Penetration of beta-lactamase inhibitors into the periplasm of gram-negative bacteria

Overcoming Beta-Lactamase-Based Antimicrobial Resistance by Nanocarrier-Loaded Clavulanic Acid and Antibiotic Cotreatments

Antimicrobial resistance (AMR) is one of the major threats to modern healthcare. Many types of bacteria have developed resistance to multiple antibiotic treatments, while additional antibiotics have not been recently brought to market. One approach to counter AMR based on the beta-lactamase enzyme has been to use cotreatments of an antibiotic and an inhibitor, to enhance the antibiotic action. Here, we aimed to enhance this technique by developing nanocarriers of two cationic beta-lactam class antibiotics, amoxicillin, and ticarcillin, combined with a beta-lactamase inhibitor, clavulanic acid, which can potentially overcome this type of AMR. We demonstrate for the first time that beta-lactamase inhibitor-loaded nanocarriers in cotreatments with either free or nanocarrier-loaded beta-lactam antibiotics can enhance their effectiveness further than when used alone. We use surface-functionalized shellac-/Poloxamer 407-stabilized antibiotic nanocarriers on Pseudomonas aeruginosa, which is susceptible to ticarcillin but is resistant to amoxicillin. We show an amplification of the antibiotic effect of amoxicillin and ticarcillin loaded in shellac nanoparticles, both alone and as a cotreatment with free or nanocarrier-loaded clavulanic acid. We also report a significant increase in the antimicrobial effects of clavulanic acid loaded in such nanocarriers as a cotreatment. We explain the increased antimicrobial activity of the cationically functionalized antibiotic-loaded nanoparticles with electrostatic attraction to the bacterial cell wall, which delivers higher local antibiotic and inhibitor concentrations. The effect is due to the accumulation of the clavulanic acid-loaded nanocarriers on the bacterial cell walls that allows a higher proportion of the inhibitor to engage with the produced intracellular beta-lactamases. These nanocarriers were also found to have a very low cytotoxic effect against human keratinocytes, which shows great potential for overcoming enzyme-based AMR.

Structural Characterization of the D179N and D179Y Variants of KPC-2 β-Lactamase: Ω-Loop Destabilization as a Mechanism of Resistance to Ceftazidime-Avibactam

Klebsiella pneumoniae carbapenemases (KPC-2 and KPC-3) present a global clinical threat, as these β-lactamases confer resistance to carbapenems and oxyimino-cephalosporins. Recent clinically identified KPC variants with substitutions at Ambler position D179, located in the Ω loop, are resistant to the β-lactam/β-lactamase inhibitor combination ceftazidime-avibactam, but susceptible to meropenem-vaborbactam. To gain insights into ceftazidime-avibactam resistance conferred by D179N/Y variants of KPC-2, crystal structures of these variants were determined. The D179N KPC-2 structure revealed that the change of the carboxyl to an amide moiety at position 179 disrupted the salt bridge with R164 present in wild-type KPC-2. Additional interactions were disrupted in the Ω loop, causing a decrease in the melting temperature. Shifts originating from N179 were also transmitted toward the active site, including ∼1-Å shifts of the deacylation water and interacting residue N170. The structure of the D179Y KPC-2 β-lactamase revealed more drastic changes, as this variant exhibited disorder of the Ω loop, with other flanking regions also being disordered. We postulate that the KPC-2 variants can accommodate ceftazidime because the Ω loop is displaced in D179Y or can be more readily displaced in D179N KPC-2. To understand why the β-lactamase inhibitor vaborbactam is less affected by the D179 variants than avibactam, we determined the crystal structure of D179N KPC-2 in complex with vaborbactam, which revealed wild-type KPC-2-like vaborbactam-active site interactions. Overall, the structural results regarding KPC-2 D179 variants revealed various degrees of destabilization of the Ω loop that contribute to ceftazidime-avibactam resistance, possible substrate-assisted catalysis of ceftazidime, and meropenem and meropenem-vaborbactam susceptibility.

