Evaluation of pooled human urine and synthetic alternatives in a dynamic bladder infection in vitro model simulating oral fosfomycin therapy

Antimicrobials: An update on new strategies to diversify treatment for bacterial infections

Ninety-five years after Fleming's discovery of penicillin, a bounty of antibiotic compounds have been discovered, modified, or synthesised. Diversification of target sites, improved stability and altered activity spectra have enabled continued antibiotic efficacy, but overwhelming reliance and misuse has fuelled the global spread of antimicrobial resistance (AMR). An estimated 1.27 million deaths were attributable to antibiotic resistant bacteria in 2019, representing a major threat to modern medicine. Although antibiotics remain at the heart of strategies for treatment and control of bacterial diseases, the threat of AMR has reached catastrophic proportions urgently calling for fresh innovation. The last decade has been peppered with ground-breaking developments in genome sequencing, high throughput screening technologies and machine learning. These advances have opened new doors for bioprospecting for novel antimicrobials. They have also enabled more thorough exploration of complex and polymicrobial infections and interactions with the healthy microbiome. Using models of infection that more closely resemble the infection state in vivo, we are now beginning to measure the impacts of antimicrobial therapy on host/microbiota/pathogen interactions. However new approaches are needed for developing and standardising appropriate methods to measure efficacy of novel antimicrobial combinations in these contexts. A battery of promising new antimicrobials is now in various stages of development including co-administered inhibitors, phages, nanoparticles, immunotherapy, anti-biofilm and anti-virulence agents. These novel therapeutics need multidisciplinary collaboration and new ways of thinking to bring them into large scale clinical use.

Raising the alarm: fosfomycin resistance associated with non-susceptible inner colonies imparts no fitness cost to the primary bacterial uropathogen

IMPORTANCE

While fosfomycin resistance is rare, the observation of non-susceptible subpopulations among clinical Escherichia coli isolates is a common phenomenon during antimicrobial susceptibility testing (AST) in American and European clinical labs. Previous evidence suggests that mutations eliciting this phenotype are of high biological cost to the pathogen during infection, leading to current recommendations of neglecting non-susceptible colonies during AST. Here, we report that the most common route to fosfomycin resistance, as well as novel routes described in this work, does not impair virulence in uropathogenic E. coli, the major cause of urinary tract infections, suggesting a re-evaluation of current susceptibility guidelines is warranted.

Ascorbate and Antibiotics, at Concentrations Attainable in Urine, Can Inhibit the Growth of Resistant Strains of Escherichia coli Cultured in Synthetic Human Urine

There are conflicting reports on the antibacterial activity of ascorbate; all at concentrations much higher than the typical in human plasma, but that can be reached in urine. The effect of 10 mM ascorbate (in itself not inhibitory) along with antibiotics, was tested both in Mueller-Hinton broth (MHb) and in synthetic human urine (SHU), against resistant isolates of Escherichia coli from lower urinary infections. The activity of nitrofurantoin and sulfamethoxazole was higher in SHU than in MHb; minimal inhibitory concentrations (MICs) in SHU with ascorbate were below typical urinary concentrations. For other antibiotics, MICs were the same in MHb vs. SHU, with no effect of ascorbate in MHb; but in SHU with ascorbate, MICs of ciprofloxacin and gentamicin also went below reported urinary concentrations, with a lesser effect with norfloxacin and trimethoprim, and none with ampicillin. The effect of ascorbate was independent of oxygen and not related to the susceptibility of each strain to oxidative stress. Ascorbate oxidizes during incubation in SHU, and bacterial growth partially prevented oxidation. These results suggest that 10 mM ascorbate can enhance the inhibitory activity of antibiotics upon resistant strains in urine. Clinical experimentation with ascorbate-antibiotic combinations against urinary infections caused by resistant bacteria is warranted.

