In Vitro Activities and Inoculum Effects of Ceftazidime-Avibactam and Aztreonam-Avibactam against Carbapenem-Resistant Enterobacterales Isolates from South Korea

Impact of the inoculum size on the in vivo activity of the aztreonam-avibactam combination in a murine model of peritonitis due to Escherichia coli expressing CTX-M-15 and NDM-1

The combination of aztreonam (ATM) and avibactam (AVI) is an attractive option to treat infections caused by extended spectrum β-lactamase plus NDM-1-producing Enterobacteriaceae. Since ATM activity was shown to be severely impacted by an increase in the inoculum size in vitro, we wondered whether ATM-AVI activity could be impaired in high-inoculum infections. We analyzed the impact of the inoculum size on ATM-AVI activity in vitro and in a murine model of peritonitis due to susceptible Escherichia coli CFT073-pTOPO and its isogenic derivatives producing NDM-1 (E. coli CFT073-NDM1) and CTX-M-15 plus NDM-1 (E. coli CFT073-CTXM15-NDM1). The impact of the inoculum size on bacterial morphology was studied by microscopic examination. In vitro, at standard (105) inoculum, E. coli CFT073-CTXM15-NDM1 was resistant to ATM but susceptible to the ATM-AVI combination. At high (107) inoculum, MICs of ATM alone and of the ATM-AVI combination reached >512 and 64 mg/L, respectively, against all tested strains. ATM led to bacterial filamentation when active against the bacteria, i.e., in monotherapy or in combination with AVI against susceptible E. coli CFT073-pTOPO and only in combination with AVI against E. coli CFT073-CTXM15-NDM1. In vivo, increase in the inoculum led to a drastic decrease in the activity of ATM alone against E. coli CFT073-pTOPO and ATM-AVI against E. coli CFT073-CTXM15-NDM1. Our results suggest a high in vivo impact of the inoculum increase on the activity of ATM alone against ATM-susceptible E. coli and of ATM-AVI against CTX-M-15 plus NDM-1 producing E. coli. Clinicians must be aware of the risk of failures when using ATM-AVI in high-inoculum infections.

Purine and pyrimidine synthesis differently affect the strength of the inoculum effect for aminoglycoside and β-lactam antibiotics

The inoculum effect has been observed for nearly all antibiotics and bacterial species. However, explanations accounting for its occurrence and strength are lacking. Previous work found that the relationship between [ATP] and growth rate can account for the strength and occurrence of the inoculum effect for bactericidal antibiotics. However, the molecular pathway(s) underlying this relationship, and therefore determining the inoculum effect, remain undiscovered. Using a combination of flux balance analysis and experimentation, we show that nucleotide synthesis can determine the relationship between [ATP] and growth and thus the strength of inoculum effect in an antibiotic class-dependent manner. If the [ATP]/growth rate is sufficiently high as determined by exogenously supplied nitrogenous bases, the inoculum effect does not occur. This is consistent for both Escherichia coli and Pseudomonas aeruginosa. Interestingly, and separate from activity through the tricarboxylic acid cycle, we find that transcriptional activity of genes involved in purine and pyrimidine synthesis can predict the strength of the inoculum effect for β-lactam and aminoglycosides antibiotics, respectively. Our work highlights the antibiotic class-specific effect of purine and pyrimidine synthesis on the severity of the inoculum effect, which may pave the way for intervention strategies to reduce the inoculum effect in the clinic.

IMPORTANCE

If a bacterial population can grow and reach a sufficiently high density, routine doses of antibiotics can be ineffective. This phenomenon, called the inoculum effect, has been observed for nearly all antibiotics and bacterial species. It has also been reported to result in antibiotic failure in the clinic. Understanding how to reduce the inoculum effect can make high-density infections easier to treat. Here, we show that purine and pyrimidine synthesis affect the strength of the inoculum effect; as the transcriptional activity of pyrimidine synthesis increases, the strength of the inoculum effect for aminoglycosides decreases. Conversely, as the transcriptional activity of purine synthesis increases, the strength of the inoculum effect for β-lactam antibiotics decreases. Our work highlights the importance of nucleotide synthesis in determining the strength of the inoculum effect, which may lead to the identification of new ways to treat high-density infections in the clinic.

