Variability in Zinc Concentration among Mueller-Hinton Broth Brands: Impact on Antimicrobial Susceptibility Testing of Metallo-β-Lactamase-Producing Enterobacteriaceae

Repurposing sunscreen as an antibiotic: zinc-activated avobenzone inhibits methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a major healthcare concern with associated healthcare costs reaching over ${\$}$1 billion in a single year in the USA. Antibiotic resistance in S. aureus is now observed against last line of defense antibiotics, such as vancomycin, linezolid, and daptomycin. Unfortunately, high throughput drug discovery approaches to identify new antibiotics effective against MRSA have not resulted in much tangible success over the last decades. Previously, we demonstrated the feasibility of an alternative drug discovery approach, the identification of metallo-antibiotics, compounds that gain antibacterial activity only after binding to a transition metal ion and as such are unlikely to be detected in standard drug screens. We now report that avobenzone, the primary active ingredient of most sunscreens, can be activated by zinc to become a potent antibacterial compound against MRSA. Zinc-activated avobenzone (AVB-Zn) potently inhibited a series of clinical MRSA isolates [minimal inhibitory concentration (MIC): 0.62-2.5 µM], without pre-existing resistance and activity without zinc (MIC: >10 µM). AVB-Zn was also active against clinical MRSA isolates that were resistant against the commonly used zinc-salt antibiotic bacitracin. We found AVB-Zn exerted no cytotoxicity on human cell lines and primary cells. Last, we demonstrate AVB-Zn can be deployed therapeutically as lotion preparations, which showed efficacy in a mouse wound model of MRSA infection. AVB-Zn thus demonstrates Zn-activated metallo-antibiotics are a promising avenue for future drug discovery.

Multicenter Evaluation of an MIC-Based Aztreonam and Ceftazidime-Avibactam Broth Disk Elution Test

Due to limited therapeutic options, there is a clinical need to assess the in vitro activity of the combination of aztreonam (ATM) and ceftazidime-avibactam (CZA) to guide the therapeutic management of multidrug-resistant (MDR) Gram-negative organism infections. We set out to develop a practical MIC-based broth disk elution (BDE) method to determine the in vitro activity of the combination ATM-CZA using readily available supplies and compare it to reference broth microdilution (BMD). For the BDE method, a 30-μg ATM disk, a 30/20-μg CZA disk, both disks in combination, and no disks were added to 4 separate 5-mL cation-adjusted Mueller-Hinton broth (CA-MHB) tubes, using various manufacturers. Three testing sites performed both BDE and reference BMD testing of bacterial isolates in parallel from a single 0.5 McFarland standard inoculum and after overnight incubation, assessed them for growth (not susceptible) or no growth (susceptible) at a final concentration of 6/6/4 μg/mL ATM-CZA. During the first phase, the precision and accuracy of the BDE were analyzed by testing 61 Enterobacterales isolates at all sites. This testing yielded 98.3% precision between sites, with 98.3% categorical agreement and 1.8% major errors (ME). During the second phase, at each site, we evaluated unique, clinical isolates of metallo-β-lactamase (MBL)-producing Enterobacterales (n = 75), carbapenem-resistant Pseudomonas aeruginosa (n = 25), Stenotrophomonas maltophilia (n = 46), and Myroides sp. (n = 1). This testing resulted in 97.9% categorical agreement, with 2.4% ME. Different results were observed for different disk and CA-MHB manufacturers, requiring a supplemental ATM-CZA-not-susceptible quality control organism to ensure the accuracy of results. The BDE is a precise and effective methodology for determining susceptibility to the combination ATM-CZA.

Increased zinc levels facilitate phenotypic detection of ceftazidime-avibactam resistance in metallo-β-lactamase-producing Gram-negative bacteria