Nitrofurantoin plasma- and urine exposure in eight healthy beagle dogs following standard nitrofurantoin dosing regimen

Bacterial cystitis is common in dogs and is usually treated with antibiotics. Nitrofurantoin is used for treatment of bacterial cystitis in humans and might provide a feasible treatment option in dogs. The aim of this study was to investigate the nitrofurantoin plasma concentration-time course and potential adverse effects in dogs. Nitrofurantoin (4.4-5.0 mg/kg) was administered orally to eight healthy beagles every 8 h for five days before repeated plasma and urine samples were collected. An additional four beagles served as untreated controls. The nitrofurantoin plasma and urine concentrations were measured using ultra high precision liquid chromatography coupled to tandem mass-spectrometry and further analysed using a non-compartmental pharmacokinetic model. In plasma, the median C_max was 2.1 μg/mL, t_max was 2 h, the terminal rate constant was 0.9 per h and the terminal half-life was 0.8 h. In urine, median C_max was 56 μg/mL, t_max was 1 h and the terminal half-life was 4.3 h. No adverse effects were observed clinically or in haematology or biochemistry. The data presented in this study combined with in vitro sensitivity data from common urine pathogens and the lack of observed adverse effects suggest that nitrofurantoin in a standard dosing regimen could be effective in sporadic bacterial cystitis treatment in dogs. Further clinical studies are highly warranted to verify the effectiveness in clinical cases.

High-Frequency Detection of fosA3 and bla CTX-M-55 Genes in Escherichia coli From Longitudinal Monitoring in Broiler Chicken Farms

Considering the worrying emergence of multidrug resistance, including in animal husbandry and especially in food-producing animals, the need to detect antimicrobial resistance strains in poultry environments is relevant, mainly considering a One Health approach. Thus, this study aimed to conduct longitudinal monitoring of antimicrobial resistance in broiler chicken farms, with an emphasis on evaluating the frequency of resistance to fosfomycin and β-lactams. Escherichia coli was isolated from broiler chicken farms (cloacal swabs, meconium, poultry feed, water, poultry litter, and Alphitobius diaperinus) in northern Paraná from 2019 to 2020 during three periods: the first period (1st days of life), the second period (20th to 25th days of life), and third period (40th to 42nd days of life). Antibiogram tests and the detection of phenotypic extended-spectrum β-lactamase (ESBL) were performed, and they were confirmed by seaching for genes from the bla_CTX–M group. The other resistance genes searched were mcr-1 and fosA3. Some ESBL bla_CTX–M–1 group strains were selected for ESBL identification by sequencing and enterobacterial repetitive intergenic consensus-polymerase chain reaction analysis. To determine the transferability of the bla_{CTX–M–1–} and fosA3-carrying plasmids, strains were subjected to conjugation experiments. A total of 507 E. coli were analyzed: 360 from cloacal swabs, 24 from meconium samples, 3 from poultry feed samples, 18 from water samples, 69 from poultry litter samples, and 33 from A. diaperinus samples. Among the strain isolate, 80% (406/507) were multidrug-resistant (MDR), and 51% (260/507) were ESBL-positive, with the bla_CTX–M–1 group being the most frequent. For the fosA3 gene, 68% (344/507) of the strains isolated were positive, deserves to be highlighted E. coli isolated from day-old chickens (OR 6.34, CI 2.34–17.17), when compared with strains isolated from other origins (poultry litter, A. diaperinus, water, and poultry feed). This work alerts us to the high frequency of the fosA3 gene correlated with the CTX-M-1 group (OR 3.57, CI 95% 2.7–4.72, p < 0.05), especially the bla_CTX–M–55 gene, in broiler chickens. This profile was observed mainly in day-old chicken, with a high percentage of E. coli that were MDR. The findings emphasize the importance of conducting longitudinal monitoring to detect the primary risk points during poultry production.

OUP accepted manuscript

Introduction

The use of oral fosfomycin for urinary tract infections (UTIs) caused by non-Escherichia coli uropathogens is uncertain, including Klebsiella pneumoniae, the second most common uropathogen.

Methods

A multicompartment bladder infection in vitro model was used with standard media and synthetic human urine (SHU) to simulate urinary fosfomycin exposure after a single 3 g oral dose (fAUC_0–72 16884 mg·h/L, t_½ 5.5 h) against 15 K. pneumoniae isolates including ATCC 13883 (MIC 2 to >1024 mg/L) with a constant media inflow (20 mL/h) and 4-hourly voiding of each bladder. The impact of the media (CAMHB + G6P versus SHU) on fosfomycin MIC measurements, drug-free growth kinetics and regrowth after fosfomycin administration was assessed. A low and high starting inoculum (5.5 versus 7.5 log₁₀ cfu/mL) was assessed in the bladder infection model.