Role of volume and inoculum in MIC assessment: a study with meropenem and Klebsiella pneumoniae

OBJECTIVES

Pharmacodynamic parameters evaluated under conditions that simulate an infection site volume and microbial load might reveal hidden risks of resistance selection and subsequent treatment failure. The study aimed to investigate the predictive potential of MICs determined at various conditions on the antimicrobial effect and emergence of resistance.

METHODS

We assessed meropenem MICs (microdilution: 0.2 mL, 5 × 105 cfu/mL; macrodilution: 2 mL, 5 × 105 cfu/mL), MICHVs (220 mL, 5 × 105 cfu/mL), MICHIs (0.2 mL, 5 × 107 cfu/mL) and MICHVIs (220 mL, 5 × 107 cfu/mL) for five Klebsiella pneumoniae strains and analysed these values alongside the results of experiments in a dynamic in vitro model. A clinically relevant meropenem dosing regimen was simulated and the starting bacterial inocula were 106 and 108 cfu/mL.

RESULTS

The effectiveness of meropenem agreed with MICHVs for the 106 cfu/mL inoculum and with MICHIs or MICHVIs for the 108 cfu/mL inoculum. Strains characterized as resistant according to these values grew during meropenem exposure, and resistant mutants were selected.

CONCLUSIONS

Our results suggest that MICHV-based parameters may be suitable for predicting antibacterial effects and the risk of resistance development when the inoculum is 106 cfu/mL, while MICHI- or MICHVI-based parameters are suitable for these purposes when the inoculum is 108 cfu/mL. Also, the correlation between resistance selection and the MICHI-based parameter was as high as one that corresponds with a mutant prevention concentration (MPC)-based parameter; this suggests that the MPC can be replaced by the more easily determined alternative parameter MICHI.

Practical Application of Aztreonam-Avibactam as a Treatment Strategy for Ambler Class B Metallo-β-Lactamase Producing Enterobacteriaceae

Carbapenem-resistant Enterobacteriaceae infections are a considerable challenge for clinicians. In recent years, novel antibiotic options have resulted in a tremendous advance in medical therapy; however, current treatment options are primarily effective for resistance derived from serine-based carbapenemases. The Ambler class B metallo-β-lactamases (MBLs) remain a critical challenge with decidedly fewer effective options. One intriguing option for these MBL pathogens is the combination of ceftazidime-avibactam with aztreonam. While clinical experience with this regimen is limited, in vitro studies are promising, and limited case reports describe success with this regimen; however, significant challenges preclude widespread adoption of this novel treatment regimen. A systemic literature review was performed to offer recommendations based on current evidence for a practical strategy on how to best integrate the use of aztreonam with avibactam combination therapy.

Antimicrobial activity of ceftazidime-avibactam against KPC-2-producing Enterobacterales: a cross-combination and dose-escalation titration study with relebactam and vaborbactam

With the introduction of ceftazidime-avibactam worldwide, the antimicrobial activity of new β-lactam/β-lactamase inhibitors (BL/BLIs) needs to be investigated. From January 2020 to June 2023, Klebsiella pneumoniae carbapenemase (KPC)-producing Enterobacterales were collected. With a broth microdilution test of new BL/BLIs, cross-activity test with nine combinations of BLs and new BLIs and dose-escalation titration test for non-susceptible isolates were conducted to investigate inhibitory activities of new BLIs. A total of 188 isolates was collected and most isolates (186/188, 98.9%) carried the KPC-2 gene exclusively, while two isolates (1.1%) co-harbored NDM-1. Among the 186 KPC-2-producing isolates, 184 (98.9%) were susceptible to ceftazidime-avibactam, 173 (93.0%) to imipenem-relebactam, and 184 (98.9%) to meropenem-vaborbactam. All isolates non-susceptible to imipenem-relebactam or meropenem-vaborbactam became susceptible when avibactam replaced relebactam or vaborbactam, with 7 of 11 (63.6%) imipenem-relebactam non-susceptible isolates and both (100.0%) of the meropenem-vaborbactam non-susceptible isolates. When the minimum inhibitory concentrations (MICs) of BLs were compared using log2 scales, combinations with avibactam showed statistically significant efficacy in lowering MICs compared to relebactam and vaborbactam (all P < 0.05). In the dose-escalation test of new BLIs, increasing dose of all new BLIs corresponded to increased susceptibility to BLs. Ceftazidime-avibactam exhibited excellent susceptibility against KPC-2-producing Enterobacterales unless co-harboring metallo-β-lactamase. The cross-combination test against non-susceptible isolates suggests that the inhibitory activity of avibactam was superior to those of relebactam or vaborbactam. Increasing the dose of new BLIs produced increased susceptibility to BLs, suggesting that high-concentration regimen need to be developed.

