Emergence of Resistance to Ceftazidime-Avibactam in a Pseudomonas aeruginosa Isolate Producing Derepressed blaPDC in a Hollow-Fiber Infection Model.

Ceftazidime (CAZ)-avibactam (AVI) is a ß-lactam/ß-lactamase inhibitor combination with activity against type A and type C ß-lactamases. Resistance emergence has been seen, with multiple mechanisms accounting for the resistance. We performed four experiments in the dynamic hollow-fiber infection model, delineating the linkage between drug exposure and both the rate of bacterial kill and resistance emergence by all mechanisms. The Pseudomonas aeruginosa isolate had MICs of 1.0 mg/liter (CAZ) and 4 mg/liter (AVI). We demonstrated that the time at ≥4.0 mg/liter AVI was linked to the rate of bacterial kill. Linkage to resistance emergence/suppression was more complex. In one experiment in which CAZ and AVI administration was intermittent and continuous, respectively, and in which AVI was given in unitary steps from 1 to 8 mg/liter, AVI at up to 3 mg/liter allowed resistance emergence, whereas higher values did not. The threshold value was 3.72 mg/liter as a continuous infusion to counterselect resistance (AVI area under the concentration-time curve [AUC] of 89.3 mg · h/liter). The mechanism involved a 7-amino-acid deletion in the Ω-loop region of the Pseudomonas-derived cephalosporinase (PDC) ß-lactamase. Further experiments in which CAZ and AVI were both administered intermittently with regimens above and below the AUC of 89.3 mg · h/liter resulted in resistance in the lower-exposure groups. Deletion mutants were not identified. Finally, in an experiment in which paired exposures as both continuous and intermittent infusions were performed, the lower value of 25 mg · h/liter by both profiles allowed selection of deletion mutants. Of the five instances in which these mutants were recovered, four had a continuous-infusion profile. Both continuous-infusion administration and low AVI AUC exposures have a role in selection of this mutation.

Spontaneous and Fungicide-Induced Genomic Variation in Sclerotinia sclerotiorum.

Stress from exposure to sublethal fungicide doses may cause genomic instability in fungal plant pathogens, which may accelerate the emergence of fungicide resistance or other adaptive traits. In a previous study, five strains of Sclerotinia sclerotiorum were exposed to sublethal doses of four fungicides with different modes of action, and genotyping showed that such exposure induced mutations. The goal of the present study was to characterize genome-wide mutations in response to sublethal fungicide stress in S. sclerotiorum and study the effect of genomic background on the mutational repertoire. The objectives were to determine the effect of sublethal dose exposure and genomic background on mutation frequency/type, distribution of mutations, and fitness costs. Fifty-five S. sclerotiorum genomes were sequenced and aligned to the reference genome. Variants were called and quality filtered to obtain high confidence calls for single nucleotide polymorphisms (SNPs), insertions/deletions (INDELs), copy number variants, and transposable element (TE) insertions. Results suggest that sublethal fungicide exposure significantly increased the frequency of INDELs in two strains from one genomic background (P value ≤ 0.05), while TE insertions were generally repressed for all genomic backgrounds and under all fungicide exposures. The frequency and/or distribution of SNPs, INDELs, and TE insertions varied with genomic background. A propensity for large duplications on chromosome 7 and aneuploidy of this chromosome were observed in the S. sclerotiorum genome. Mutation accumulation did not significantly affect the overall in planta strain aggressiveness (P value > 0.05). Understanding factors that affect pathogen mutation rates can inform disease management strategies that delay resistance evolution.

Identifying key factors of the transmission dynamics of drug-resistant malaria.

Development of resistance to malaria treatments remains a great threat to continued malaria burden reduction and elimination. Quantifying the impact of key factors which increase the emergence and spread of drug resistance can guide intervention strategies. Whilst modelling provides a framework to understand these factors, we show that a simple of model with a sensitive-resistant dichotomy leads to incorrectly focusing on reducing the treatment rate as a means to prevent resistance. Instead we present a model that considers the development of resistance within hosts as a scale, and we then quantify the number of resistant infections that would arise from a single sensitive infection. By including just one step before full resistance, the model highlights that disrupting this development is more effective than reducing treatment rate. This result is compounded when the model includes the more realistic scenario of several intermediary steps. An additional comparison to transmission probabilities, where resistant infections are less likely to be transmitted (cost of resistance), confirms that preventing the establishment of resistance is more effective than controlling the spread. Our work strongly advocates for further studies into within-host models of resistance, including the potential of combination therapies to disrupt emergence.

Correlations Between the Hollow Fiber Model of Tuberculosis and Therapeutic Events in Tuberculosis Patients: Learn and Confirm.

