible and antibiotic-resistant bacteria, mechanisms of resistance and development of resistance during treatment, synergy and antagonism with other antibiotics, clinical effectiveness, and adverse events. Issues that need to be studied further are also discussed.

Efficacy of Linezolid and Fosfomycin in Catheter-Related Biofilm Infection Caused by Methicillin-Resistant Staphylococcus aureus

As long-standing clinical problems, catheter-related infections and other chronic biofilm infections are more difficult to treat due to the high antibiotic resistance of biofilm. Therefore, new treatments are needed for more effective bacteria clearance. In this study, we evaluated the antibacterial activities of several common antibiotics alone and their combinations against biofilm-embedded methicillin-resistant staphylococcus aureus (MRSA) infections, both in vitro and in vivo. In brief, fosfomycin, levofloxacin, and rifampin alone or in combination with linezolid were tested in vitro against planktonic and biofilm-embedded MRSA infection in three MRSA stains. The synergistic effects between linezolid and the other three antibiotics were assessed by fractional inhibitory concentration index (FICI) and time-kill curves, where the combination of linezolid plus fosfomycin showed the best synergistic effect in all strains. For further evaluation in vivo, we applied the combination of linezolid and fosfomycin in a catheter-related biofilm rat model and found that viable bacteria counts in biofilm were significantly reduced after treatment (P < 0.05). In summary, we have shown here that the combination of linezolid and fosfomycin treatment had improved therapeutic effects on biofilm-embedded MRSA infection both in vitro and in vivo, which provided important basis for new clinical therapy development.

Prevalence of Fosfomycin Resistance and Mutations in murA, glpT, and uhpT in Methicillin-Resistant Staphylococcus aureus Strains Isolated from Blood and Cerebrospinal Fluid Samples

In China, fosfomycin alone or in combination with other antibiotics is commonly used in the treatment of infections caused by Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus (MRSA). Although fosfomycin-resistant S. aureus strains have continued to emerge and increase, the research on them is rare. In order to determine the prevalence and mechanisms of fosfomycin resistance in MRSA clinical isolates, a total of 96 non-duplicate MRSA isolates were collected from blood and cerebrospinal fluid samples at Huashan Hospital in Shanghai, China between 2004 and 2014. Antimicrobial susceptibility testing was performed by agar dilution. Meanwhile, the fosfomycin-resistance-related genes, fosB, murA, glpT, and uhpT, were amplified by PCR and subjected to sequencing analysis. Multilocus sequence typing (MLST) was conducted to assess strain types. The minimum inhibitory concentration (MIC) of fosfomycin for the 96 MRSA strains ranged from 1.0 to >1,024 mg/L, and approximately 70% (67/96) of the isolates were resistant to fosfomycin (MIC ≥ 64.0 mg/L). Nine isolates with MICs ≥ 128 mg/L carried fosB gene. Twenty-five distinct mutations were detected in the murA (7), glpT (10), and uhpT (8) genes. While five of the murA mutations and five of the glpT mutations were observed only in fosfomycin-sensitive isolates and one of the murA mutation was found both in fosfomycin-resistant and fosfomycin-sensitive isolates, the remaining 14 mutations (1 murA, 5 glpT, and all uhpT mutations) were present only in fosfomycin-resistant isolates. MLST analysis demonstrated that the majority (46/67) of the glpT and/or uhpT mutants belong to ST5, the predominant sequence type among the fosfomycin-resistant MRSA isolates. In conclusion, there is a high rate of fosfomycin resistance in MRSA strains. The mutations in the murA, glpT, and uhpT genes are common in fosfomycin-resistant MRSA strains, and may play a greater role in the development of fosfomycin resistance than the presence of the fosB gene in these organisms.

Fosfomycin plus β-Lactams as Synergistic Bactericidal Combinations for Experimental Endocarditis Due to Methicillin-Resistant and Glycopeptide-Intermediate Staphylococcus aureus