IMPORTANCE

This study investigated 188 Klebsiella pneumoniae carbapenemase (KPC)-2-producing Enterobacterales collected from January 2020 to June 2023 in a tertiary care hospital of Korea. Most isolates were susceptible to ceftazidime-avibactam (98.9%) and meropenem-vaborbactam (98.9%), while susceptibility to imipenem-relebactam was lower (93.0%). The cross-combination test using nine combinations of the individual β-lactams (BLs) and new β-lactamase inhibitors (BLIs) showed that the inhibitory activity of avibactam was significantly superior to relebactam or vaborbactam when the Log2 MIC of BLs were compared for each combination with BLIs (all P < 0.05). The dose-escalation test of new BLIs demonstrated that increasing doses of new BLIs corresponded to increased susceptibility to BLs. Taken together, this study illustrates the excellent activity of ceftazidime-avibactam against KPC-2-producing Enterobacterales and suggests further investigation into high-concentration regimens for potentially non-susceptible clinical isolates.

Purine and pyrimidine synthesis differently affect the strength of the inoculum effect for aminoglycoside and β-lactam antibiotics

The inoculum effect has been observed for nearly all antibiotics and bacterial species. However, explanations accounting for its occurrence and strength are lacking. We previously found that growth productivity, which captures the relationship between [ATP] and growth, can account for the strength of the inoculum effect for bactericidal antibiotics. However, the molecular pathway(s) underlying this relationship, and therefore determining the inoculum effect, remain undiscovered. We show that nucleotide synthesis can determine the relationship between [ATP] and growth, and thus the strength of inoculum effect in an antibiotic class-dependent manner. Specifically, and separate from activity through the tricarboxylic acid cycle, we find that transcriptional activity of genes involved in purine and pyrimidine synthesis can predict the strength of the inoculum effect for β-lactam and aminoglycosides antibiotics, respectively. Our work highlights the antibiotic class-specific effect of purine and pyrimidine synthesis on the severity of the inoculum effect and paves the way for intervention strategies to reduce the inoculum effect in the clinic.

In Vitro Activities and Inoculum Effects of Cefiderocol and Aztreonam-Avibactam against Metallo-β-Lactamase-Producing Enterobacteriaceae

Cefiderocol and aztreonam-avibactam (ATM-AVI) both had activity against carbapenem-resistant Gram-negative bacilli, including those that produce metallo-β-lactamases (MBLs). We compared the in vitro activities and inoculum effects of these antibiotics against carbapenemase-producing Enterobacteriaceae (CPE), especially MBL-producing isolates. The MICs of cefiderocol and ATM-AVI were determined using broth microdilution method for a 2016 to 2021 collection of Enterobacteriaceae isolates which produced MBL, KPC, or OXA-48-like carbapenemases. MICs with high bacteria inoculum were also evaluated for susceptible isolates. A total of 195 CPE were tested, including 143 MBL- (74 NDM, 42 IMP, and 27 VIM), 38 KPC-, and 14 OXA-48-like-producing isolates. The susceptible rates of MBL-, KPC-, and OXA-48-like producers to cefiderocol were 86.0%, 92.1%, and 92.9%, respectively, and that to ATM-AVI were 95.8%, 100%, and 100%, respectively. NDM producers displayed lower susceptibility and higher MIC50s/MIC90s of cefiderocol (78.4%, 2/16 mg/L) than IMP (92.9%, 0.375/4 mg/L) and VIM (96.3%, 1/4 mg/L) producers. NDM- and VIM-producing Escherichia coli showed lower susceptibility to ATM-AVI (77.3% and 75.0%, respectively) compared to MBL-CPE of other species (100% susceptible). Inoculum effects for cefiderocol and ATM-AVI were observed among 95.9% and 95.2% of susceptible CPE, respectively. A switch from susceptible to resistant category was observed in 83.6% (143/171) of isolates for cefiderocol and 94.7% (179/189) for ATM-AVI. Our results revealed that NDM-producing Enterobacteriaceae had lower susceptibility to cefiderocol and ATM-AVI. Prominent inoculum effects on both antibiotics were observed for CPE, which suggested a risk of microbiological failure when they were used for CPE infections with high bacteria burden. IMPORTANCE The prevalence of infections caused by carbapenem-resistant Enterobacteriaceae is increasing worldwide. Currently, therapeutic options for metallo-β-lactamase (MBL)-producing Enterobacteriaceae remain limited. We demonstrated that clinical metallo-β-lactamase (MBL)-producing Enterobacteriaceae isolates were highly susceptible to cefiderocol (86.0%) and aztreonam-avibactam (ATM-AVI) (95.8%). However, inoculum effects on cefiderocol and ATM-AVI were observed for over 90% of susceptible carbapenemase-producing Enterobacteriaceae (CPE) isolates. Our findings highlight a potential risk of microbiological failure when using monotherapy with cefiderocol or ATM-AVI to treat severe CPE infection.

