Proteolysis and multimerization regulate signaling along the two-component regulatory system AdeRS

Cryo-EM structure and complementary drug efflux activity of the Acinetobacter baumannii multidrug efflux pump AdeG

Multidrug-resistant Acinetobacter baumannii has emerged as one of the most antibiotic-resistant bacterial pathogens associated with nosocomial infection, with its resistance highly depending on multiple multidrug efflux pumps. Here, we report the cryoelectron microscopy (cryo-EM) structure of Acinetobacter drug efflux G (AdeG), the inner membrane component of one of three important resistance-nodulation-cell division (RND) pump family members in A. baumannii, which is involved in drug resistance to chloramphenicol, trimethoprim, ciprofloxacin, and clindamycin. We systematically compare the structures and substrate binding specificities of AdeG, AdeB, and AdeJ multidrug efflux pumps via molecular docking, revealing potential determinants for drug binding. Knockout experiments demonstrate a functional complementarity between AdeABC, AdeFGH, and AdeIJK. Our study provides a structural understanding of A. baumannii multidrug efflux pump AdeG and reveals complementary drug efflux activity between AdeG and other RND efflux pumps, which may promote further rational drug discovery efforts targeting multidrug efflux pumps.

Structural and biophysical characterization of the cytoplasmic domains of HprS kinase and its interactions with the cognate regulator HprR

The HprSR constitutes the bacterial two-component regulatory system engaged by Escherichia coli to reduce the damaging effects of reactive chlorine and oxygen species present in its cytosol. Hypochlorous acid (HOCl) has been shown to be the molecule capable of activating of the HprSR system. HOCl is produced upon pathogen invasion by phagocytic cells of the human innate immune system, particularly neutrophils, to take advantage of its powerful antimicrobial attributes. Therefore, comprehensive studies concerning bacterial sensing and regulatory HprSR system are indispensable in understanding and effectively eliminating pathogens. Here we present the first crystal structure, solved at 1.7 Å resolution, of the HprS cytoplasmic domains arranged as a homodimer. In both protomers, the catalytic ATP-binding domain contains a non-hydrolysable ATP analog coordinated by a magnesium ion. This structure allowed us to provide a detailed characterization of kinase-substrate interaction. Furthermore, the structural data are supported by biophysical studies of kinase interaction with cognate response regulator HprR and substrate ATP. The kinase activity is also assessed in the presence or absence of HprR.

Targeting Acinetobacter baumannii resistance-nodulation-division efflux pump transcriptional regulators to combat antimicrobial resistance

Regulatory elements controlling gene expression fine-tune bacterial responses to environmental cues, including antimicrobials, to optimize survival. Acinetobacter baumannii, a pathogen notorious for antimicrobial resistance, relies on efficient efflux systems. Though the role of efflux systems in antibiotic expulsion are well recognized, the regulatory mechanisms controlling their expression remain understudied. This review explores the current understanding of these regulators, aiming to inspire strategies to combat bacterial resistance and improve therapeutic outcomes.

Molybdate uptake interplay with ROS tolerance modulates bacterial pathogenesis

The rare metal element molybdenum functions as a cofactor in molybdoenzymes that are essential to life in almost all living things. Molybdate can be captured by the periplasmic substrate-binding protein ModA of ModABC transport system in bacteria. We demonstrate that ModA plays crucial roles in growth, multiple metabolic pathways, and ROS tolerance in Acinetobacter baumannii. Crystal structures of molybdate-coordinated A. baumannii ModA show a noncanonical disulfide bond with a conformational change between reduced and oxidized states. Disulfide bond formation reduced binding affinity to molybdate by two orders of magnitude and contributes to its substrate preference. ModA-mediated molybdate binding was important for A. baumannii infection in a murine pneumonia model. Together, our study sheds light on the structural and functional diversity of molybdate uptake and highlights a potential target for antibacterial development.