The urgent need of effective therapies for methicillin-resistant Staphylococcus aureus (MRSA) infective endocarditis (IE) is a cause of concern. We aimed to ascertain the in vitro and in vivo activity of the older antibiotic fosfomycin combined with different beta-lactams against MRSA and glycopeptide-intermediate-resistant S. aureus (GISA) strains. Time-kill tests with 10 isolates showed that fosfomycin plus imipenem (FOF+IPM) was the most active evaluated combination. In an aortic valve IE model with two strains (MRSA-277H and GISA-ATCC 700788), the following intravenous regimens were compared: fosfomycin (2 g every 8 h [q8h]) plus imipenem (1 g q6h) or ceftriaxone (2 g q12h) (FOF+CRO) and vancomycin at a standard dose (VAN-SD) (1 g q12h) and a high dose (VAN-HD) (1 g q6h). Whereas a significant reduction of MRSA-227H load in the vegetations (veg) was observed with FOF+IPM compared with VAN-SD (0 [interquartile range [IQR], 0 to 1] versus 2 [IQR, 0 to 5.1] log CFU/g veg; P = 0.01), no statistical differences were found with VAN-HD. In addition, FOF+IPM sterilized more vegetations than VAN-SD (11/15 [73%] versus 5/16 [31%]; P = 0.02). The GISA-ATCC 700788 load in the vegetations was significantly lower after FOF+IPM or FOF+CRO treatment than with VAN-SD (2 [IQR, 0 to 2] and 0 [IQR, 0 to 2] versus 6.5 [IQR, 2 to 6.9] log CFU/g veg; P < 0.01). The number of sterilized vegetations after treatment with FOF+CRO was higher than after treatment with VAN-SD or VAN-HD (8/15 [53%] versus 4/20 [20%] or 4/20 [20%]; P = 0.03). To assess the effect of FOF+IPM on penicillin binding protein (PBP) synthesis, molecular studies were performed, with results showing that FOF+IPM treatment significantly decreased PBP1, PBP2 (but not PBP2a), and PBP3 synthesis. These results allow clinicians to consider the use of FOF+IPM or FOF+CRO to treat MRSA or GISA IE.

No Outbreak of Vancomycin and Linezolid Resistance in Staphylococcal Pneumonia over a 10-Year Period

BACKGROUND

Staphylococci can cause wound infections and community- and nosocomial-acquired pneumonia, among a range of illnesses. Staphylococcus aureus and methicillin-resistant S. aureus (MRSA) have been rapidly increasing as a cause of infections worldwide in recent decades. Numerous reports indicate that S. aureus and MRSA are becoming resistant to many antibiotics, which makes them very dangerous. Therefore, this study retrospectively investigated the resistance to antimicrobial agents in all hospitalized patients suffering from community- or nosocomial-acquired pneumonia due to S. aureus and MRSA.

METHODS

Information from the study groups suffering from either community- or nosocomial-acquired pneumonia caused by S. aureus or MRSA was gathered by searching records from 2004 to 2014 at the HELIOS Clinic Wuppertal, Witten/Herdecke University, Germany. The findings of antibiotic resistance were analyzed after the evaluation of susceptibility testing for S. aureus and MRSA.

RESULTS

Total of 147 patients (63.9%, 95% CI 57.5%-69.8%), mean age 67.9 ± 18.5 years, with pneumonia triggered by S. aureus, and 83 patients (36.1%, 95% CI 30.2%-42.5%), mean age 72.3 ± 13.8 years, with pneumonia due to MRSA. S. aureus and MRSA developed no resistance to vancomycin (P = 0.019 vs. < 0.0001, respectively) or linezolid (P = 0.342 vs. < 0.0001, respectively). MRSA (95.3%) and S. aureus (56.3%) showed a high resistance to penicillin. MRSA (87.7%) was also found to have a high antibiotic resistance against ß-lactam antibiotics, compared to S. aureus (9.6%). Furthermore, MRSA compared to S. aureus, respectively, had increased antibiotic resistance to ciprofloxacin (90.1% vs. 17.0%), cefazolin (89.7% vs. 10.2%), cefuroxime (89.0% vs. 9.1%), levofloxacin (88.2% vs. 18.4%), clindamycin (78.0% vs. 14.7%), and erythromycin (76.5% vs. 20.8%).

CONCLUSION

No development of resistance was found to vancomycin and linezolid in patients with pneumonia caused by S. aureus and MRSA.

In vitro pharmacodynamics of fosfomycin against clinical isolates of Pseudomonas aeruginosa

BACKGROUND

The use of fosfomycin for treatment of systemic infections due to MDR Pseudomonas aeruginosa is increasing. However, pharmacodynamic data for fosfomycin are limited.

METHODS

Sixty-four clinical isolates of P. aeruginosa (MDR and non-MDR) from two Australian hospitals were collected; 59 isolates were from patients with cystic fibrosis and 5 isolates were from critically ill patients. The in vitro pharmacodynamic properties of fosfomycin (disodium) were investigated via MICs (all isolates) and, for selected isolates, via time-kill kinetics (static and dynamic models; concentration range, 1-1024 mg/L), population analysis profiles (PAPs) and post-antibiotic effect (PAE). Two inocula (∼10(6) and ∼10(8) cfu/mL) were included in static time-kill studies to examine the effect of inocula on bacterial killing.

RESULTS

MICs ranged from 1 to >512 mg/L, with 61% of isolates considered fosfomycin susceptible (MIC ≤64 mg/L). The MIC distributions for MDR and non-MDR isolates were similar. Baseline PAPs indicated heteroresistance in all isolates tested. Time-kill studies showed moderate (maximum killing ∼3 log10 cfu/mL), time-dependent killing at the low inoculum with regrowth at 24 h. Most concentrations resulted in complete replacement of fosfomycin-susceptible colonies by fosfomycin-resistant colonies. Bacterial killing was virtually eliminated at the high inoculum. The PAE ranged from 0.3 to 5.5 h.