Meropenem MICs at Standard and High Inocula and Mutant Prevention Concentration Inter-Relations: Comparative Study with Non-Carbapenemase-Producing and OXA-48-, KPC- and NDM-Producing Klebsiella pneumoniae

The minimal inhibitory concentration (MIC) is conventionally used to define in vitro levels of susceptibility or resistance of a specific bacterial strain to an antibiotic and to predict its clinical efficacy. Along with MIC, other measures of bacteria resistance exist: the MIC determined at high bacterial inocula (MIC_HI) that allow the estimation of the occurrence of inoculum effect (IE) and the mutant prevention concentration, MPC. Together, MIC, MIC_HI and MPC represent the bacterial "resistance profile". In this paper, we provide a comprehensive analysis of such profiles of K. pneumoniae strains that differ by meropenem susceptibility, ability to produce carbapenemases and specific carbapenemase types. In addition, we have analyzed inter-relations between the MIC, MIC_HI and MPC for each tested K. pneumoniae strain. Low IE probability was detected with carbapenemase-non-producing K. pneumoniae, and high IE probability was detected with those that were carbapenemase-producing. MICs did not correlate with the MPCs; significant correlation was observed between the MIC_HIs and the MPCs, indicating that these bacteria/antibiotic characteristics display similar resistance properties of a given bacterial strain. To determine the possible resistance-related risk due to a given K. pneumoniae strain, we propose determining the MIC_HI. This can more or less predict the MPC value of the particular strain.

Antimicrobial resistance in Gram-negative bacilli in Spain: an experts' view

BACKGROUND

Antibiotic resistance in Gram-negative bacilli poses a serious problem for public health. In hospitals, in addition to high mortality rates, the emergence and spread of resistance to practically all antibiotics restricts therapeutic options against serious and frequent infections.

OBJECTIVE

The aim of this work is to present the views of a group of experts on the following aspects regarding resistance to antimicrobial agents in Gram-negative bacilli: 1) the current epidemiology in Spain, 2) how it is related to local clinical practice and 3) new therapies in this area, based on currently available evidence.

METHODS

After reviewing the most noteworthy evidence, the most relevant data on these three aspects were presented at a national meeting to 99 experts in infectious diseases, clinical microbiology, internal medicine, intensive care medicine, anaesthesiology and hospital pharmacy.

RESULTS AND CONCLUSIONS

Subsequent local debates among these experts led to conclusions in this matter, including the opinion that the approval of new antibiotics makes it necessary to train the specialists involved in order to optimise how they use them and improve health outcomes; microbiology laboratories in hospitals must be available throughout a continuous timetable; all antibiotics must be available when needed and it is necessary to learn to use them correctly; and the Antimicrobial Stewardship Programs (ASP) play a key role in quickly allocating the new antibiotics within the guidelines and ensure appropriate use of them.

Growth productivity as a determinant of the inoculum effect for bactericidal antibiotics

Understanding the mechanisms by which populations of bacteria resist antibiotics has implications in evolution, microbial ecology, and public health. The inoculum effect (IE), where antibiotic efficacy declines as the density of a bacterial population increases, has been observed for multiple bacterial species and antibiotics. Several mechanisms to account for IE have been proposed, but most lack experimental evidence or cannot explain IE for multiple antibiotics. We show that growth productivity, the combined effect of growth and metabolism, can account for IE for multiple bactericidal antibiotics and bacterial species. Guided by flux balance analysis and whole-genome modeling, we show that the carbon source supplied in the growth medium determines growth productivity. If growth productivity is sufficiently high, IE is eliminated. Our results may lead to approaches to reduce IE in the clinic, help standardize the analysis of antibiotics, and further our understanding of how bacteria evolve resistance.