BACKGROUND: The hollow fiber system model of tuberculosis (HFS-TB) is designed to perform pharmacokinetics/pharmacodynamics (PK/PD) experiments, and hence the design of optimal doses and dose schedules for the treatment of tuberculosis. To determine if this model is useful for deriving PK/PD data relevant to clinical outcomes, we compared its quantitative output to that from clinical trials. METHODS: We performed a PubMed search to identify clinical studies performed with antituberculosis therapy in which PK/PD data and/or parameters were documented or a dose-scheduling study design was employed. The search period was from January 1943 to December 2012. All clinical studies were published prior to HFS-TB experiments. Bias minimization was done according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses. Clinical publications were scored for quality of evidence, with 1 as the highest score (randomized controlled trials or meta-analyses of such studies), and 4 as the lowest score. RESULTS: We identified 17 studies that examined the same parameters as in 8 HFS-TB studies. Fifteen of 17 studies had a quality-of-evidence score of 1. The sterilizing and bactericidal effect rates for isoniazid, rifampin, pyrazinamide, and ethambutol were the same in the HFS-TB as in patients. Time to emergence of resistance for monotherapy was the same as in patients. The PK/PD indices associated with efficacy were the same in HFS-TB as in patients for all drugs examined. CONCLUSIONS: The HFS-TB model is highly accurate at identifying optimal drug exposures, doses, and dosing schedules for use in the clinic.

Risk Factors for the Development of Colistin Resistance during Colistin Treatment of Carbapenem-Resistant Klebsiella pneumoniae Infections.

Colistin is one of the last-resort options for carbapenem-resistant Klebsiella pneumoniae (CRKP) infections if novel antibiotics are unavailable, where the development of colistin resistance during treatment represents a major challenge for clinicians. We aimed to investigate the risk factors associated with the development of colistin resistance in patients with CRKP infections following colistin treatment. We conducted a retrospective case-control study of patients with CRKP strains available before and after colistin treatment at a medical center in Taiwan, between October 2016 and November 2020. Cases (n = 35) included patients with an initial colistin-susceptible CRKP (ColS-CRKP) strain and a subsequent colistin-resistant CRKP (ColR-CRKP) strain. Controls (n = 18) included patients with ColS-CRKP as both the initial and subsequent strains. The 30-day mortality rate after the subsequent CRKP isolation was not different between cases and controls (12/35 [34%] versus 5/18 [28%] [P = 0.631]). blaKPC (n = 38) and blaOXA-48 (n = 11) accounted for the major mechanisms of carbapenem resistance. Alterations in mgrB were found in 18/35 (51%) ColR-CRKP strains, and mcr-1 was not detected in any of the strains. More patients received combination therapy in the control group than in the case group (17/18 versus 21/35 [P = 0.008]). The logistic regression model indicated that combination therapy with tigecycline was protective against the acquisition of colistin resistance (odds ratio, 0.17; 95% confidence interval, 0.05 to 0.62 [P = 0.008]). We observed that the inclusion of tigecycline in colistin treatment mitigated the risk of acquiring colistin resistance. These results offer insight into using the combination of tigecycline and colistin for the treatment of CRKP infections in antimicrobial stewardship. IMPORTANCE Treatment of carbapenem-resistant Klebsiella pneumoniae (CRKP) infections is challenging due to the limited options of antibiotics. Colistin is one of the last-resort antibiotics if novel antimicrobial agents are not available. It is crucial to identify modifiable clinical factors associated with the emergence of resistance during colistin treatment. Here, we found that the addition of tigecycline to colistin treatment prevented the acquisition of colistin resistance. Colistin-tigecycline combination therapy is therefore considered a hopeful option in antimicrobial stewardship to treat CRKP infections.

Contrasting patterns of isotype-1 ß-tubulin allelic diversity in Haemonchus contortus and Haemonchus placei in the southern USA are consistent with a model of localised emergence of benzimidazole resistance.

The benzimidazoles are one of the most important broad-spectrum anthelmintic drug classes for parasitic nematode control in domestic animals and humans. They have been widely used in livestock, particularly in small ruminants for over 40 years. This has resulted in widespread resistance in small ruminant gastrointestinal nematode parasite species, especially Haemonchus contortus. Benzimidazole resistance mutations have also been reported in Haemonchus placei, but only at low frequencies, suggesting resistance is at a much earlier stage of emergence than is the case for H. contortus. Here, we investigate the haplotype diversity of isotype-1 ß-tubulin benzimidazole resistance mutations and the population genetic structure of H. contortus and H. placei populations from sheep and cattle from the southern USA. Microsatellite genotyping revealed a low level of genetic differentiation in six H.placei and seven H. contortus populations examined. This is consistent with several previous studies from other regions, mainly in H. contortus, supporting a model of high gene flow between parasite populations. There was a single F200Y(TAC) haplotype present in all six H. placei populations across Georgia, Florida and Arkansas. In contrast, there were at least two different F200Y(TAC) haplotypes (up to four) and two different F167Y(TAC) haplotypes across the seven H. contortus populations studied. These results provide further evidence to support a model for benzimidazole resistance in Haemonchus spp, in which resistance mutations arise from a single, or the small number of locations, in a region during the early phases of emergence, and subsequently spread due to animal movement.