CONCLUSIONS

These data suggest monotherapy with fosfomycin may be problematic for the treatment of infections caused by P. aeruginosa. Further investigation of fosfomycin combination therapy is warranted.

Enterococcal endocarditis revisited

The Enterococcus species is the third main cause of infective endocarditis (IE) worldwide, and it is gaining relevance, especially among healthcare-associated cases. Patients with enterococcal IE are older and have more comorbidities than other types of IE. Classical treatment options are limited due to the emergence of high-level aminoglycosides resistance (HLAR), vancomycin resistance and multidrug resistance in some cases. Besides, few new antimicrobial alternatives have shown real efficacy, despite some of them being recommended by major guidelines (including linezolid and daptomycin). Ampicillin plus ceftriaxone 2 g iv./12 h is a good option for Enterococcus faecalis IE caused by HLAR strains, but randomized clinical trials are essential to demonstrate its efficacy for non-HLAR EFIE and to compare it with ampicillin plus short-course gentamicin. The main mechanisms of resistance and treatment options are also reviewed for other enterococcal species.

Treatment of methicillin-resistant Staphylococcus aureus infections: Importance of high vancomycin minumum inhibitory concentrations

Staphylococcus aureus (SA) infections remain a major cause of morbidity and mortality despite the availability of numerous effective anti-staphylococcal antibiotics. This organism is responsible for both nosocomial and community-acquired infections ranging from relatively minor skin and soft tissue infections to life-threatening systemic infections. The increasing incidence of methicillin-resistant strains has granted an increasing use of vancomycin causing a covert progressive increase of its minimum inhibitory concentration (MIC) (dubbed the MIC “creep”). In this way, the emergence of vancomycin-intermediate SA (VISA) strains and heteroresistant-VISA has raised concern for the scarcity of alternative treatment options. Equally alarming, though fortunately less frequent, is the emergence of vancomycin-resistant SA. These strains show different mechanisms of resistance but have similar problems in terms of therapeutic approach. Ultimately, various debate issues have arisen regarding the emergence of SA strains with a minimum inhibitory concentration sitting on the superior limit of the sensitivity range (i.e., MIC = 2 μg/mL). These strains have shown certain resilience to vancomycin and a different clinical behaviour regardless of vancomycin use, both in methicillin-resistant SA and in methicillin-sensitive SA. The aim of this text is to revise the clinical impact and consequences of the emergence of reduced vancomycin susceptibility SA strains, and the different optimal treatment options known.

Improved treatment of multidrug-resistant bacterial infections: utility of clinical studies

In a time of increasing antibacterial resistance and limited availability of new antibiotics, clinical studies are much needed to assess treatment options against multidrug-resistant organisms (MDROs). In this review, we describe the clinical challenge caused by MDROs and present recent evidence on how clinical studies may generate quality data to improve antibiotic treatment of MDRO infections. To this aim, we critically assess the current status, gaps and challenges associated with observational and interventional studies performed to assess MDRO treatment options. We address why observational studies are useful, which treatment options for MDRO have been explored by observational studies and how to improve quality and usefulness of observational studies. Furthermore, the utility of clinical pharmacokinetic/pharmacodynamic studies for improving MDRO treatment is described. Finally, we discuss interventional study designs, end points and margins, as well as ethical, logistic and statistical challenges, and current regulatory changes proposed to foster the development of new antibiotics.

Testing the mutant selection window hypothesis in vitro and in vivo with Staphylococcus aureus exposed to fosfomycin

The purpose of this study was to test the mutant selection window (MSW) hypothesis in vitro and in vivo with Staphylococcus aureus exposed to fosfomycin. With the in vitro time-kill studies, S. aureus ATCC 29213 [with a minimal concentration that inhibits colony formation by 99% (MIC99) of 2.2 μg/mL and a mutant prevention concentration (MPC) of 57.6 μg/mL] lost fosfomycin susceptibility at antibiotic concentrations (2×, 4×, and 8× MIC) that are between the lower and upper boundaries of the MSW. In the tissue-cage model, S. aureus was exposed to fosfomycin pharmacokinetics at concentrations below the MIC99, between the MIC99 and the MPC, and above the MPC, respectively. Changes in susceptibility and counts of total and resistant viable bacteria were monitored in tissue-cage fluid obtained daily. However, the selection of resistant mutants was not observed during antibacterial treatment and 48 h after the termination of fosfomycin treatment, regardless of the fosfomycin dosage. Besides, we found no differences between the in vitro-isolated mutant and its sensitive parental strain, which indicates the absence of fitness cost of fosfomycin resistance in S. aureus ATCC 29213. These findings demonstrate that agar plate determinations do not fit the MSW for fosfomycin treatment of rabbits infected with S. aureus ATCC 29213; therefore, the existence of the window must be demonstrated not only in vitro but also in vivo. Further research is needed on the exact mechanism of resistance.