Inoculum effect of Enterobacterales co-expressing OXA-48 and CTX-M on the susceptibility to ceftazidime/avibactam and meropenem

BACKGROUND

The treatment of infections caused by OXA-48/CTX-M-coproducing Enterobacterales may be based on new beta-lactam/beta-lactamase inhibitors, such as ceftazidime/avibactam (CZA), or on high dose of meropenem (MER). However, bacterial density at the infection site may vary widely, and the inoculum effect of such antimicrobial strategies has never been specifically investigated. To determine if CZA or MER susceptibilities are impacted by high inocula of Enterobacterales co-expressing both enzymes: OXA-48 like and CTX-M.

METHODS

Determination of an inoculum effect was performed with a standard inoculum of 10⁸ CFU/mL (0.5 McFarland) as recommended by EUCAST guidelines and compared to a twofold increase as well as a tenfold increase (1 McFarland and 5 McFarland respectively).

RESULTS

Thirty-nine isolates of ceftazidime-resistant Enterobacterales were included of which 27 (70%) co-expressed OXA-48 + CTX-M-15, 6 (15%) OXA-48 + CTX-M-14, and 6 (15%) OXA-181 + CTX-M-15. The susceptibility to the CZA combination was preserved whatever the inoculum used. Regarding MER, 24 (61.5%) of the isolates were susceptible to MER with the standard inoculum, 19 (48.7%) with a twofold increase, and only 15 (38.5%) with a tenfold increase.

CONCLUSION

We showed that in vitro inoculum effect was observed with meropenem but not with CZA for OXA-48- combined with CTX-M-producing Enterobacterales.

Successful treatment of Verona integron-encoded metallo-β-lactamase-producing Klebsiella pneumoniae infection using the combination of ceftazidime/avibactam and aztreonam

The case of a female who had an accident that caused an open fracture is reported. During hospitalisation, Verona integron-encoded metallo-β-lactamase (VIM)-producing Klebsiella pneumoniae was isolated. Antimicrobial susceptibility testing revealed resistance to β-lactam antibiotics, quinolones, trimethoprim/sulfamethoxazole, and susceptibility to tigecycline, colistin, fosfomycin and aminoglycosides. Synergistic activity of ceftazidime-avibactam and aztreonam was proved in vitro and a combined therapy with tigecycline was started. Combination with aminoglycosides was ruled out as it was not described in the literature and also in order to avoid side effects. Colistin was rejected because of its nephrotoxicity profile. The antibiotic treatment was assessed by a multidisciplinary team with a pharmacist who closely monitored adverse effects and interactions with other drugs. The total duration of this combination was 25 days, without any adverse events reported. Fourteen weeks after the accident the patient was discharged. After 2 months of follow-up neither relapses nor reinfections have been reported.

The Role of Colistin in the Era of New β-Lactam/β-Lactamase Inhibitor Combinations

With the current crisis related to the emergence of carbapenem-resistant Gram-negative bacteria (CR-GNB), classical treatment approaches with so-called “old-fashion antibiotics” are generally unsatisfactory. Newly approved β-lactam/β-lactamase inhibitors (BLBLIs) should be considered as the first-line treatment options for carbapenem-resistant Enterobacterales (CRE) and carbapenem-resistant Pseudomonas aeruginosa (CRPA) infections. However, colistin can be prescribed for uncomplicated lower urinary tract infections caused by CR-GNB by relying on its pharmacokinetic and pharmacodynamic properties. Similarly, colistin can still be regarded as an alternative therapy for infections caused by carbapenem-resistant Acinetobacter baumannii (CRAB) until new and effective agents are approved. Using colistin in combination regimens (i.e., including at least two in vitro active agents) can be considered in CRAB infections, and CRE infections with high risk of mortality. In conclusion, new BLBLIs have largely replaced colistin for the treatment of CR-GNB infections. Nevertheless, colistin may be needed for the treatment of CRAB infections and in the setting where the new BLBLIs are currently unavailable. In addition, with the advent of rapid diagnostic methods and novel antimicrobials, the application of personalized medicine has gained significant importance in the treatment of CRE infections.

