Expression of CTX-M-15 limits the efficacy of ceftolozane/tazobactam against Escherichia coli in a high-inoculum murine peritonitis model

Evaluation of the inoculum effect of new antibiotics against carbapenem-resistant enterobacterales

OBJECTIVES

New antibiotics have been developed to treat multidrug-resistant Enterobacterales. We evaluated the impact of the inoculum size on minimal inhibitory concentrations (MICs) of recently commercialized antibiotics.

METHODS

We focused on 40 clinical carbapenemase-producing Enterobacterales and evaluated the impact of the inoculum size on the MICs to cefiderocol and to new β-lactams/β-lactamase inhibitors (ceftolozane-tazobactam, ceftazidime-avibactam, imipenem-relebactam, and meropenem-vaborbactam) at usual and high inocula (105 and 107 CFU/mL, respectively).

RESULTS

At usual inoculum, 15% were resistant to cefiderocol (n = 6), 30% to meropenem-vaborbactam (n = 12), 42.5% to ceftazidime-avibactam (n = 17), 55% to imipenem-relebactam (n = 22), and 90% to ceftolozane-tazobactam (n = 36). At higher inoculum, a switch from susceptible to resistant category was observed for 88% (n = 30/34; CI, 71.6-96.2), 75% (n = 3/4; CI, 21.9-98.7), 72% (n = 13/18; CI, 46.4-89.3), 50% (n = 14/28; CI, 31.1-68.9), and 8.7% (n = 2/23; CI, 1.5-29.5) isolates regarding cefiderocol, ceftolozane-tazobactam, imipenem-relebactam, meropenem-vaborbactam, and ceftazidime-avibactam, respectively.

DISCUSSION

Cefiderocol and meropenem-vaborbactam were the most efficient against carbapenemase-producing Enterobacterales at usual inoculum. When increasing inoculum to 107 CFU/mL, all of the molecules were impacted, particularly cefiderocol and imipenem-relebactam, while others, such as ceftazidime-avibactam, remain mildly affected. Our in vitro results deserved to be confirmed in vivo.

β-Lactam Therapeutic Drug Monitoring in Critically Ill Patients: Weighing the Challenges and Opportunities to Assess Clinical Value

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Spread of hypervirulent multidrug-resistant ST147 Klebsiella pneumoniae in patients with severe COVID-19: an observational study from Italy, 2020-21

Objectives

To report an outbreak of hypervirulent Klebsiella pneumoniae (hvKp) in COVID-19 patients.

Methods

Prospective, observational study including consecutive COVID-19 patients with hvKp infections admitted to the University Hospital of Pisa (Italy). Clinical data and outcome of patients were collected. All patients were followed-up to 30 days from the diagnosis of infection. Mortality within 30 days of the diagnosis of hvKp infection was reported. The hypermucoviscous phenotype was determined by the ‘string test’. Molecular typing was performed on three strains collected during different periods of the outbreak. The strains underwent whole genome sequencing using the Illumina MiSeq instrument. The complete circular assemblies were also obtained for the chromosome and a large plasmid using the Unicycler tool.

Results

From November 2020 to March 2021, hvKp has been isolated from 36 COVID-19 patients: 29/36 (80.6%) had infections (15 bloodstream infections, 8 ventilator-associated pneumonias and 6 complicated urinary tract infections), while 7/36 (19.4%) had colonization (3 urine, 2 rectal and 2 skin). The isolates belonged to ST147 and their plasmid carried three replicons of the IncFIB (Mar), IncR and IncHI1B types and several resistance genes, including the rmpADC genes encoding enhancers of capsular synthesis. The hvKp isolates displayed an ESBL phenotype, with resistance to piperacillin/tazobactam and ceftolozane/tazobactam and susceptibility only to meropenem and ceftazidime/avibactam. The majority of patients were treated with meropenem alone or in combination with fosfomycin. Thirty-day mortality was 48.3% (14/29).

Conclusions

ST147 ESBL-producing hvKp is associated with high mortality in COVID-19 patients. Strict microbiological surveillance and infection control measures are needed in this population.

In Vitro Activities of Ceftazidime-Avibactam and Aztreonam-Avibactam at Different Inoculum Sizes of Extended-Spectrum β-Lactam-Resistant Enterobacterales Blood Isolates

β-lactam–avibactam combinations have been proposed as carbapenem-sparing therapies, but little data exist on their in vitro activities in infections with high bacterial inocula. We investigated the in vitro efficacies and the inoculum effects of ceftazidime–avibactam and aztreonam–avibactam against extended-spectrum β-lactam-resistant Enterobacterales blood isolates. A total of 228 non-repetitive extended-spectrum β-lactam-resistant Escherichia coli and Klebsiella pneumoniae blood isolates were prospectively collected in a tertiary center. In vitro susceptibilities to ceftazidime, aztreonam, meropenem, ceftazidime–avibactam, and aztreonam–avibactam were evaluated by broth microdilution method using standard and high inocula. An inoculum effect was defined as an eightfold or greater increase in MIC when tested with the high inoculum. Of the 228 isolates, 99% were susceptible to ceftazidime–avibactam and 99% had low aztreonam–avibactam MICs (≤8 mg/L). Ceftazidime–avibactam and aztreonam–avibactam exhibited good in vitro activities; MIC₅₀/MIC₉₀ values were 0.5/2 mg/L, 0.125/0.5 mg/L, and ≤0.03/0.25 mg/L, respectively, and aztreonam–avibactam was more active than ceftazidime–avibactam. The frequencies of the inoculum effect with ceftazidime–avibactam and aztreonam–avibactam were lower than with meropenem (14% vs. 38%, p < 0.001 and 30% vs. 38%, p = 0.03, respectively). The β-lactam-avibactam combinations could be useful as carbapenem-sparing strategies, and aztreonam–avibactam has the better in vitro activity but is more subject to the inoculum effect than ceftazidime–avibactam.