Cefazolin and imipenem enhance AmpC expression and resistance in NagZ-dependent manner in Enterobacter cloacae complex

BACKGROUND

Enterobacter cloacae complex (ECC) is a common opportunistic pathogen and is responsible for causing various infections in humans. Owing to its inducible chromosomal AmpC β-lactamase (AmpC), ECC is inherently resistant to the 1st- and 2nd- generation cephalosporins. However, whether β-lactams antibiotics enhance ECC resistance remains unclear.

RESULTS

In this study, we found that subinhibitory concentrations (SICs) of cefazolin (CFZ) and imipenem (IMP) can advance the expression of AmpC and enhance its resistance towards β-lactams through NagZ in Enterobacter cloacae (EC). Further, AmpC manifested a substantial upregulation in EC in response to SICs of CFZ and IMP. In nagZ knockout EC (ΔnagZ), the resistance to β-lactam antibiotics was rather weakened and the effect of CFZ and IMP on AmpC induction was completely abrogated. NagZ ectopic expression can rescue the induction effects of CFZ and IMP on AmpC and increase ΔnagZ resistance. More importantly, CFZ and IMP have the potential to induce the expression of AmpR's target genes in a NagZ-dependent manner.

CONCLUSIONS

Our findings suggest that NagZ is a critical determinant for CFZ and IMP to promote AmpC expression and resistance and that CFZ and IMP should be used with caution since they may aggravate ECC resistance. At the same time, this study further improves our understanding of resistance mechanisms in ECC.

Impact of Peptidoglycan Recycling Blockade and Expression of Horizontally Acquired β-Lactamases on Pseudomonas aeruginosa Virulence

In the current scenario of antibiotic resistance magnification, new weapons against top nosocomial pathogens like Pseudomonas aeruginosa are urgently needed. The interplay between β-lactam resistance and virulence is considered a promising source of targets to be attacked by antivirulence therapies, and in this regard, we previously showed that a peptidoglycan recycling blockade dramatically attenuated the pathogenic power of P. aeruginosa strains hyperproducing the chromosomal β-lactamase AmpC. Here, we sought to ascertain whether this observation could be applicable to other β-lactamases. To do so, P. aeruginosa wild-type or peptidoglycan recycling-defective strains (ΔampG and ΔnagZ) harboring different cloned β-lactamases (transferable GES, VIM, and OXA types) were used to assess their virulence in Galleria mellonella larvae by determining 50% lethal doses (LD50s). A mild yet significant LD50 increase was observed after peptidoglycan recycling disruption per se, whereas the expression of class A and B enzymes did not impact virulence. While the production of the narrow-spectrum class D OXA-2 entailed a slight attenuation, its extended-spectrum derivatives OXA-226 (W159R [bearing a change of W to R at position 159]), OXA-161 (N148D), and principally, OXA-539 (D149 duplication) were associated with outstanding virulence impairments, especially in recycling-defective backgrounds (with some LD50s being >1,000-fold that of the wild type). Although their exact molecular bases remain to be deciphered, these results suggest that mutations affecting the catalytic center and, therefore, the hydrolytic spectrum of OXA-2-derived enzymes also drastically impact the pathogenic power of P. aeruginosa. This work provides new and relevant knowledge to the complex topic of the interplay between the production of β-lactamases and virulence that could be useful to build future therapeutic strategies against P. aeruginosa. IMPORTANCE Pseudomonas aeruginosa is one of the leading nosocomial pathogens whose growing resistance makes the development of therapeutic options extremely urgent. The resistance-virulence interplay has classically aroused researchers' interest as a source of therapeutic targets. In this regard, we describe a wide array of virulence attenuations associated with different transferable β-lactamases, among which the production of OXA-2-derived extended-spectrum β-lactamases stood out as a dramatic handicap for pathogenesis, likely as a side effect of mutations causing the expansion of their hydrolytic spectrums. Moreover, our results confirm the validity of disturbing peptidoglycan recycling as a weapon to attenuate P. aeruginosa virulence in class C and D β-lactamase production backgrounds. In the current scenario of dissemination of horizontally acquired β-lactamases, this work brings out new data on the complex interplay between the production of specific enzymes and virulence attenuation that, if complemented with the characterization of the underlying mechanisms, will likely be exploitable to develop future virulence-targeting antipseudomonal strategies.