Assessment of Ceftazidime-Avibactam 30/20-μg Disk, Etest versus Broth Microdilution Results When Tested against Enterobacterales Clinical Isolates

The objective of this research was to evaluate the correlation between inhibitory zones and MIC when testing ceftazidime-avibactam using disk diffusion, Etest, and broth microdilution method established by the Clinical and Laboratory Standards Institute (CLSI). Four-hundred and 58 isolates of Enterobacterales isolated from 54 medical centers from the China Antimicrobial Surveillance Network (CHINET) in 2016 to 2020 were collected. Antimicrobial susceptibility testing using broth microdilution, Etest, and disk diffusion were performed according to the CLSI. Of the 458 Enterobacterales, 17.2% (79/458) and 82.8%(379/458) were resistant or susceptible to ceftazidime-avibactam by broth microdilution, respectively. Compared with the broth microdilution method, the categorical agreement (CA) and essential agreement (EA) of the Etest were 99.6% (456/458) and 94.8% (434/458), respectively; the major error (ME) and very major error (VME) were both 0.2% (1/458). For disk diffusion, the CA and VME were 99.8% (457/458) and 0.2% (1/458), respectively. For Escherichia coli, the CA and EA of the Etest were 100% and 97.1% (135/139), respectively. The CA of the disk diffusion was 100%. For Klebsiella pneumoniae, the CA and EA of the Etest were 99.3% (288/290) and 93.4% (271/290), respectively, the ME and VME were both 0.3% (1/290). The CA and VME of disk diffusion were 99.7% (289/290) and 0.3% (1/290), respectively. For other Enterobacterales, the CA and EA of the Etest were 100% and 96.6% (28/29), respectively. The CA of the disk diffusion was 100%. Ceftazidime-avibactam disk diffusion (30/20-μg disks) and Etest demonstrated good performance for ceftazidime-avibactam susceptibility testing against Enterobacterales clinical isolates. IMPORTANCE Multidrug-resistant Gram-negative bacteria, especially for extended-spectrum β-lactamases-producing and carbapenemase-producing Enterobacterales, are disseminating rapidly around the world. Treatment options for these infections are limited, which prompt the development of novel or combinational therapies to combat the infections caused by multidrug-resistant pathogens. The newly available β-lactam combination agent ceftazidime-avibactam has been demonstrated good in vitro and in vivo activity against ESBL, AmpC, KPC-2, or OXA-48-like-producing isolates and has shown promise in treating carbapenem-resistant Enterobacterales infections. Concerningly, there are few available automated systems for ceftazidime-avibactam susceptibility testing, and the broth microdilution method is hard to perform in most routine laboratories. Therefore, we urgently need an economical and practical method for the accurate detection of ceftazidime-avibactam activity against Gram-negative bacilli. Here, we evaluate the performance of the disk diffusion and Etest compared with the reference broth microdilution method against Enterobacterales clinical strains.

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.

A new PKPD model to characterize the inoculum effect of Acinetobacter baumannii on polymyxin B in vitro

The inoculum effect (i.e., reduction in antimicrobial activity at large starting inoculum) is a phenomenon described for various pathogens. Since limited data exist regarding inoculum effect of Acinetobacter baumannii, we evaluated killing of A. baumannii by polymyxin B, a last-resort antibiotic, at several starting inocula and developed a PKPD model to capture this phenomenon. In vitro static time-kill experiments were performed using polymyxin B at concentrations ranging from 0.125 to 128 mg/L against a clinical A. baumannii isolate at four starting inocula from 10⁵ to 10⁸ CFU/mL. Samples were collected up to 30 h to quantify the viable bacterial burden and were simultaneously modeled in the NONMEM software program. The expression of polymyxin B resistance genes (lpxACD, pmrCAB and wzc), and genetic modifications were studied by RT-qPCR and DNA sequencing experiments, respectively. The PKPD model included a single homogeneous bacterial population with adaptive resistance. Polymyxin B effect was modelled as a sigmoidal E_max model and the inoculum effect as an increase of polymyxin B EC₅₀ with increasing starting inoculum using a power function. Polymyxin B displayed a reduced activity as the starting inoculum increased: a 20-fold increase of polymyxin B EC₅₀ was observed between the lowest and the highest inoculum. No effects of polymyxin B and inoculum size were observed on the studied genes. The proposed PKPD model successfully described and predicted the pronounced in vitro inoculum effect of A. baumannii on polymyxin B activity. These results should be further validated using other bacteria/antibiotic combinations and in vivo models.