Results

Compared with CAMHB, isolates in SHU had a slower growth rate doubling time (37.7 versus 24.1 min) and reduced growth capacity (9.0 ± 0.3 versus 9.4 ± 0.3 log₁₀ cfu/mL), which was further restricted with increased inflow rate (40 mL/h) and more frequent voids (2-hourly). Regrowth was commonly observed in both media with emergence of fosfomycin resistance promoted by a high starting inoculum in CAMHB (MIC rise to ≥1024 mg/L in 13/14 isolates). Resistance was rarely detected in SHU, even with a high starting inoculum (MIC rise to ≥1024 mg/L in 2/14 isolates).

Conclusions

Simulated in an in vitro UTI model, the regrowth of K. pneumoniae urinary isolates was inadequately suppressed following oral fosfomycin therapy. Efficacy was further reduced by a high starting inoculum.

Role of inorganic phosphate concentrations in in vitro activity of fosfomycin

OBJECTIVES

The objective of this study was to evaluate the in vitro activity of fosfomycin under different physiological concentrations of inorganic phosphate (P_i).

METHODS

The wild-type BW25113 strain, four isogenic mutants (ΔglpT, ΔuhpT, ΔglpT-uhpT, and ΔphoB) and six clinical isolates of Escherichia coli with different fosfomycin susceptibilities were used. EUCAST breakpoints were used. Susceptibility was evaluated by agar dilution using standard Mueller-Hinton agar (P_i concentration of 1 mM similar to human plasma concentration) and supplemented with P_i (13 and 42 mM, minimum and maximum urinary P_i concentrations) and/or glucose-6-phosphate (25 mg/L). Fosfomycin transporter promoter activity was assayed using PglpT::gfpmut2 or PuhpT::gfpmut2 promoter fusions in standard Mueller-Hinton Broth (MHB), supplemented with P_i (13 or 42 mM) ± glucose-6-phosphate. Fosfomycin activity was quantified, estimating fosfomycin EC₅₀ under different P_i concentrations (1, 13 and 42 mM + glucose-6-phosphate) and in time-kill assays using fosfomycin concentrations of 307 (maximum plasma concentration (C_max)), 1053 and 4415 mg/L (urine C_max range), using MHB with 28 mM P_i (mean urine P_i concentration) + 25 mg/L glucose-6-phosphate.

RESULTS

All the strains showed decreased susceptibility to fosfomycin linked to increased P_i concentrations: 1-4 log₂ dilution differences from 1 to 13 mM, and 1-8 log₂ dilution differences at 42 mM P_i. Changes in phosphate concentration did not affect the expression of fosfomycin transporters. By increasing P_i concentrations higher fosfomycin EC₅₀ bacterial viability was observed, except against ΔglpT-uhpT. The increase in P_i reduced the bactericidal effect of fosfomycin.

DISCUSSION

P_i variations in physiological fluids may reduce fosfomycin activity against E. coli. Elevated P_i concentrations in urine may explain oral fosfomycin failure in non-wild-type but fosfomycin-susceptible E. coli strains.

Pharmacokinetic/pharmacodynamic analysis of oral fosfomycin against Enterobacterales, Pseudomonas aeruginosa and Enterococcus spp. in an in vitro bladder infection model: impact on clinical breakpoints

OBJECTIVES

Fosfomycin is an established treatment for uncomplicated urinary tract infections (UTIs), yet evidence supporting susceptibility breakpoints is limited. We examine the UTI susceptibility criteria.

METHODS

Fosfomycin susceptibility, heteroresistance and in vitro growth in a bladder infection model, after a single 3 g dose of oral fosfomycin, were bridged to human pharmacokinetics with pharmacokinetic/pharmacodynamic and Monte Carlo analyses. Data from common uropathogens (24 Escherichia coli, 20 Klebsiella pneumoniae, 4 Enterobacter cloacae, 14 Pseudomonas aeruginosa, 8 Enterococcus faecalis and 8 Enterococcus faecium) were compared and analysed to ascertain species-specific PTA.