In Vivo Validation of Peptidoglycan Recycling as a Target to Disable AmpC-Mediated Resistance and Reduce Virulence Enhancing the Cell-Wall-Targeting Immunity

BACKGROUND

Searching for new strategies to defeat Pseudomonas aeruginosa is of paramount importance. Previous works in vitro showed that peptidoglycan recycling blockade disables AmpC-dependent resistance and enhances susceptibility against cell-wall-targeting immunity. Our objective was to validate these findings in murine models.This study shows for the first time in different murine models of infection that blocking the peptidoglycan recycling in Pseudomonas aeruginosa causes an important virulence impairment and disables AmpC-mediated resistance, being hence validated as a promising therapeutic target.

METHODS

Wildtype PAO1, recycling-defective AmpG and NagZ mutants, an AmpC hyperproducer dacB mutant, and their combinations were used to cause systemic/respiratory infections in mice. Their survival, bacterial burden, inflammation level, and effectiveness of ceftazidime or subtherapeutic colistin to treat the infections were assessed.

RESULTS

Inactivation of AmpG or NagZ significantly attenuated the virulence in terms of mice mortality, bacterial load, and inflammation. When inactivating these genes in the dacB-defective background, the β-lactam resistance phenotype was abolished, disabling the emergence of ceftazidime-resistant mutants, and restoring ceftazidime for treatment. Subtherapeutic colistin was shown to efficiently clear the infection caused by the recycling-defective strains, likely due to the combined effect with the mice cell-wall- targeting immunity.

CONCLUSIONS

This study brings us one step closer to new therapies intended to disable P. aeruginosa AmpC-mediated resistance and dampen its virulence, and strongly support the interest in developing efficient AmpG and/or NagZ inhibitors.

Role of Low-Molecular-Mass Penicillin-Binding Proteins, NagZ and AmpR in AmpC β-lactamase Regulation of Yersinia enterocolitica

Yersinia enterocolitica encodes a chromosomal AmpC β-lactamase under the regulation of the classical ampR-ampC system. To obtain a further understanding to the role of low-molecular-mass penicillin-binding proteins (LMM PBPs) including PBP4, PBP5, PBP6, and PBP7, as well as NagZ and AmpR in ampC regulation of Y. enterocolitica, series of single/multiple mutant strains were systematically constructed and the ampC expression levels were determined by luxCDABE reporter system, reverse transcription-PCR (RT-PCR) and β-lactamase activity test. Sequential deletion of PBP5 and other LMM PBPs result in a continuously growing of ampC expression level, the β-lactamse activity of quadruple deletion strain YEΔ4Δ5Δ6Δ7 (pbp4, pbp5, pbp6, and pbp7 inactivated) is approached to the YEΔD123 (ampD1, ampD2, and ampD3 inactivated). Deletion of nagZ gene caused two completely different results in YEΔD123 and YEΔ4Δ5Δ6Δ7, NagZ is indispensable for YEΔ4Δ5Δ6Δ7 ampC derepression phenotype but dispensable for YEΔD123. AmpR is essential for ampC hyperproduction in these two types of strains, inactivation of AmpR notable reduced the ampC expression level in both YEΔD123 and YEΔ4Δ5Δ6Δ7.

Conformational flexibility of the glycosidase NagZ allows it to bind structurally diverse inhibitors to suppress β-lactam antibiotic resistance