Activity and resistance mechanisms of the third generation tetracyclines tigecycline, eravacycline and omadacycline against nationwide Spanish collections of carbapenemase-producing Enterobacterales and Acinetobacter baumannii

INTRODUCTION

The rise in multidrug-resistant bacteria challenges clinical microbiology. Tigecycline, eravacycline, and omadacycline show promise against carbapenem-resistant Enterobacterales and Acinetobacter baumannii. This study evaluates their activity and resistance mechanisms.

METHODS

Two Spanish multicentre collections of WGS-characterized carbapenemase-producing Enterobacterales (n=399) and A. baumannii (n=118) were tested. The MICs of tigecycline, eravacycline, omadacycline and classic tetracyclines were determined. WGS-guided resistome analysis, expression studies, efflux pump inhibition and cloning assays identified potential mechanisms of resistance.

RESULTS

Tigecycline and eravacycline exhibited the highest activity against the whole set of Enterobacterales (MIC50/MIC90 0.5/1 mg/L and 1/2 mg/L, respectively), and A. baumannii (MIC50/MIC90 1/2 mg/L and ≤0.25/1 mg/L, respectively). Omadacycline showed no improvement relative to classic tetracyclines (MIC50/MIC90 values of 8/32 mg/L and 8/16 mg/L for Enterobacterales and A. baumannii, respectively). Resistance mechanisms in Enterobacterales included efflux pumps (AcrAB-TolC, OqxAB) and mutation in their regulatory genes. In A. baumannii, adeS gene mutations and adeABC upregulation decreased tigecycline activity. The prevalent sequence types with reduced susceptibility to eravacycline and tigecycline were ST307 in K. pneumoniae and ST2 in A. baumannii.

CONCLUSION

Eravacycline remains a key agent for the treatment of bacterial infections exhibiting promising efficacy against multidrug-resistant pathogens. As an empirical antibiotic it could be a good alternative for severe infections caused by CPE or CRAB, however, its clinical use is limited by the absence of standardized breakpoints. Resistance mechanisms, including efflux pumps and gene mutations, vary among isolates. High-risk clones like K. pneumoniae ST307 and A. baumannii ST2 underscore the necessity of prudent antibiotic use.

Mechanistic and biophysical characterization of polymyxin resistance response regulator PmrA in Acinetobacter baumannii

Introduction

Acinetobacter baumannii PmrAB is a crucial two-component regulatory system (TCS) that plays a vital role in conferring resistance to polymyxin. PmrA, a response regulator belonging to the OmpR/PhoB family, is composed of a C-terminal DNA-binding effector domain and an N-terminal receiver domain. The receiver domain can be phosphorylated by PmrB, a transmembrane sensor histidine kinase that interacts with PmrA. Once phosphorylated, PmrA undergoes a conformational change, resulting in the formation of a symmetric dimer in the receiver domain. This conformational change facilitates the recognition of promoter DNA by the DNA-binding domain of PmrA, leading to the activation of adaptive responses.

Methods

X-ray crystallography was carried out to solve the structure of PmrA receiver domain. Electrophoretic mobility shift assay and Isothermal titration calorimetry were recruited to validate the interaction between the recombinant PmrA protein and target DNA. Field-emission scanning electron microscopy (FE-SEM) was employed to characterize the surface morphology of A. baumannii in both the PmrA knockout and mutation strains.

Results

The receiver domain of PmrA follows the canonical α5β5 response regulator assembly, which undergoes dimerization upon phosphorylation and activation. Beryllium trifluoride is utilized as an aspartate phosphorylation mimic in this process. Mutations involved in phosphorylation and dimerization significantly affected the expression of downstream pmrC and naxD genes. This impact resulted in an enhanced cell surface smoothness with fewer modifications, ultimately contributing to a decrease in colistin (polymyxin E) and polymyxin B resistance. Additionally, a conservative direct-repeat DNA PmrA binding sequence TTTAAGNNNNNTTTAAG was identified at the promoter region of the pmrC and naxD gene. These findings provide structural insights into the PmrA receiver domain and reveal the mechanism of polymyxin resistance, suggesting that PmrA could be a potential drug target to reverse polymyxin resistance in Acinetobacter baumannii.