Role of TEM-1 β-Lactamase in the Predominance of Ampicillin-Sulbactam-Nonsusceptible Escherichia coli in Japan

We investigated the epidemiology and resistance mechanisms of ampicillin-sulbactam-nonsusceptible Escherichia coli, focusing on the role of the TEM-1 β-lactamase. We collected all nonduplicate E. coli clinical isolates at 10 Japanese hospitals during December 2014 and examined their antimicrobial susceptibility, β-lactamases, TEM-1 transferability, TEM-1 β-lactamase activity, outer membrane protein profile, membrane permeability, and clonal genotypes. Among the 329 isolates collected, 95 were ampicillin-sulbactam nonsusceptible. Of these ampicillin-sulbactam-nonsusceptible isolates, β-lactamases conferring resistance to sulbactam, such as AmpC, were present in 33%. Hyperproduction of sulbactam-susceptible β-lactamases, TEMs with a strong promoter, were rare (5%). The remaining 59 isolates (62%) had only sulbactam-susceptible β-lactamases, including TEM-1 with a wild-type promoter (n = 28), CTX-Ms (n = 13), or both (n = 17). All 45 transconjugants from 96 donors with TEM-1 had higher ampicillin-sulbactam MICs (4 to 96 mg/liter) than the recipient (2 mg/liter). In donors with only TEM-1, TEM-1 activity correlated with the 50% inhibitory concentration of sulbactam and ampicillin-sulbactam MICs. The decreased membrane permeation of sulbactam was associated with an increased ampicillin-sulbactam MIC. The reduced permeation was partly attributable to deficient outer membrane proteins, which were observed in 57% of the ampicillin-sulbactam-nonsusceptible isolates with only TEM-1 and a wild-type promoter. Sequence type 131 (ST131) was the most common clonal type (52%). TEM-1 with a wild-type promoter primarily contributed to ampicillin-sulbactam nonsusceptibility in E. coli, with the partial support of other mechanisms, such as reduced permeation. Conjugative TEM-1 and the clonal spread of ST131 may contribute to the prevalence of Japanese ampicillin-sulbactam-nonsusceptible isolates.

Real-Time Monitoring of NDM-1 Activity in Live Bacterial Cells by Isothermal Titration Calorimetry: A New Approach To Measure Inhibition of Antibiotic-Resistant Bacteria

The "superbug" infection caused by New Delhi metallo-β-lactamase (NDM-1) has become an emerging threat. Monitoring NDM-1 has proven challenging due to its shuttling between pathogenic bacteria. Here, we report an isothermal titration calorimetry (ITC) method that can monitor activity and inhibition of NDM-1 in live bacterial cells in real time. This method has been exemplified by monitoring of the activity and inhibition of the target enzyme and evaluating the breakdown of antibiotics by pathogenic bacteria expressing β-lactamases. Cell-based studies demonstrate that the NDM-1 expressed in bacterial cells was inhibited by four known inhibitors ethylene diamine tetraacetic acid (EDTA), d-captopril, ebselen and azolylthioacetamide with fifty percent inhibitory concentration (IC₅₀) values of 3.8, 48, 0.55, and 17.5 μM, respectively, which are in good agreement with the data from inhibition kinetics using UV-vis and NMR spectroscopy in vivo. This approach could be applied to screen and evaluate small molecule inhibitors of metallo-β-lactamases (MβLs) in whole cells or to identify drug resistant bacteria.

Outer membrane vesicles from β-lactam-resistant Escherichia coli enable the survival of β-lactam-susceptible E. coli in the presence of β-lactam antibiotics

Outer membrane vesicles (OMVs) containing various bacterial compounds are released from mainly gram-negative bacteria. Secreted OMVs play important roles in the ability of a bacterium to defend itself, and thus contribute to the survival of bacteria in a community. In this study, we collected OMVs from β-lactam antibiotic-resistant Escherichia coli established by conjugation assay and the parental β-lactam antibiotic-susceptible strain, and performed comparative proteomic analysis to examine whether these OMVs carried β-lactam-resistant compounds. We also investigated whether both types of OMVs could protect susceptible cells from β-lactam-induced death and/or directly degrade β-lactam antibiotics. Several proteins that can be involved in degrading β-lactam antibiotics were more abundant in OMVs from β-lactam-resistant E. coli, and thus OMVs from β-lactam resistant E. coli could directly and dose-dependently degrade β-lactam antibiotics and fully rescue β-lactam-susceptible E. coli and other bacterial species from β-lactam antibiotic-induced growth inhibition. Taken together, present study demonstrate that OMVs from β-lactam-resistant E. coli play important roles in survival of antibiotic susceptible bacteria against β-lactam antibiotics. This finding may pave the way for new efforts to combat the current global spread of antibiotic resistances, which is considered to be a significant public health threat.