RESULTS

Glucose-6-phosphate (G6P) increased MICs of E. coli, K. pneumoniae and E. cloacae (median 2-fold dilutions 3-5), but not of P. aeruginosa and Enterococcus. Atypical E. coli lacking G6P potentiation were killed in the bladder infection model despite high MICs (32-128 mg/L). Fosfomycin heteroresistance was uncommon in E. coli (MIC > 2 mg/L) but was detected in the majority of K. pneumoniae (MIC > 1 mg/L) and P. aeruginosa (MIC >8 mg/L). For these species, baseline heteroresistance was a strong predictor for treatment failure in the model. No heteroresistance was found in Enterococcus. The fAUC/MIC targets for stasis were 1935, 3393, 9968, 2738 and 283 for typical E. coli, K. pneumoniae, E. cloacae, P. aeruginosa and E. faecalis, respectively (synthetic human urine medium alone promoted a 1 log10 kill in E. faecium). A >95% PTA for stasis was only found at MIC ≤ epidemiological cut-off (ECOFF) for E. coli (4 mg/L). For other species, PTAs were low for WT populations.

CONCLUSIONS

With the exception of E. coli, fosfomycin is a poor target for other uropathogen species. A reduction in oral fosfomycin UTI breakpoints is supported.

Antimicrobial pharmacokinetics and preclinical in vitro models to support optimized treatment approaches for uncomplicated lower urinary tract infections

INTRODUCTION

Urinary tract infections (UTIs) are extremely common. Millions of people, particularly healthy women, are affected worldwide every year. One-in-two women will have a recurrence within 12-months of an initial UTI. Inadequate treatment risks worsening infection leading to acute pyelonephritis, bacteremia and sepsis. In an era of increasing antimicrobial resistance, it is critical to provide optimized antimicrobial treatment.

AREAS COVERED

Literature was searched using PubMed and Google Scholar (up to 06/2020), examining the etiology, diagnosis and oral antimicrobial therapy for uncomplicated UTIs, with emphasis on urinary antimicrobial pharmacokinetics (PK) and the application of dynamic in vitro models for the pharmacodynamic (PD) profiling of pathogen response.

EXPERT OPINION

The majority of antimicrobial agents included in international guidelines were developed decades ago without well-described dose-response relationships. Microbiology laboratories still apply standard diagnostic methodology that has essentially remained unchanged for decades. Furthermore, it is uncertain how relevant standard in vitro susceptibility is for predicting antimicrobial efficacy in urine. In order to optimize UTI treatments, clinicians must exploit the urine-specific PK of antimicrobial agents. Dynamic in vitro models are valuable tools to examine the PK/PD and urodynamic variables associated with UTIs, while informing uropathogen susceptibility reporting, optimized dosing schedules, clinical trials and treatment guidelines.

Fosfomycin, Applying Known Methods and Remedies to A New Era

The exponential increase in the numbers of isolates of Carbapenem-Resistant Enterobacteriaceae (CRE) creates the need for using novel therapeutic approaches to save the lives of patients. Fosfomycin has long been considered a rational option for the treatment of CRE to be used as part of a combined therapy scheme. However, the assessment of fosfomycin susceptibility in the laboratory presents a great challenge due to the discrepancies found between different methodologies. Thus, our goal was to evaluate fosfomycin susceptibility in a group of 150 Enterobacteriaceae bacterial isolates using agar dilution as the gold standard technique to compare the results with those obtained by disk diffusion. We found a fosfomycin susceptibility of 79.3% in general terms. By comparing both methodologies, we reported a categorical agreement of 96% without Very Major Errors (VMEs) or Major Errors (MEs) and 4% of minor Errors (mEs). Our results suggest that fosfomycin could provide a rational alternative treatment for those patients that are infected by a Multidrug-Resistant (MDR) microorganism that is currently untreatable and that the disk diffusion and classical agar dilution techniques are adequate to assess the resistance profile of CRE to fosfomycin.

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.