Ceftazidime-avibactam is one of the last resort antimicrobial agents for the treatment of carbapenem-resistant, Gram-negative bacteria. Metallo-β-lactamase-producing bacteria are considered to be ceftazidime-avibactam resistant. Here, we evaluated a semi-automated antimicrobial susceptibility testing system regarding its capability to detect phenotypic ceftazidime-avibactam resistance in 176 carbapenem-resistant, metallo-β-lactamase-producing Enterobacterales and Pseudomonas aeruginosa isolates. Nine clinical isolates displayed ceftazidime-avibactam susceptibility in the semi-automated system and six of these isolates were susceptible by broth microdilution, too. In all nine isolates, metallo-β-lactamase-mediated hydrolytic activity was demonstrated with the EDTA-modified carbapenemase inactivation method. As zinc is known to be an important co-factor for metallo-β-lactamase activity, test media of the semi-automated antimicrobial susceptibility testing system and broth microdilution were supplemented with zinc. Thereby, the detection of phenotypic resistance was improved in the semi-automated system and in broth microdilution. Currently, ceftazidime-avibactam is not approved as treatment option for infections by metallo-β-lactamase-producing, Gram-negative bacteria. In infections caused by carbapenem-resistant Gram-negatives, we therefore recommend to rule out the presence of metallo-β-lactamases with additional methods before initiating ceftazidime-avibactam treatment.

Comparison of carbapenem minimum inhibitory concentrations of OXA-48-like Klebsiella pneumoniae by Sensititre, Vitek 2, MicroScan and Etest

OBJECTIVES

The aim of this laboratory-based study was to compare carbapenem minimum inhibitory concentrations (MICs) yielded by Sensititre, Vitek 2, MicroScan WalkAway plus and Etest for OXA-48-like Klebsiella pneumoniae isolates.

METHODS

Analysis was performed for categorical agreement for ertapenem, meropenem and imipenem, and the proportion of isolates with MICs ≤8μg/mL and the MIC50/MIC90 for meropenem and imipenem, from a convenience sample of 82 deduplicated blood culture OXA-48-like K. pneumoniae isolates.

RESULTS

The proportion of isolates testing susceptible to ertapenem by Etest (19/82, 23.1%) differed from Sensititre/Vitek (0/82) and MicroScan (2/82, 2.4%) (p<0.001 for all). For meropenem the proportion of isolates susceptible by Etest (31/82, 37.8%) differed from Sensititre/Vitek (16/82, 19.5%) (p=0.015). There was variation in the proportion of isolates that tested imipenem susceptible when comparing Sensititre (9/82, 11%) and Vitek (8/82, 9.8%) to MicroScan (27/82, 32.9%), p=0.001 and p<0.001, respectively, Sensititre and Vitek to Etest (45/82, 54.9%), p<0.001 for both, and MicroScan to Etest, p=0.007. The proportion of isolates with meropenem MICs ≤8μg/mL with Sensititre and Vitek differed significantly from Etest, 58.5% and 85.4%, respectively, p<0.001. A 2-fold difference between the Sensititre and Vitek meropenem and imipenem MIC at which ≥50% of isolates were inhibited compared to the MicroScan, and a 4-fold difference compared to Etest, was present.

CONCLUSIONS

Substantial variability in carbapenem MICs for OXA-48-like CPE isolates by the four methods was demonstrated. Performance characteristics verification of MIC methods in use for the predominant CPE type is required by laboratories to optimise accuracy of carbapenem reporting.

Rational Design of Silicon-Based Zinc Ionophores

Ionophores transport ions across biological membranes and have wide-ranging applications, but a platform for their rapid development does not exist. We report a platform for developing ionophores from metal-ion chelators, which are readily available with wide-ranging affinities and specificities, and structural data that can aid rational design. Specifically, we fine-tuned the binding affinity and lipophilicity of a Zn^II -chelating ligand by introducing silyl groups proximal to the Zn^II -binding pocket, which generated ionophores that performed better than most of the currently known Zn^II ionophores. Furthermore, these silicon-based ionophores were specific for Zn^II over other metals and exhibited better antibacterial activity and less toxicity to mammalian cells than several known Zn^II ionophores, including pyrithione. These studies establish rational design principles for the rapid development of potent and specific ionophores and a new class of antibacterial agents.