Role of the Outer Membrane and Porins in Susceptibility of β-Lactamase-Producing Enterobacteriaceae to Ceftazidime-Avibactam

This study examined the activity of the novel antimicrobial combination ceftazidime-avibactam against Enterobacteriaceae exhibiting different outer membrane permeability profiles, specifically with or without porins and with or without expression of the main efflux pump (AcrAB-TolC). The addition of the outer membrane permeabilizer polymyxin B nonapeptide increased the antibacterial activities of avibactam alone, ceftazidime alone, and ceftazidime-avibactam against the characterized clinical isolates of Escherichia coli, Enterobacter aerogenes, and Klebsiella pneumoniae. This enhancement of activities was mainly due to increased passive penetration of compounds since inhibition of efflux by the addition of phenylalanine-arginine β-naphthylamide affected the MICs minimally. OmpF (OmpK35) or OmpC (OmpK36) pores were not the major route by which avibactam crossed the outer membranes of E. coli and K. pneumoniae. In contrast, Omp35 and Omp36 allowed diffusion of avibactam across the outer membrane of E. aerogenes, although other diffusion channels for avibactam were also present in that species. It was clear that outer membrane permeability and outer membrane pore-forming proteins play a key role in the activity of ceftazidime-avibactam. Nevertheless, the MICs of ceftazidime-avibactam (with 4 mg/liter avibactam) against the ceftazidime-resistant clinical isolates of the three species of Enterobacteriaceae studied were ≤ 8 mg/liter, regardless of outer membrane permeability changes resulting from an absence of defined porin proteins or upregulation of efflux.

In Vitro Activity of β-Lactams in Combination with β-Lactamase Inhibitors against Multidrug-Resistant Mycobacterium tuberculosis Isolates

The combination of β-lactams and β-lactamase inhibitors has been shown to have potent in vitro activity against multidrug-resistant tuberculosis (MDR-TB) isolates. In order to identify the most potent β-lactam-β-lactamase inhibitor combination against MDR-TB, we selected nine β-lactams and three β-lactamase inhibitors, which belong to different subgroups. A total of 121 MDR-TB strains were included in this study. Out of the β-lactams used herein, biapenem was the most effective against MDR-TB and had an MIC50 value of 8 μg/ml. However, after the addition of clavulanate or sulbactam, meropenem exhibited the most potent anti-MDR-TB activity with an MIC50 value of 4 μg/ml. For meropenem, 76 (62.8%), 41 (33.9%), and 22 (18.2%) of the 121 MDR-TB strains were subjected to a synergistic effect when the drug was combined with sulbactam, tazobactam, or clavulanate, respectively. Further statistical analysis revealed that significantly more strains experienced a synergistic effect when exposed to the combination of meropenem with sulbactam than when exposed to meropenem in combination with tazobactam or clavulanate, respectively (P < 0.01). In addition, a total of 10.7% (13/121) of isolates harbored mutations in the blaC gene, with two different nucleotide substitutions: AGT333AGG and ATC786ATT. For the strains with a Ser111Arg substitution in BlaC, a better synergistic effect was observed in the meropenem-clavulanate and in the amoxicillin-clavulanate combinations than that in a synonymous single nucleotide polymorphism (SNP) group. In conclusion, our findings demonstrate that the combination of meropenem and sulbactam shows the most potent activity against MDR-TB isolates. In addition, the Ser111Arg substitution of BlaC may be associated with an increased susceptibility of MDR-TB isolates to meropenem and amoxicillin in the presence of clavulanate.

Following drug uptake and reactions inside Escherichia coli cells by Raman microspectroscopy

Raman microspectroscopy combined with Raman difference spectroscopy reveals the details of chemical reactions within bacterial cells. The method provides direct quantitative data on penetration of druglike molecules into Escherichia coli cells in situ along with the details of drug-target reactions. With this label-free technique, clavulanic acid and tazobactam can be observed as they penetrate into E. coli cells and subsequently inhibit β-lactamase enzymes produced within these cells. When E. coli cells contain a β-lactamase that forms a stable complex with an inhibitor, the Raman signature of the known enamine acyl-enzyme complex is detected. From Raman intensities it is facile to measure semiquantitatively the number of clavulanic acid molecules taken up by the lactamase-free cells during growth.