Alteration of adeS Contributes to Tigecycline Resistance and Collateral Sensitivity to Sulbactam in Acinetobacter baumannii

The treatment of extensively drug-resistant (XDR) A. baumannii has emerged as a major problem. Tigecycline (TGC) and sulbactam (SUL) are both effective antibiotics against XDR A. baumannii. Here, we investigated the in-host evolution and mechanism of collateral sensitivity (CS) phenomenon in development of tigecycline resistance accompanied by a concomitant increase of sulbactam susceptibility. A total of four XDR A. baumannii strains were sequentially isolated from the same patient suffering from bacteremia. Core-genome multilocus sequence typing separated all the strains into two clusters. Comparative analysis of isolate pair 1 revealed that multiplication of blaOXA-23 within Tn2006 on the chromosome contributed to the change in the antimicrobial susceptibility phenotype of isolate pair 1. Additionally, we observed the emergence of CS to sulbactam in isolate pair 2, as demonstrated by an 8-fold increase in the TGC MIC with a simultaneous 4-fold decrease in the SUL MIC. Compared to the parental strain Ab-3557, YZM-0406 showed partial deletion in the two-component system sensor adeS. Reconstruction of the adeS mutant in Ab-3557 in situ suggested that TGC resistance and CS to SUL were mainly caused by the mutation of adeS. Overall, our study reported a novel CS combination of TGC and SUL in A. baumannii and further revealed a mechanism of CS attributed to the mutation of adeS. This study provides a valuable foundation for developing effective regimens and sequential combinations of tigecycline and sulbactam against XDR A. baumannii. IMPORTANCE Collateral sensitivity (CS) has become an increasingly common evolutionary trade-off during adaptive bacterial evolution. Here, we report a novel combination of tigecycline (TGC) resistance and CS to sulbactam (SUL) in A. baumannii. TGC and SUL are both effective antibiotics against XDR A. baumannii, and it is essential to reveal the mechanism of CS between TGC and SUL. In our study, the partial deletion of adeS, a two-component system sensor, was confirmed to be the key factor contributing to this CS phenomenon. This study provides a valuable foundation for developing effective regimens and sequential combinations of tigecycline and sulbactam against XDR A. baumannii.

Insights into the structure and function of the histidine kinase ComP from Bacillus amyloliquefaciens based on molecular modeling

The ComPA two-component signal transduction system (TCS) is essential in Bacillus spp. However, the molecular mechanism of the histidine kinase ComP remains unclear. Here, we predicted the structure of ComP from Bacillus amyloliquefaciens Q-426 (BaComP) using an artificial intelligence approach, analyzed the structural characteristics based on the molecular docking results and compared homologous proteins, and then investigated the biochemical properties of BaComP. We obtained a truncated ComPS protein with high purity and correct folding in solution based on the predicted structures. The expression and purification of BaComP proteins suggested that the subdomains in the cytoplasmic region influenced the expression and stability of the recombinant proteins. ComPS is a bifunctional enzyme that exhibits the activity of both histidine kinase and phosphotransferase. We found that His571 played an obligatory role in the autophosphorylation of BaComP based on the analysis of the structures and mutagenesis studies. The molecular docking results suggested that the HATPase_c domain contained an ATP-binding pocket, and the ATP molecule was coordinated by eight conserved residues from the N, G1, and G2 boxes. Our study provides novel insight into the histidine kinase BaComP and its homologous proteins.