Comparison of Zinc Concentrations in the Broth of Commercial Automated Susceptibility Testing Devices (Vitek 2, MicroScan, BD Phoenix, and Sensititre)

Up to 4-fold differences in zinc concentrations have been observed in commercial broth routinely utilized for susceptibility testing via manual broth microdilution. Herein, we report the concentration of zinc in the broth of common automated susceptibility testing (AST) platforms (Vitek, MicroScan, BD Phoenix, and Sensititre). For AST platforms with lyophilized broth contents (Vitek and MicroScan), wells were rehydrated with appropriate diluent, and contents were aliquoted out for zinc assay. Aliquots from the manufacturer-specific broth (premade cation-adjusted Mueller-Hinton broth [caMHB]) for BD Phoenix and Sensititre were also assayed by inductively coupled plasma mass spectrometry. Up to a 10-fold difference in zinc concentrations was observed across the 4 platforms (MicroScan: 0.46 mg/L; BD Phoenix: 1.16 mg/L; Vitek: 1.22 mg/L; Sensititre: 4.49 mg/L). Attention should be given to the supraphysiologic and variable zinc concentrations observed in broth used in automated platforms and the subsequent implications for susceptibility testing of metallo-β-lactamase (MBL)-harboring isolates. This variability also hampers efforts to develop a standardized method to uniformly reduce zinc concentrations in broth and mimic physiologic zinc conditions.

IMPORTANCE Growing data on the impact of extracellular zinc concentration on metallo-β-lactamase-mediated resistance has shed light on the importance of susceptibility testing media. However, there are no studies documenting the amount of zinc in commonly utilized automated susceptibility testing (AST) platforms. This study reveals supraphysiologic zinc concentrations as well as large zinc variability among AST platforms and highlights the challenges this raises in the development of zinc-limited media.

Metallo-β-lactamases and a tug-of-war for the available zinc at the host-pathogen interface

Metallo-β-lactamases (MBLs) are zinc-dependent hydrolases that inactivate virtually all β-lactam antibiotics. The expression of MBLs by Gram-negative bacteria severely limits the therapeutic options to treat infections. MBLs bind the essential metal ions in the bacterial periplasm, and their activity is challenged upon the zinc starvation conditions elicited by the native immune response. Metal depletion compromises both the enzyme activity and stability in the periplasm, impacting on the resistance profile in vivo. Thus, novel inhibitory approaches involve the use of chelating agents or metal-based drugs that displace the native metal ion. However, newer MBL variants incorporate mutations that improve their metal binding abilities or stabilize the metal-depleted form, revealing that metal starvation is a driving force acting on MBL evolution. Future challenges require addressing the gap between in cell and in vitro studies, dissecting the mechanism for MBL metalation and determining the metal content in situ.

Evaluation of Metallo-β-Lactamase Susceptibility Testing in a Physiologic Medium

Research in identifying alternative growth media that better mimic host conditions is gaining ground. Relative to nutrient-rich Mueller-Hinton broth (MHB), data on the influence of physiologic or host-mimicking media on metallo-β-lactamase (MBL) resistance are lacking. The objective was to evaluate meropenem susceptibility against clinical and engineered MBL-harboring Enterobacterales strains in a physiologic medium (urine). Antimicrobial susceptibility testing (AST) by broth microdilution was conducted with a wild-type Klebsiella pneumoniae strain and two engineered isogenic variants harboring K. pneumoniae carbapenemase 2 (KPC-2) or New Delhi MBL 1 (NDM-1), as well as two clinical K. pneumoniae isolates (harboring NDM-1 and VIM-1). MICs were determined in conventional cation-adjusted MHB (caMHB) and sterile-filtered urine samples (18 patients). All KPC- and MBL-harboring isolates were meropenem resistant (MICs of ≥16 mg/liter) in caMHB. AST of the KPC isolate in urine resulted in 50% (9/18 urine samples) essential agreement (i.e., within ±1 dilution, relative to the caMHB MIC), highlighting challenges with the use of urine as a medium capable of supporting AST. In the 9 AST-viable urine samples, meropenem MICs were 2- to 9-fold lower than that in caMHB (MIC of 32 mg/liter) among MBL-harboring isolates. Zinc concentrations determined by inductively coupled plasma mass spectrometry averaged 1.25 mg/liter and ranged from 0.12 to 1.14 mg/liter in caMHB and 18 urine samples, respectively. The full extent of MBL-mediated resistance among K. pneumoniae isolates appears to be attenuated in urine. Factors influencing free bioactive zinc levels warrant further investigation. IMPORTANCE Studies assessing antibiotic susceptibility profiles in nonconventional media are lacking. MBL-mediated resistance has come under scrutiny due to the dependence on extracellular zinc concentrations, which makes the choice of testing medium influential for β-lactam MICs. This study explores human urine as a physiologically relevant matrix with which susceptibility profiles of MBL-harboring isolates can be assessed, relative to conventional broth.