NagZ is an N-acetyl-β-d-glucosaminidase that participates in the peptidoglycan (PG) recycling pathway of Gram-negative bacteria by removing N-acetyl-glucosamine (GlcNAc) from PG fragments that have been excised from the cell wall during growth. The 1,6-anhydromuramoyl-peptide products generated by NagZ activate β-lactam resistance in many Gram-negative bacteria by inducing the expression of AmpC β-lactamase. Blocking NagZ activity can thereby suppress β-lactam antibiotic resistance in these bacteria. The NagZ active site is dynamic and it accommodates distortion of the glycan substrate during catalysis using a mobile catalytic loop that carries a histidine residue which serves as the active site general acid/base catalyst. Here, we show that flexibility of this catalytic loop also accommodates structural differences in small molecule inhibitors of NagZ, which could be exploited to improve inhibitor specificity. X-ray structures of NagZ bound to the potent yet non-selective N-acetyl-β-glucosaminidase inhibitor PUGNAc (O-(2-acetamido-2-deoxy-d-glucopyranosylidene) amino-N-phenylcarbamate), and two NagZ-selective inhibitors - EtBuPUG, a PUGNAc derivative bearing a 2-N-ethylbutyryl group, and MM-156, a 3-N-butyryl trihydroxyazepane, revealed that the phenylcarbamate moiety of PUGNAc and EtBuPUG completely displaces the catalytic loop from the NagZ active site to yield a catalytically incompetent form of the enzyme. In contrast, the catalytic loop was found positioned in the catalytically active conformation within the NagZ active site when bound to MM-156, which lacks the phenylcarbamate extension. Displacement of the catalytic loop by PUGNAc and its N-acyl derivative EtBuPUG alters the active site conformation of NagZ, which presents an additional strategy to improve the potency and specificity of NagZ inhibitors.

The sentinel role of peptidoglycan recycling in the β-lactam resistance of the Gram-negative Enterobacteriaceae and Pseudomonas aeruginosa

The peptidoglycan is the structural polymer of the bacterial cell envelope. In contrast to an expectation of a structural stasis for this polymer, during the growth of the Gram-negative bacterium this polymer is in a constant state of remodeling and extension. Our current understanding of this peptidoglycan "turnover" intertwines with the deeply related phenomena of the liberation of small peptidoglycan segments (muropeptides) during turnover, the presence of dedicated recycling pathways for reuse of these muropeptides, β-lactam inactivation of specific penicillin-binding proteins as a mechanism for the perturbation of the muropeptide pool, and this perturbation as a controlling mechanism for signal transduction leading to the expression of β-lactamase(s) as a key resistance mechanism against the β-lactam antibiotics. The nexus for many of these events is the control of the AmpR transcription factor by the composition of the muropeptide pool generated during peptidoglycan recycling. In this review we connect the seminal observations of the past decades to new observations that resolve some, but certainly not all, of the key structures and mechanisms that connect to AmpR.

E. coli sabotages the in vivo production of O-linked β-N-acetylglucosamine-modified proteins

The O-linked β-N-acetylglucosamine (O-GlcNAc) post-translational modification is an important, regulatory modification of cytosolic and nuclear enzymes. To date, no 3-dimensional structures of O-GlcNAc-modified proteins exist due to difficulties in producing sufficient quantities with either in vitro or in vivo techniques. Recombinant co-expression of substrate protein and O-GlcNAc transferase in Escherichia coli was used to produce O-GlcNAc-modified domains of human cAMP responsive element-binding protein (CREB1) and Abelson tyrosine-kinase 2 (ABL2). Recombinant expression in E. coli is an advantageous approach, but only small quantities of insoluble O-GlcNAc-modified protein were produced. Adding β-N-acetylglucosaminidase inhibitor, O-(2-acetamido-2-dexoy-D-glucopyranosylidene)amino-N-phenylcarbamate (PUGNAc), to the culture media provided the first evidence that an E. coli enzyme cleaves O-GlcNAc from proteins in vivo. With the inhibitor present, the yields of O-GlcNAc-modified protein increased. The E. coli β-N-acetylglucosaminidase was isolated and shown to cleave O-GlcNAc from a synthetic O-GlcNAc-peptide in vitro. The identity of the interfering β-N-acetylglucosaminidase was confirmed by testing a nagZ knockout strain. In E. coli, NagZ natively cleaves the GlcNAc-β1,4-N-acetylmuramic acid linkage to recycle peptidoglycan in the cytoplasm and cleaves the GlcNAc-β-O-linkage of foreign O-GlcNAc-modified proteins in vivo, sabotaging the recombinant co-expression system.