The Revival of Aztreonam in Combination with Avibactam against Metallo-β-Lactamase-Producing Gram-Negatives: A Systematic Review of In Vitro Studies and Clinical Cases

Infections caused by metallo-β-lactamase (MBL)-producing Enterobacterales and Pseudomonas are increasingly reported worldwide and are usually associated with high mortality rates (>30%). Neither standard therapy nor consensus for the management of these infections exist. Aztreonam, an old β-lactam antibiotic, is not hydrolyzed by MBLs. However, since many MBL-producing strains co-produce enzymes that could hydrolyze aztreonam (e.g., AmpC, ESBL), a robust β-lactamase inhibitor such as avibactam could be given as a partner drug. We performed a systematic review including 35 in vitro and 18 in vivo studies on the combination aztreonam + avibactam for infections sustained by MBL-producing Gram-negatives. In vitro data on 2209 Gram-negatives were available, showing the high antimicrobial activity of aztreonam (MIC ≤ 4 mg/L when combined with avibactam) in 80% of MBL-producing Enterobacterales, 85% of Stenotrophomonas and 6% of MBL-producing Pseudomonas. Clinical data were available for 94 patients: 83% of them had bloodstream infections. Clinical resolution within 30 days was reported in 80% of infected patients. Analyzing only patients with bloodstream infections (64 patients), death occurred in 19% of patients treated with aztreonam + ceftazidime/avibactam. The combination aztreonam + avibactam appears to be a promising option against MBL-producing bacteria (especially Enterobacterales, much less for Pseudomonas) while waiting for new antimicrobials.

The Potential Use of Ceftazidime-Avibactam Against Carbapenem Resistant Klebsiella pneumoniae Clinical Isolates Harboring Different Carbapenemase Types in a Thai University Hospital

Purpose

MBL and OXA-48 genes in carbapenem-resistant Enterobacterales (CRE) have emerged as a major public health problem worldwide, including Thailand. Due to the lack of susceptibility data and dosing regimens of ceftazidime-avibactam (CZA) against CRE in Thailand, especially in colistin-resistant era, we aimed to demonstrate in vitro susceptibility data of CZA and optimal dose based on Monte Carlo simulation of CZA to expand the treatment options.

Patients and Methods

We collected 49 carbapenem-resistant Klebsiella pneumoniae (CRKP) clinical isolates from unique patients at Phramongkutklao Hospital (June–October 2020). CZA disk diffusion and E-test testing were performed to obtain minimum inhibitory concentration (MIC). Each drug regimen was simulated using the Monte Carlo technique to calculate the probability of target attainment (PTA) and the cumulative fraction of response (CFR).

Results

The most common genotypes of CRKP were bla_OXA-48 (53.1%) and bla_OXA-48 +bla_NDM (42.8%). CZA showed 47.7% and 90.5% susceptible rate against all genotypes of carbapenemases and OXA-48 type CRKP isolates. The MIC₅₀ and MIC₉₀ of CZA against CRKP were 2 and >256 µg/mL. The categorical agreement (CA) between disk diffusion and E-test testing of CZA against CRKP was 95.4%. The CZA dosing regimens of 2.5 g infused 2–3 h every 8 h achieved ≥90% of the target of free ceftazidime plasma concentration over MIC (%fTime >MIC) ≥50% and 100% against isolates MICs of ≤8 and ≤8 µg/mL, respectively. The avibactam regimens also provided 100%fTime at 0.5 µg/mL. Based on CFR ≥90%, no CZA regimens were effective against all of the studied CRKP isolates except CRKP carrying OXA-48.

Conclusion

CZA exhibited a fairly susceptible rate among the OXA-48-positive isolates in Thailand. The current suggested dose of CZA with prolonged infusion appears appropriate to achieve the pharmacokinetic/pharmacodynamic targets of ceftazidime and avibactam against CRKP carrying bla_OXA-48.

Present and Future Perspectives on Therapeutic Options for Carbapenemase-Producing Enterobacterales Infections

Carbapenem-resistant Enterobacterales (CRE) are included in the list of the most threatening antibiotic resistance microorganisms, being responsible for often insurmountable therapeutic issues, especially in hospitalized patients and immunocompromised individuals and patients in intensive care units. The enzymatic resistance to carbapenems is encoded by different β-lactamases belonging to A, B or D Ambler class. Besides compromising the activity of last-resort antibiotics, CRE have spread from the clinical to the environmental sectors, in all geographic regions. The purpose of this review is to present present and future perspectives on CRE-associated infections treatment.