Targeting the Holy Triangle of Quorum Sensing, Biofilm Formation, and Antibiotic Resistance in Pathogenic Bacteria

Chronic and recurrent bacterial infections are frequently associated with the formation of biofilms on biotic or abiotic materials that are composed of mono- or multi-species cultures of bacteria/fungi embedded in an extracellular matrix produced by the microorganisms. Biofilm formation is, among others, regulated by quorum sensing (QS) which is an interbacterial communication system usually composed of two-component systems (TCSs) of secreted autoinducer compounds that activate signal transduction pathways through interaction with their respective receptors. Embedded in the biofilms, the bacteria are protected from environmental stress stimuli, and they often show reduced responses to antibiotics, making it difficult to eradicate the bacterial infection. Besides reduced penetration of antibiotics through the intricate structure of the biofilms, the sessile biofilm-embedded bacteria show reduced metabolic activity making them intrinsically less sensitive to antibiotics. Moreover, they frequently express elevated levels of efflux pumps that extrude antibiotics, thereby reducing their intracellular levels. Some efflux pumps are involved in the secretion of QS compounds and biofilm-related materials, besides being important for removing toxic substances from the bacteria. Some efflux pump inhibitors (EPIs) have been shown to both prevent biofilm formation and sensitize the bacteria to antibiotics, suggesting a relationship between these processes. Additionally, QS inhibitors or quenchers may affect antibiotic susceptibility. Thus, targeting elements that regulate QS and biofilm formation might be a promising approach to combat antibiotic-resistant biofilm-related bacterial infections.

Characterization of Amino Acid Substitutions in the Two-Component Regulatory System AdeRS Identified in Multidrug-Resistant Acinetobacter baumannii

In Acinetobacter baumannii, resistance-nodulation-cell division (RND)-type efflux is a resistance mechanism of great importance since it contributes to reduced susceptibility to multiple antimicrobial compounds. Some mutations within the genes encoding the two-component regulatory system AdeRS appear to play a major role in increased expression of the RND efflux pump AdeABC and, consequently, in reduced antimicrobial susceptibility, as they are commonly observed in multidrug-resistant (MDR) A. baumannii. In the present study, the impact of frequently identified amino acid substitutions, namely, D21V and D26N in AdeR and T156M in AdeS, on adeB expression, efflux activity, and antimicrobial susceptibility was investigated. Reverse transcription-quantitative PCR (qRT-PCR) studies revealed significantly increased adeB expression caused by D26N (AdeR) and T156M (AdeS). In addition, accumulation assays have shown that these mutations induce increased efflux activity. Subsequently, antimicrobial susceptibility testing via agar dilution and broth microdilution confirmed the importance of these substitutions for the MDR phenotype, as the MICs for various antimicrobials of different classes were increased. In contrast, the amino acid substitution D21V in AdeR did not lead to increased adeB expression and did not reduce antimicrobial susceptibility. This study demonstrates the impact of the D26N (AdeR) and T156M (AdeS) amino acid substitutions, highlighting that these regulators represent promising targets for interfering with efflux activity to restore antimicrobial susceptibility.

IMPORTANCE The active efflux of antimicrobials by bacteria can lead to antimicrobial resistance and persistence and can affect multiple different classes of antimicrobials. Efflux pumps are tightly regulated, and their overexpression can be mediated by changes in their regulators. Identifying these changes is one step in the direction of resistance prediction, but it also opens the possibility of targeting efflux pump regulation as a strategy to overcome antimicrobial resistance. Here, we have investigated commonly found changes in the regulators of the main efflux pumps in Acinetobacter baumannii.

Evaluation of small molecule kinase inhibitors as novel antimicrobial and antibiofilm agents

Antibiotic resistance is a global and pressing concern. Our current therapeutic arsenal is increasingly limited as bacteria are developing resistance at a rate that far outpaces our ability to create new treatments. Novel approaches to treating and curing bacterial infections are urgently needed. Bacterial kinases have been increasingly explored as novel drug targets and are poised for development into novel therapeutic agents to combat bacterial infections. This review describes several general classes of bacterial kinases that play important roles in bacterial growth, antibiotic resistance, and biofilm formation. General features of these kinase classes are discussed and areas of particular interest for the development of inhibitors will be highlighted. Small molecule kinase inhibitors are described and organized by phenotypic effect, spotlighting particularly interesting inhibitors with novel functions and potential therapeutic benefit. Finally, we provide our perspective on the future of bacterial kinase inhibition as a viable strategy to combat bacterial infections and overcome the pressures of increasing antibiotic resistance.