Trade-Offs between Antibacterial Resistance and Fitness Cost in the Production of Metallo-β-Lactamases by Enteric Bacteria Manifest as Sporadic Emergence of Carbapenem Resistance in a Clinical Setting

Meropenem is a clinically important antibacterial reserved for treatment of multiresistant infections. In meropenem-resistant bacteria of the family Enterobacterales, NDM-1 is considerably more common than IMP-1, despite both metallo-β-lactamases (MBLs) hydrolyzing meropenem with almost identical kinetics. We show that bla_NDM-1 consistently confers meropenem resistance in wild-type Enterobacterales, but bla_IMP-1 does not. The reason is higher bla_NDM-1 expression because of its stronger promoter. However, the cost of meropenem resistance is reduced fitness of bla_NDM-1-positive Enterobacterales. In parallel, from a clinical case, we identified multiple Enterobacter spp. isolates carrying a plasmid-encoded bla_NDM-1 having a modified promoter region. This modification lowered MBL production to a level associated with zero fitness cost, but, consequently, the isolates were not meropenem resistant. However, we identified a Klebsiella pneumoniae isolate from this same clinical case carrying the same bla_NDM-1 plasmid. This isolate was meropenem resistant despite low-level NDM-1 production because of a ramR mutation reducing envelope permeability. Overall, therefore, we show how the resistance/fitness trade-off for MBL carriage can be resolved. The result is sporadic emergence of meropenem resistance in a clinical setting.

The paradoxical in vivo activity of β-lactams against metallo-β-lactamase-producing Enterobacterales is not restricted to carbapenems

BACKGROUND

Using murine models of infection, we previously reported the potent in vivo activity of carbapenems against MBL-producing Enterobacterales despite the observed resistance in vitro. In the current study, we examined the in vivo activity of a cefepime human-simulated regimen against MBL-producing Enterobacterales in a murine thigh infection model.

METHODS

A population of clinical isolates and isogenic engineered MBL-producing Enterobacterales transformants expressing MBLs but no detectable cefepime-hydrolysing serine β-lactamases were utilized. KPC-producing isolates were included as positive controls. Cefepime, piperacillin/tazobactam and meropenem MICs were determined using broth microdilution in conventional CAMHB and EDTA-supplemented (zinc-limited) broth. In vivo efficacy of a cefepime human-simulated regimen (2 g q8h as a 2 h infusion) was determined in the neutropenic murine thigh infection model against the test strains. Efficacy was measured as the change in log10 cfu/thigh at 24 h compared with 0 h controls.

RESULTS

MBL-producing Enterobacterales strains were found to be cefepime, piperacillin/tazobactam and meropenem non-susceptible in conventional broth. Supplementation with EDTA at a concentration of 300 mg/L resulted in multi-fold reduction in the MICs and restoration of susceptibility. In accordance with the MICs generated in zinc-limited broth, administration of a cefepime human-simulated regimen was associated with substantial bacterial reductions among mice infected with MBL-producing Enterobacterales. Absence of MIC reduction in zinc-limited broth and lack of efficacy among mice infected with KPC-producing isolates were observed.

CONCLUSIONS

For MBL-producing Enterobacterales, susceptibility testing with Mueller-Hinton broth, a zinc-rich testing medium, is flawed since it does not recapitulate the host environment, in which zinc concentrations are low.

Activity of β-Lactam Antibiotics against Metallo-β-Lactamase-Producing Enterobacterales in Animal Infection Models: a Current State of Affairs

Metallo-β-lactamases (MBLs) result in resistance to nearly all β-lactam antimicrobial agents, as determined by currently employed susceptibility testing methods. However, recently reported data demonstrate that variable and supraphysiologic zinc concentrations in conventional susceptibility testing media compared with physiologic (bioactive) zinc concentrations may be mediating discordant in vitro-in vivo MBL resistance. While treatment outcomes in patients appear suggestive of this discordance, these limited data are confounded by comorbidities and combination therapy. To that end, the goal of this review is to evaluate the extent of β-lactam activity against MBL-harboring Enterobacterales in published animal infection model studies and provide contemporary considerations to facilitate the optimization of current antimicrobials and development of novel therapeutics.